Iodine

Endpoint summary

Description of key information

Key animal data:

A reliable subacute oral toxicity study in rats showed a NOAEL of 10 mg/kg bw/day of iodine for repeated dose toxicity (highest tested dose level). The study was performed according to OECD guideline 422 using rats.

A subchronic toxicity study to assess the effects of iodine and iodide was conducted in male and female Sprague-Dawley rats. Animals were treated with 0, 1, 3, 10 and 100 mg/L of either iodine or iodide (as NaI) in drinking water for 100 days. A NOAEL  of 0.375 mg/kg body weight/day and a LOAEL of 1.25 mg/kg bw/day for thyroid imbalance in female rats was found (calculated from 3 and 10 mg/L in drinking water, assuming a daily consumption of 25 mL per animal and a body weight of 200 grams). Specifically, in this study, a significant increase in T4 levels in females and an increase T4/T3 ratio in both sexes at the highest iodine concentration, was seen . In the case of iodide, there was an increase in T3 levels at 10 and 100 mg/L in male rats. T4/T3 ratio was increased at the highest iodide concentration in females and decreased at 10 mg/L in males. After 100 days of treatment, T4 values were decreased and the T4/T3 ratio increased significantly at 10 and 100 mg/L in female rats, this effect was only significant at the highest concentration in male rats. With sodium iodide, results were less predictable, T4 level increased at 1 mg/mL in males and T3 levels as well as T4/T3 ratio were affected at the lowest and highest concentrations. The fact that also iodide was tested in this study has no impact on the acceptability of the study, since iodine is quickly and quantitatively reduced to iodide in the small intestine prior to absorption and then converted to iodine in the thyroid and / or excreted as iodide via the urine.

 

Key human data:

In addition, there is an extensive set of literature on the long-term effects of iodine, iodide, and iodate salts in humans - see expert opinions below and section 7.10 for further informaiton on the key epidemiological tudies used for endpoint conclusion and dose descriptor. Based on the studies by Boyages et al., 1989 and Li et al., 1987 a chronic NOAEL in humans can be set at 0.01 mg/kg bw/day for a sensitive sub populations based on subclinical hypothyroidism in healthy human children.

The same data has been used by the ATSDR and WHO as the basis for setting a mimimum risk level (ATSDR) and a Tolerable Daily Intake (WHO) of 0.01 mg/kg bw/day which are values for acceptable oral chronic exposure to iodine.

Key value for chemical safety assessment

Toxic effect type:

dose-dependent

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Reference 1

Endpoint:

short-term repeated dose toxicity: oral

Remarks:

combined repeated dose and reproduction / developmental screening

Type of information:

experimental study

Adequacy of study:

key study

Study period:

From 2010-02-09 to 2010-05-14

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 422 (Combined Repeated Dose Toxicity Study with the Reproduction / Developmental Toxicity Screening Test)

Version / remarks:

March 1996

Deviations:

no

GLP compliance:

yes

Limit test:

no

Species:

rat

Strain:

other: Crl:WI (Han)

Details on species / strain selection:

This species and strain of rat has been recognized as appropriate for general and reproduction toxicity studies. NOTOX BV has general and reproduction/developmental historical data in this species from the same strain and source. This animal model has been proven to be susceptible to the effects of reproductive toxicants.

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Laboratories, L'Arbresle Cedex, France
- Age at study initiation: Approximately 11 weeks
- Weight at study initiation: Males 329 g ± 20% of the sex mean, Females 192 g ± 20% of the sex mean
- Fasting period before study: Overnight
- Housing:
Pre-mating: Animals were housed in groups of 5 animals/sex/cage in Macrolon cages (MIV type, height 18 cm), except for Female 81 which was single housed.
Mating: Females were caged together with males on a one-to-one-basis in Macrolon cages (MIII type, height 18 cm).
Post-mating: Males were housed in their home cage (Macrolon cages, MIV type, height 18 cm) with a maximum of 5 animals/cage. Females were individually housed in Macrolon cages (MIII type, height 18 cm).
Lactation: Pups were kept with the dam until termination in Macrolon cages (MIII type, height 18 cm).
- Diet (e.g. ad libitum): Free access to pelleted rodent diet (SM R/M-Z from SSNIFF® Spezialdiäten GmbH, Soest, Germany)
- Water (e.g. ad libitum): Free access to tap-water.
- Acclimation period: At least 5 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 21.0 ± 3.0
- Humidity (%): 40-70
- Air changes (per hr): 15
- Photoperiod (hrs dark / hrs light): 12/12

Route of administration:

oral: gavage

Details on route of administration:

using a plastic feeding tube

Vehicle:

DMSO

Details on oral exposure:

PREPARATION OF DOSING SOLUTIONS:
Formulations of Group 4 and/or a stock solution (w/w) were prepared daily within 6 hours prior to dosing and were homogenised to a visually acceptable level. Adjustment was made for specific gravity of the vehicle. The Group 2 and 3 formulations were prepared by dilution of the Group 4 formulation or the stock solution. Solutions were stored at ambient temperature.

VEHICLE
- Justification for use and choice of vehicle: Dimethyl sulphoxide (DMSO), specific gravity 1.1 (Merck, Darmstadt, Germany). DMSO was selected based on trial formulations performed at NOTOX.
- Amount of vehicle (if gavage): 1 mL/kg body weight. Actual dose volumes were calculated according to the latest body weight. This dose volume was based on results of a 13 week oral study in the rat in which a NOEL of 1100 mg/kg body weight/day was established (information from MSDS).

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Analyses were conducted on a single occasion during the treatment phase (24 March 2010), according to a validated method (NOTOX project 492834). Samples of formulations were analyzed for homogeneity (highest* and lowest concentration) and accuracy of preparation (all concentrations). Stability in vehicle over 6 hours at room temperature under protection from light and stability over 7 days in a refrigerator was also determined for the highest* and lowest concentrations.

*The highest concentration was 30 mg/mL (analyses were performed before change to 10 mg/mL).

Duration of treatment / exposure:

Males were exposed for 29 days, ie. 2 weeks prior to mating, during mating, and up to termination. Females were exposed for 41-47 days, i.e. during 2 weeks prior to mating, during mating, during post-coitum, and during at least 4 days of lactation.

Frequency of treatment:

Once daily

Dose / conc.:

10 mg/kg bw/day (actual dose received)

Remarks:

Initially, Group 4 animals received 30 mg/kg. However, the dose level was lowered to 10 mg/kg from Day 4 of treatment onwards after one female was euthanized in extremis.

Dose / conc.:

3 mg/kg bw/day (actual dose received)

Dose / conc.:

0.3 mg/kg bw/day (actual dose received)

No. of animals per sex per dose:

10 males and 10 females per group

Control animals:

yes, concurrent vehicle

Details on study design:

- Dose selection rationale: In order to set the dose levels for the main study, a dose range finding study was performed. Groups of 3 females (11-13 weeks old) were dosed at 1, 10 or 100 mg/kg/day for 10 days by oral gavage. At 1 and 10 mg/kg no toxicologically significant toxicity was noted. Animals treated at 100 mg/kg from Day 2 onwards showed clinical signs consisted of lethargy, hunched posture, piloerection, lean appearance, and/or rales. All three female rats were killed in extremis on Day 4 of treatment.
- Rationale for animal assignment (if not random): by computer-generated random algorithm according to body weight, with all animals within ± 20% of the sex mean.

Positive control:

None

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Twice daily.

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: Immediately after each dosing, once prior to start of treatment and at weekly intervals.

BODY WEIGHT: Yes
- Time schedule for examinations: Males and females were weighed on the first day of exposure and weekly thereafter. Mated females were weighed on days 0, 4, 7, 11, 14, 17 and 20 post-coitum, and during lactation on days 1 and 4.

FOOD CONSUMPTION AND COMPOUND INTAKE
- Time schedule for examinations: Weekly, except for males and females which were housed together for mating and for females without evidence of mating. Food consumption of mated females was measured on Days 0, 4, 7, 11, 14, 17 and 20 post-coitum and on Days 1 and 4 of lactation.

FOOD EFFICIENCY:
- Body weight gain in kg/food consumption in kg per unit time X 100 calculated as time-weighted averages from the consumption and body weight gain data: No

WATER CONSUMPTION AND COMPOUND INTAKE: Yes, Subjective appraisal was maintained during the study, but no quantitative investigation was introduced as no effect was suspected.

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: Yes
- Time schedule for collection of blood: at termination
- Anaesthetic used for blood collection: Yes (isoflurane)
- Animals fasted: Yes, fasted overnight
- How many animals: 5 animals/sex/group
- Parameters checked in table No. 1 were examined.

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: at termination
- Animals fasted: Yes, fasted overnight
- How many animals: 5 animals/sex/group
- Parameters checked in table No. 1 were examined.

URINALYSIS: No


NEUROBEHAVIOURAL EXAMINATION: Yes
- Time schedule for examinations: Selected males were tested during week 4 and the selected females were tested towards the end of the scheduled lactation period (all before blood sampling).
- Dose groups that were examined: All groups (5 animals/sex/group)
- Battery of functions tested: sensory activity (hearing ability, pupillary reflex, static righting reflex), grip strength and motor activity

Sacrifice and pathology:

GROSS PATHOLOGY: Yes (see table 2)
HISTOPATHOLOGY: Yes (see table 2)

The following slides were examined by a pathologist:
- The preserved organs and tissues of the selected 5 animals/sex of Groups 1 and 4.
- The additional slides of the testes of the selected 5 males of Groups 1 and 4 to examine staging of spermatogenesis.
- The preserved organs and tissues of the animal that was killed in extremis (no. 80).
- All gross lesions of all animals (all dose groups).
- The reproductive organs of all animals that failed to mate, conceive, sire or deliver healthy pups: Group 1 Female 46 and Male 6, and Group 2 Female 56 and Male 16 (both females were non-pregnant).

Other examinations:

Organ Weights: The following organ weights and terminal body weight were recorded from the fllowing animals on the scheduled day of necropsy. Selected 5 animals/sex/qroup: Adrenal glands, Spleen, Brain, Testes, Epididymides, Thymus, Heart, Uterus (including cervix), Kidneys, Prostate, Liver, Seminal vesicles including coagulating glands, Ovaries, Thyroid including parathyroid; All remaining males: Epididymides, Testes

Statistics:

- If the variables could be assumed to follow a normal distribution, the Dunnett-test based on a pooled variance estimate was applied for the comparison of the treated groups and the control groups for each sex.
- The Steel-test was applied if the data could not be assumed to follow a normal distribution.
- The Fisher Exact-test was applied to frequency data.
The following additional methods of statistical analysis were used:
The number of corpora lutea was transformed by using 1/x to obtain a normal distribution. This was followed by an ANOVA. The Dunnett-test (many-to-one t-test) based on a pooled variance estimate was applied for the comparison of the treated groups and the control group.
All tests were two-sided and in all cases p < 0.05 was accepted as the lowest level of significance.

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

no effects observed

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

not examined

Haematological findings:

effects observed, non-treatment-related

Clinical biochemistry findings:

effects observed, non-treatment-related

Urinalysis findings:

not examined

Behaviour (functional findings):

effects observed, non-treatment-related

Organ weight findings including organ / body weight ratios:

effects observed, non-treatment-related

Gross pathological findings:

effects observed, non-treatment-related

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

not examined

Details on results:

CLINICAL SIGNS AND MORTALITY

One female at 30 mg/kg Group 4, Female no.80) was killed in extremis on Day 2 of the treatment period. This animal showed hunched posture, piloerection, ptosis and pale appearance, and a body weight loss of 10% over one day. At necropsy, many dark red foci on the thymus were noted and reddish discoloration of the gastro-intestinal tract that was also distended with gas. At microscopic examination, this animal had minimal to severe degrees of villous atrophy in the duodenum, jejunum and cecum along with moderate atrophy of the gastric mucosa. These findings indicate a functional disturbance of the gastrointestinal tract which was considered to have contributed to moribundity in this animal. As this animal had been dosed at 30 mg/kg which resulted in a 10% loss of body weight within 24 hours, a relationship to treatment cannot be excluded.

Clinical signs of toxicity were noted at 30 mg/kg for females. No treatment related clinical signs were noted up to 10 mg/kg. At 30 mg/kg, hunched posture, piloerection, pale appearance and ptosis were noted for female no. 80 on the day she was euthanized in extremis (Day 2 of treatment). In addition, Female 75 showed hunched posture on Days 3-5 of treatment. This was considered to have been caused by treatment at 30 mg/kg on Days 1-3 (not from treatment at 10 mg/kg which she received from Day 4 onwards).

Redness and swelling of both ears was noted for female no. 79 (Group 4, 10 mg/kg) over the repro period. Microscopic examination revealed slight unilateral lymphohistiocytic inflammation with cartilage necrosis. At this single incidence it was considered to have occurred by chance and not to be treatment related.
Incidental findings that were noted included scabs and alopecia. These findings occurred within the range of background findings to be expected for rats of this age and strain which are housed and treated under the conditions in this study.

Partly based on these findings, the dose level of Group 4 was lowered on Day 4 of the treatment period from 30 mg/kg to 10 mg/kg.

BODY WEIGHT AND WEIGHT GAIN

Body weights and body weight gain of treated animals remained in the same range as controls over the treatment period up to 10 mg/kg.
At 30 mg/kg, body weight loss was noted for five out of ten females (range of 1-12% body weight loss in two days).
Partly based on these findings, the dose level of Group 4 was lowered on Day 4 of the treatment period from 30 mg/kg to 10 mg/kg.

FOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study)

Food consumption before or after allowance for body weight was similar between treated and control animals. Food consumption was slightly reduced for the first week for females of Group 4 (cages 15-16; not statistically significant). This was considered to have been caused by the high dose level of 30 mg/kg given for the first three days of dosing.


HAEMATOLOGY

No toxicologically relevant changes occurred in haematological parameters of treated rats.
The statistically significant reduction in Prothrombin time (PT) and increase in lymphocytes for males at 10 mg/kg (Group 4) were not considered to be toxicologically relevant as they remained within the range considered normal for rats of this age and strain and were considered to have arisen as a result of slightly high or low control values.
Individual increases of neutrophil counts with concurrently reduced lymphocyte counts were noted among the dose groups (both sexes) without a treatment related distribution. This shift in type of white blood cells was considered to be a secondary non-specific response to stress and of no toxicological relevance.

CLINICAL CHEMISTRY

Significantly lower Aspartate aminotransferase (ASAT) values and significantly higher inorganic phosphate values were seen for males at 10 mg/kg (Group 4). Furthermore, significantly lower bile acid counts were noted for males at 3 and 10 mg/kg. Means for these values only just exceeded or remained within the range considered normal for rats of this age and strain.
T4/T3 ratio was increased at 10 mg/kg for the males and at 3 and 10 mg/kg for the females (not statistical significant).
For the females, TSH was reduced for all dose groups however without a dose response relationship and not statistical significant (high variation in control group). Total T3 was reduced for females at 3 mg/kg (not statistical significant) and at 10 mg/kg (p < 0.05), and Total T4 was increased for females at 3 mg/kg (p < 0.05) and at 10 mg/kg (not statistical significant).
For males at 3 mg/kg, an increase in chloride was noted. At 3 mg/kg for females, the statistically significantly lower ASAT and the higher total bilirubin counts occurred in the absence of a treatment-related distribution. Furthermore, the increase in total bilirubin was attributed to a high value noted for animal no. 64. These values were within normal and occurred in the absence of a dose response relationship. Taken together, the changes in chloride, total bilirubin, and ASAT were not considered toxicologically significant.

NEUROBEHAVIOUR

No toxicologically relevant effects on hearing ability, pupillary reflex, static righting reflex and grip strength were observed.
One female treated at 0.3 mg/kg (Group 2, no. 54) showed absent grip strength. As it concerning only one animal of the low dose group, it was not considered toxicologically relevant.
The variation in motor activity did not indicate a relation with treatment. Females at 10 mg/kg (Group 4) had higher activity counts (low sensor; not statistically significant). As the values were well within normal limits and it occurred in the absence of any corroborative findings like hyperactivity, the increase was not considered to be toxicologically relevant.

ORGAN WEIGHTS

No toxicologically relevant changes were noted in organ weights and organ to body weight ratios.
At 10 mg/kg (Group 4), the higher liver to body weight ratio seen for males was not reflective of treatment related toxicity because the mean and individual values remained well within the range of data considered normal for this age and strain and no microscopic correlate was noted.
Two females treated at 10 mg/kg (nos. 73 and 75) showed relatively high thymus weights when compared to the concurrent control values. However, these values were still within normal limits and in the absence of microscopic findings, were not regarded toxicologically relevant.
Other organ weights and organ to body weight ratios among the dose groups were similar to control levels.

GROSS PATHOLOGY

Necropsy did not reveal any toxicologically relevant alterations up to 10 mg/kg.
At 30 mg/kg, the female that was euthanized in extremis (no. 80) was noted with many dark red foci on the thymus and reddish discoloration of the gastro-intestinal tract that was also distended with gas.
Incidental findings included dark red discoloration of the mandibular lymph nodes, black discoloration of the popliteal lymph nodes, an isolated red focus or many dark red foci on the thymus, soft yellow nodule on the tail and body of the left epididymide, thickening of the ears, pelvic dilation of the kidneys, and alopecia over various body regions. The incidence of these findings was within the background range of findings that are encountered among rats of this age and strain, and did not show a dose-related incidence trend. These necropsy findings were therefore not considered to be toxicologically relevant.

HISTOPATHOLOGY: NON-NEOPLASTIC

There were no treatment-related microscopic findings.
One female Group 4 animal (30 mg/kg, animal 80) was sacrificed moribund after one day on test. This animal had minimal to severe degrees of villous atrophy in the duodenum, jejunum and cecum along with moderate atrophy of the gastric mucosa. These findings indicate a functional disturbance of the gastrointestinal tract which was considered to have contributed to moribundity in this animal. As this animal had been dosed at 30 mg/kg which resulted in a 10% loss of bodyweight within 24 hours, a relationship to treatment cannot be excluded.
No abnormalities were seen in the reproductive organs of the suspected non-fertile animals (Group 1 female 46 and male 6 and Group 2 female 56 and male 16) which could account for their infertility.
All microscopic findings recorded were considered to be within the normal range of background pathology encountered in Wistar-Han rats of this age and strain.

Dose descriptor:

NOAEL

Remarks:

Parental

Effect level:

10 mg/kg bw/day (actual dose received)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: Based on the absence of functional or morphological disturbances supporting the changes noted for clinical biochemistry parameters

Critical effects observed:

not specified

Analysis of Dose Preparations

The concentrations analysed in the formulations of Group 2 (0.273 mg/g), Group 3 (2.73 mg/g) and Group 4 (27.3 mg/g) were in agreement with target concentrations (i.e. mean accuracies between 90% and 110%). No test substance was detected in the Group 1 formulations.

The formulations of Group 2 and Group 4 were homogeneous (i.e. coefficient of variation ≤ 10%).

Formulations at the highest concentration level (i.e. 27.3 mg/g) were stable when stored in a refrigerator for at least 7 days. Formulations at the lowest concentration level (i.e. 0.273 mg/g) were stable when stored at room temperature for at least 6 hours, but not stable when stored in a refrigerator for 7 days.

Conclusions:

A combined 28-day repeated dose toxicity study with the reproduction/developmental toxicity screening test was conducted in rats by oral gavage following OECD 422. Rats were given 0, 0.3, 3 and 10 mg/kg (initially 30 mg/kg) of Iodine. Based on the absence of functional or morphological disturbances supporting the changes noted for clinical biochemistry parameters under testing conditions, a NOAEL of 10 mg/kg was established after repeated doses of Iodine in Wistar rats.

Executive summary:

A combined 28-day repeated dose toxicity study with the reproduction/developmental toxicity screening test was conducted in rats by oral gavage following OECD 422.

 

Based on the results of a 10-day dose range finding study, the dose levels for this combined 28-day oral gavage study with reproduction/developmental toxicity screening test were selected to be 0.3, 3 and 30 mg/kg of Iodine.

 

After acclimatisation, four groups of ten male and ten female Wistar Han rats were exposed by oral gavage to Iodine at 0, 0.3, 3 and 30 mg/kg. Due to severe toxicity at 30 mg/kg, the dose level of Group 4 was adjusted to 10 mg/kg from Day 4 of the study onwards. Males were exposed for 29 days, i.e. 2 weeks prior to mating, during mating, and up to termination. Females were exposed for 41-47 days, i.e. during 2 weeks prior to mating, during mating, during post-coitum, and during at least 4 days of lactation.

 

The following parameters were evaluated: mortality/viability, clinical signs, functional observations, body weights, food consumption, reproduction/developmental parameters, observations pups, clinical pathology (including thyroid hormones), macroscopy, organ weights, and histopathology. Chemical analyses of formulations were conducted once during the study to assess accuracy, homogeneity and stability.

 

No relevant parental toxicity was observed up to 10 mg/kg. Initially, Group 4 animals received 30 mg/kg. However, the dose level was lowered to 10 mg/kg from Day 4 of treatment onwards after one female was euthanized in extremis. This animal had hunched posture, piloerection, pale appearance and ptosis after one day of dosing. Many dark red foci on the thymus and reddish discoloration of the gastro-intestinal tract that was distended with gas was noted upon macroscopic examination, and minimal to severe degrees of villous atrophy in the duodenum, jejunum and cecum along with moderate atrophy of the gastric mucosa were noted at microscopic examination. These findings indicate a functional disturbance of the gastrointestinal tract which was considered to have contributed to moribundity in this animal. In addition, one female treated at 30 mg/kg showed hunched posture on Days 3-5 of treatment, and body weight loss was noted for five out of ten females treated at 30 mg/kg. Food consumption was slightly reduced for females treated at 30 mg/kg. These findings were considered to have been caused by irritating properties of the test substance as noted during the dose range finding study at 100 mg/kg.

A number of clinical biochemistry changes were noted at 3 and/or 10 mg/kg which included lower aspartate aminotransferase and higher inorganic phosphate levels in blood for males at 10 mg/kg, and lower bile acid counts for males at 3 and 10 mg/kg. Means of these changes only just exceeded or remained within the range considered normal for rats of this age and strain. Moreover, there were no histopathological correlates that would support these changes. Therefore, these changes were considered not to be of toxicological relevance. The changes noted for thyroid hormone levels were very slight, not statistical significant and/or without a dose response relationship. In addition, as no effects were noted for thyroid weights and histopathology these slight changes were not considered to be toxicologically significant. Up to 10 mg/kg, no treatment-related toxicologically significant changes were noted in any of the remaining parental parameters investigated in this study (i.e. clinical appearance, functional observations, body weight, food consumption, haematology, macroscopic examination, organ weights, and microscopic examination).

Based on the absence of functional or morphological disturbances supporting the changes noted for clinical biochemistry parameters, a parental NOAEL of 10 mg/kg was established.

Reference 2

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Published study with deficiencies (compared to current OECD guideline): - Lower number of animals in treated groups - Clinical observation were not reported, ophthalmological examination not performed. - Limited haematological examinations and body weights were not determined as frequent as recommended. - Limited histopathological examination.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)

Deviations:

yes

Remarks:

relevant deviations are indicated under reliability rationale.

GLP compliance:

not specified

Limit test:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Simonsen Laboratories (Gilroi, Calif., USA)
- Age at study initiation: 34-38 days
- Weight at study initiation: Not reported
- Housing: 3 animals per cage.
- Diet: Purina rodent chow ad libitum
- Water: Ad libitum
- Acclimation period: Not reported

ENVIRONMENTAL CONDITIONS
- Temperature (°C): Not reported
- Humidity (%): Not reported
- Air changes (per hr): Not reported
- Photoperiod (hrs dark / hrs light): 12/12

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

PREPARATION OF DOSING SOLUTIONS:
Disolution of tested substance in drinking water (iodine and sodium iodide, in separate groups). No further details reported.


VEHICLE
- Vehicle: Water
- Concentration in vehicle: 1, 3, 10, and 100 mg/L

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

Samples for analysis were collected during the middle of the lights-on phase.

Duration of treatment / exposure:

100 days

Frequency of treatment:

Continuous treatment (administration in drinking water)

Dose / conc.:

1 mg/L drinking water

Dose / conc.:

3 mg/L drinking water

Dose / conc.:

10 mg/L drinking water

Dose / conc.:

100 mg/L drinking water

No. of animals per sex per dose:

Control group: 12 females and 12 males
Treated groups (iodine and iodide): 6 females and 6 females

Control animals:

yes

Details on study design:

- Dose selection rationale: Not reported
- Rationale for animal assignment: Random

Positive control:

No positive control

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: No data
- Time schedule: No data
- Cage side observations are not reported.

DETAILED CLINICAL OBSERVATIONS: No data
- Time schedule: No data

BODY WEIGHT: Yes
- Time schedule for examinations: 100 days

FOOD EFFICIENCY:
- Body weight gain in kg/food consumption in kg per unit time X 100 calculated as time-weighted averages from the consumption and body weight gain data: No data

WATER CONSUMPTION AND COMPOUND INTAKE : Yes
- Time schedule for examinations: Every other day

OPHTHALMOSCOPIC EXAMINATION: No data

HAEMATOLOGY: Yes
- Time schedule for collection of blood: 100 days
- Anaesthetic used for blood collection: No data
- Animals fasted: No data
- How many animals: All
- Parameters checked in table 1 were examined.

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: 100 days
- Animals fasted: No data
- How many animals: All
- Parameters checked in table 2 were examined.

URINALYSIS: Not performed


NEUROBEHAVIOURAL EXAMINATION: No


OTHER:
ORGAN WEIGHTS
- Time schedule for collection of samples: 100 days
- How many animals: All
- Parameters checked in table 3 were examined.

THYROID HORMONES
- Time schedule for collection of samples: 10 days and at termination (100 days)
- How many animals: All
- Parameters checked in table 4 were examined.

Sacrifice and pathology:

HISTOPATHOLOGY: Yes, but only thyroid

Statistics:

One-way analysis of variance (ANOVA) was used to test for statistical differences among treatment groups. Groups with a p value ≤ 0.05 (ANOVA) were subjected to pairwise comparisons using Turkey's multiple range test for thyroid hormone levels and two-sample t-test for all others.

Clinical signs:

not examined

Mortality:

not examined

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not examined

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not specified

Ophthalmological findings:

not examined

Haematological findings:

no effects observed

Description (incidence and severity):

Hematocrit, hemoglobin, and blood urea nitrogen (BUN) values are relatively consistent and do not statistically vary with treatment. There were variations in AST, ALT, cholesterol, and triglyceride values between treatment groups, but there are no consistent treatment-related effects. No statistically significant differences were seen among any of the hematological parameters.

Clinical biochemistry findings:

effects observed, treatment-related

Description (incidence and severity):

produce significant increases in the T4/T3 ratio

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

no effects observed

Description (incidence and severity):

revealed no pathological alterations that were treatment related.

Gross pathological findings:

not examined

Histopathological findings: non-neoplastic:

no effects observed

Description (incidence and severity):

restricted to thyroid exclusively

Histopathological findings: neoplastic:

not examined

Details on results:

CLINICAL SIGNS AND MORTALITY: Not reported

BODY WEIGHT: Unaffected by iodine and iodide treatment

WATER CONSUMPTION AND COMPOUND INTAKE (if drinking water study); Not reported

OPHTHALMOSCOPIC EXAMINATION: Not performed

HAEMATOLOGY: No statiscally significant difference between treated and control groups.

CLINICAL CHEMISTRY: No consistent treatment-related effects.

URINALYSIS: Not performed

NEUROBEHAVIOUR: Not performed

ORGAN WEIGHTS: Males had increased thyroid weight in a in a concentration- dependant manner with iodide. When thyroid weight was corrected for body weight the ratio significantly increased at the 10 and 100 ppm level. Thyroid weight was decreased in female rats at the highest iodide dose. Kidney weight was affected by iodide treatment in female rats but was not dose related and was less marked when adjusted to body weight. Liver weight was affected (increased) in highest dose iodine dose but not statistically significant when corrected to body weight.

GROSS PATHOLOGY: Not performed

HISTOPATHOLOGY: No pathological alterations of thyroid

OTHER FINDINGS: See table 5 and 6 below for thyroid hormones measurements

Dose descriptor:

NOAEL

Effect level:

3 mg/L drinking water

Based on:

test mat.

Remarks:

iodine

Sex:

female

Basis for effect level:

gross pathology

histopathology: neoplastic

histopathology: non-neoplastic

Dose descriptor:

LOAEL

Effect level:

10 mg/L drinking water

Sex:

female

Basis for effect level:

other: Thyroid hormone imbalance: decrease in T3 levels; increase in T4/T3 ratio (100 days treatment)

Dose descriptor:

NOAEL

Effect level:

10 mg/L drinking water

Based on:

test mat.

Remarks:

iodine

Sex:

male

Basis for effect level:

gross pathology

histopathology: neoplastic

histopathology: non-neoplastic

Dose descriptor:

LOAEL

Effect level:

100 mg/L drinking water

Based on:

test mat.

Sex:

male

Basis for effect level:

other: Thyroid hormone imbalance: decrease in T3 levels; increase in T4/T3 ratio (100 days treatment)

Critical effects observed:

not specified

After 10 days, there was dose-related trend of increased plasma T4 concentrations in both sexes treated with iodine. However, only significant in female rats at the highest dose. T4 values did not show a consistent change in iodide-treated rats. In females treated with 100 mg/L iodine there was a significant increase in T4 (statistically significant) with a decrease in T3 (not statistically significant), resulting in an increased T4/T3 ratio (statistically significant). A similar increase was observed in male rats at the highest iodine dose.

 

After 100 days, thyroid hormone levels showed similar trends in the T4/T3 ratio in iodine-treated animals. The ratio was increased with iodine and iodide treatment in males and females. However, decreased T3 was more responsible for this increase in the ratio than T4. While iodine lead to significant increase in the T4/T3 ratio in male animals, iodide did not. In females, both iodine and iodide treatment resulted in statistically significant elevations in T4/T3 ratios.

 

According to the authors the effect of iodide was much less predictable and was smaller than that with iodine.

Table 5: Effects of 10 and 100-days treatment with varying concentrations of iodine on thyroid hormones in rats.

Time schedule/Sex/Dose	T4	T3	T4/T3
10 days
Females
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	Increased	NSC	Increased
Males
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	NSC	NSC	Increased
100 days
Females
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	Decreased	Increased
100 mg/L	NSC	Decreased	Increased
Males
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	NSC	Decreased	Increased

NSC: No statistical significant change.

Table 6: Effects of 10 and 100-days treatment with varying concentrations of sodium iodide on thyroid hormones in rats.

Time schedule/Sex/Dose	T4	T3	T4/T3
10 days
Females
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	NSC	NSC	Increased
Males
1 mg/L	NSC	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	Increased	Decreased
100 mg/L	NSC	Increased	NSC
100 days
Females
1 mg/L	NSC	Decreased	Increased
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	NSC	Decreased	Increased
Males
1 mg/L	Increased	NSC	NSC
3 mg/L	NSC	NSC	NSC
10 mg/L	NSC	NSC	NSC
100 mg/L	NSC	NSC	NSC

NSC: No statistical significant change.

Conclusions:

A subchronic toxicity study to assess the effects of iodine and iodide was conducted in male and female Sprague-Dawley rats. Animals were treated with 0, 1, 3, 10 and 100 mg/L of either iodine or iodide (as NaI) in drinking water for 100 days.

After 100 days of treatment with iodine, T3 levels and the T4/T3 were significantly affected at the higher concentrations (10 and 100 mg/L in females and 100 mg/L in male rats). The same effect was observed with iodine, but at the highest and lowest concentrations in females.

A NOAEL  of 0.375 mg/kg body weight/day and a LOAEL of 1.25 mg/kg bw/day for thyroid imbalance in female rats (calculated from 3 and 10 mg/L in drinking water, assuming a daily consumption of 25 mL per animal and a body weight of 200 grams), was derived.

Executive summary:

A subchronic toxicity study to assess the effects of iodine and iodide was conducted in male and female Sprague-Dawley rats. Animals were treated with 0, 1, 3, 10 and 100 mg/L of either iodine or iodide (as NaI) in drinking water for 100 days.

No significant or treatment-related effects were observed on organ body weights, haematological parameters and clinical chemistry at termination. After 10 days of treatment there was a significant increase in T4 levels in females and an increase T4/T3 ratio in both sexes at the highest iodine concentration. In the case of iodide, there was an increase in T3 levels at 10 and 100 mg/L in male rats. T4/T3 ratio was increased at the highest iodide concentration in females and decreased at 10 mg/L in males. After 100 days of treatment, T4 values were decreased and the T4/T3 ratio increased significantly at 10 and 100 mg/L in female rats, this effect was only significant at the highest concentration in male rats. With sodium iodide, results were less predictable, T4 level increased at 1 mg/mL in males and T3 levels as well as T4/T3 ratio were affected at the lowest and highest concentrations. The fact that also iodide was tested in this study has no impact on the acceptability of the study, since iodine is quickly and quantitatively reduced to iodide in the small intestine prior to absorption and then converted to iodine in the thyroid and / or excreted as iodide via the urine.

A NOAEL  of 0.375 mg/kg body weight/day and a LOAEL of 1.25 mg/kg bw/day for thyroid imbalance in female rats (calculated from 3 and 10 mg/L in drinking water, assuming a daily consumption of 25 mL per animal and a body weight of 200 grams), was derived.

Reference 3

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

GLP compliance:

not specified

Limit test:

no

Species:

rat

Strain:

Wistar

Details on test animals or test system and environmental conditions:

Sixty female diabetes-prone BB Wistar rats (13 separate litters) from the Health Protection Branch colony were block randomized according to body weight into 4 groups of 15 animals each. The animals were housed in individual stainless steel mesh-bottomed cages and allowed free access to food and water. Each of the groups was fed a basal diet which was a modification of the AIN-76A diet containing 10% com oil instead of 5%.
Composition of the basal diet:
Casein: 20%; Corn oil: 10%; Nonnutritive fiber: 3%; Starch: 25%; Dextrose: 37%; AIN-76A vitamin mix: 1%; AIN-76 mineral mix: 3.5%; Choline bitartrate: 0.2 %; DL-methionine: 0.3%.

Route of administration:

oral: feed

Details on route of administration:

This diet was supplemented with KIO3 so that the final concentrations of iodine in the experimental diets were 0.2, 1.0, 2.0, and 3.0 mg/kg of diet.

Vehicle:

unchanged (no vehicle)

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

10 weeks

Dose / conc.:

0.2 mg/kg diet

Dose / conc.:

1 mg/kg diet

Dose / conc.:

2 mg/kg diet

Dose / conc.:

3 mg/kg diet

No. of animals per sex per dose:

15

Control animals:

no

Observations and examinations performed and frequency:

The onset of diabetes was monitored by estimating the level of glucose in the urine daily (Testape, Eli Lilly). Food consumption and body weights were determined weekly. At 2-wk intervals, the rats were moved into metabolic cages (Nalgene, Rochester, NY) and 24 h urine and fecal samples were collected. These were analyzed for iodine concentration.

Sacrifice and pathology:

After 10 wk, the animals were killed by exsanguination from the abdominal aorta while under pentobarbital anesthesia (6 mg/100 g body wt, i.p.).
- Histology:
Thyroid lobes were resected at necropsy, fixed in 10% phosphate buffered formalin, embedded in paraffin, and cut at intervals of 5 μm. Sections from all thyroids were stained with hematoxylin and eosin. Additional sections from animals with lymphocytic thyroiditis were stained with methyl green pyronin to differentiate plasma cells from mast cells, by ammoniacal silver impregnation for reticulum and by phosphotungstic acid hematoxylin, Van Gieson's technique for collagenous and other mesenchymal cellular components. Using slides stained with hematoxylin and eosin, the median of maximum and minimum diameters of the largest tissue section per rat was measured by Vernier calipers.

Statistics:

All analyses were carried out in triplicate using at least 13 animals per group. Results were expressed as mean ± SEM. The data were tested for significant differences by one-way analysis of variance (ANOVA) for unbalanced data, using the Human System Dynamics (North Ridge, CA) PC ANOVA program. Significant differences between means were calculated by the Least Significant Difference (LSD) method at 95 and 99% confidence levels, using the same program.

Details on results:

During the 10-wk course of the experiment, only three rats developed diabetes: 2 fed 0.2 mg of iodine/kg (wk 9) and 1 fed 1.0 mg/kg (wk 8). The data for these animals were not included in calculating the mean values for these groups. By 4 wk, the mean body weight of the rats fed 0.2 mg/kg was significantly lower than that of the animals fed 3.0 mg/kg (P < 0.05). By wk 6, this weight also became significantly lower than that of the rats fed 2.0 mg/kg and by wk 9, less than that of the animals fed 1.0 mg/kg. The rats fed 0.2 mg of iodine/ kg of diet had a significantly lower food consumption starting at wk 2.
Urinary iodine excretion was directly related to iodine intake, with correlation coefficients between 0.984 and 1.000 during the 10-wk experiment. Fecal excretion, in comparison, was significantly correlated to iodine intakes only at wk 4 (r = 0.999) and at wk 10 (r = 0.9772).
Lymphocytic thyroiditis was found in one rat fed 0.2 mg iodine/kg, and in 2 in each of the groups fed 2.0 and 3.0 mg/kg. In other rats, thyromegaly, related to iodine intake, appeared without other associated histopathological changes visible by standard unassisted light microscopy. The mean thyroid weights increased with increasing dietary iodine levels, so that the weight observed in the group fed 0.2 mg/kg was significantly less than that in the group fed 3 mg/kg (P < 0.05). Similarly the mean thyroid diameters in the groups fed 2.0 and 3.0 mg iodine/ kg were significantly greater than the diameter observed in the group fed 0.2 mg/kg (P < 0.01). If an abnormally large thyroid is defined as one that is greater than the mean + 2 so of the diameters observed in the group fed 0.2 mg of iodine/kg, then 2 abnormal glands were found in the group fed 1.0 mg/kg, and 6 in each of the groups fed 2.0 and 3.0 mg/kg.
No autoantibodies to either T3 or T4 could be detected. This may have been due to the fact that rats have thyroxine-binding globulin which might reduce the sensitivity of the assay. A positive control serum was not available to test this. There was, however, an increase in autoantibodies to thyroglobulin with increasing iodine intakes. The A405 observed in the thyroglobulin assay of the serum from animals fed the highest amount of iodine was significantly greater than that observed when analyzing serum from rats fed 1.0 mg/kg. If an absorbance in excess of the mean + 2 so of the values observed in the group fed 0.2 mg/kg is taken as indicative of the presence of thyroglobulin autoantibodies, then 2 rats fed 0.2 mg/kg, none fed 1.0 mg/kg, 2 fed 2.0 mg/kg and 6 fed 3.0 mg/kg tested positive for these antibodies. The absorbances observed for the two rats fed 0.2 mg/kg which were considered to have antibodies to thyroglobulin were included in the mean.

Conclusions:

High iodine intakes increase Tg antibodies, which may be associated with an increase in autoimmune thyroiditis in these animals.

Executive summary:

Diabetes-prone BB Wistar rats were fed a modified AIN-76 diet providing the following amounts of iodine for 10 wk: 0.2 mg/kg diet (recommended amount): 1.0 mg/kg; 2.0 mg/kg; or 3.0 mg/kg. The thyroids were examined for gross and microscopic changes and sera were assayed for antibodies to triiodothyronine (T3), thyroxine (T4), and thyroglobulin (Tg). The body weights and food consumption of the rats fed 0.2 mg of iodine/kg were significantly lower than those of the animals fed higher amounts, urinary iodine excretion reflected dietary intakes. The thyroids from animals fed 2.0 and 3.0 mg/kg were significantly (P < 0.01 ) larger than those from animals fed 0.2 mg/kg. One rat fed 0.2 mg/kg and 2 rats in each group fed 2.0 and 3.0 mg/kg had extensive lymphocytic thyroiditis. Three rats fed 1.0 mg/kg. 6 fed 2.0 mg/kg and 6 fed 3.0 mg/kg had enlarged thyroids. Two rats fed 0.2 mg/ kg, 2 fed 2.0 mg/kg and 6 fed 3.0 mg/kg had detectable Tg antibodies. These data suggest that high iodine intakes increase Tg antibodies, which may be associated with an increase in autoimmune thyroiditis in these animals.

Reference 4

Endpoint:

short-term repeated dose toxicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Studies were carried out to determine whether a far smaller increase in iodine intake would also affect thyroid function. A small increase in dietary iodine can induce subtle changes (all values remaining within the normal range) in pituitary-thyroid function, probably by inhibiting thyroid hormone release. The smaller iodine supplements of 500 and 250 μg daily, quantities that may easily be achieved under normal conditions, did not, however, affect thyroid function.

GLP compliance:

not specified

Limit test:

no

Species:

other: Human

Sex:

male/female

Details on test animals or test system and environmental conditions:

Nine euthyroid men between the ages of 26 and 56 years (34 ± 3, mean ± SE) and 23 euthyroid women between the ages of 23 and 44 years (32 ± 2) were studied.

Careful history and physical examination revealed no evidence of thyroid disease in any, and none had detectable quantities of antithyroid antibodies.

Route of administration:

other: po

Details on route of administration:

All subjects were then treated with various doses of iodine in 0.5 mL of water po (Nal and 5 mg ascorbic acid/mL) every 12 hours for the following 14 days.

Vehicle:

water

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

14 days

Frequency of treatment:

Twice daily

Dose / conc.:

1 500 other: μg/day

Remarks:

men and women

Dose / conc.:

500 other: μg/day

Remarks:

women

Dose / conc.:

250 other: μg/day

Remarks:

women

No. of animals per sex per dose:

1500 μg/day: 9 men and 9 women; 500 and 250 μg/day: 9 women each

Control animals:

yes, concurrent no treatment

Details on study design:

A 24-hour urine was collected for baseline urinary iodine (I) and creatinine (Cr) excretion. Urinary creatinine was measured to help assure that the 24-hour urine collection was complete. At 9 AM on day 0, an indwelling venous catheter was inserted into an antecubital vein and blood collected for baseline serum protein bound (PBI) and total iodine (TI), thyroxine (T4), triiodothyronine (T3), and thyrotropin (TSH) concentrations, as well as for determinations of the resin T3 uptake and free T4 index (FT4I). Thyrotropin releasing hormone (TRH, 500 μg, Relefact, Hoechst-Roussel, Somerville, NJ) was then administered as a bolus through the intravenous catheter, and blood was obtained 15, 30, 45, and 60 minutes later for the measurement of serum TSH concentrations. All subjects were then treated with various doses of iodine in 0.5 mL of water po (Nal and 5 mg ascorbic acid/mL) every 12 hours for the following 14 days. The men received 750 μg iodide twice daily and the women received 750, 250, or 125 μg twice daily. Some of the women were studied at two dose levels at least one year apart. A 24-hour urine collection was obtained for measurement of I and Cr content on day 7, and at 9 AM on day 8, prior to iodine ingestion, a sample of blood was obtained for serum iodine analyses. Another 24-hour urine specimen was collected on day 14 and the TRH test repeated at 9 AM on day 15, as outlined above.
Five additional age-matched men were studied as outlined above, but were not given the iodine supplement. No changes in serum T4 and T3 concentrations, the FT4I, and basal and TRH stimulated serum TSH concentrations were observed in these five volunteers during the 14-day study period.

Observations and examinations performed and frequency:

Samples of serum and urine were frozen at - 20℃ until analyzed. All measurements were made in duplicate and all serum samples for a given test were assayed in the same assay. Serum T4, T3, and TSH concentrations were measured by radioimmunoassay (ARIA-HT, Bectin Dickinson Immuno Diagnostics, Orangeburg, NY) and the FT4I was calculated as the product of the total T4 and the T3 resin uptake (ARIA). Normal ranges for these thyroid function tests are: T4 (4.3 to 9.5 μg/dL); FT4I (4.4 to 9.4); T3 (100 to 205 ng/dL); TSH (0.5 to 5μU/mL). Antithyroglobulin (anti-Tg) and antimicrosomal (anti-M) antibodies were measured by a tanned red blood cell technique (Ames Division, Miles Laboratories, Elkhart, IN). Serum PBI and TI and urine I concentrations were measured by the method of Benotti et al and urine Cr by antoanalyzer. THE serum inorganic iodide concentration was calculated as the difference between the TI and PBI concentrations. The 24-hour urinary I excretion is reported as μg I/24 h.

Statistics:

All values are reported as the mean ± SE and statistical significance of differences observed was assessed by the Student's paired t-test.

Details on results:

- Urine and Serum Iodine:
In the men and women receiving 1500 μg iodine daily, the mean 24-hour urine I excretion increased from 211 ± 41 μE I/24 h on day 0 to 1360 ± 74 on day 7 (P < 0.001) and 1308 ± 58 on day 14 (P < 0.001). The increase in I excretion approximated the quantity of additional iodine administered, confirming subject compliance. Losses of iodine in the feces, sweat, salivary glands, choroid plexus, and gastric mucosa could not be evaluated. In these subjects, the increase in iodine intake induced a small, but significant, increase in serum TI concentrations from baseline values of 5.9 ± 0.2 μg/dL to 7.6 ± 0.4 on day 8 and 7.6 ± 0.4 on day 15. Since the serum PBI did not change, calculated values of the serum inorganic I concentration increased significantly, from baseline values of 0.7 ± 0.1μg/dL to 2.4 ± 0.3 on day 8 (P < 0.01) and 2.7 ± 0.4 on day 15 (P < 0.01).
The mean 24-hour urine I excretion in the women receiving 500 μg iodine daily increased significantly from 177 ± 21 μg daily to 410 ± 36 μg on day 8 and 506 ± 33 μg on day 15. In the women receiving 250nμg iodine daily, 24-hour urine I excretion also increased significantly from 186 ± 37 μg daily to 332 ± 28 μg on day 8 and 314 ± 39 μg on day 15. There was no change in serum TI, PBI, and inorganic I concentrations during administration of these two doses of iodine.
- Serum Thyroid Hormone and Basal TSH Concentrations:
Ingestion of 1500 μg iodine daily induced small but significant decreases in the serum T4 and T3 concentrations and in values of the FT4I and a significant increase in the TSH concentration on day 15. The administration of 500 μg or 250 μg iodine daily did not significantly affect the serum T4, T3, and TSH concentrations or the FT4I. The maximum increment in the serum TSH concentration following TRH administration was significantly greater in both men and women receiving 1500 μg iodine daily, compared with that observed on day 0. The increase in the TSH response to TRH was also highly significant when integrated TSH responses (μU/mL x min) over one hour were calculated (men, 534 ± 49 vs 752 ± 58, P < 0.001; women, 1244 ± 112 vs 1653 ± 202, P < 0.02). The serum TSH response to TRH was significantly greater in the women than in the men, both before and during iodine administration (P < 0.01). The administration of 500 μg iodine daily to nine women or 250 μg iodine to another nine women did not significantly affect the serum TSH response to TRH.

Conclusions:

A small increase in dietary iodine can induce subtle changes (all values remaining within the normal range) in pituitary-thyroid function, probably by inhibiting thyroid hormone release. The smaller iodine supplements of 500 and 250 μg daily did not affect thyroid function.

Executive summary:

Studies were carried out to determine whether a far smaller increase in iodine intake would also affect thyroid function. Normal volunteers received 1500, 500, or 250 μg supplemental iodine daily for 14 days. Following the administration of 1500 μg iodine daily, there were small but significant decreases in the serum T4 and T3 concentrations and a small compensatory increase in the serum TSH concentration and the serum TSH response to TRH. In contrast. no changes in pituitary-thyroid function occurred during the administration of 500 or 250 μg iodine daily. These findings indicate that a small increase in dietary iodine can induce subtle changes (all values remaining within the normal range) in pituitary-thyroid function, probably by inhibiting thyroid hormone release. The smaller iodine supplements of 500 and 250 μg daily, quantities that may easily be achieved under normal conditions, did not, however, affect thyroid function.

Reference 5

Endpoint:

chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 453 (Combined Chronic Toxicity / Carcinogenicity Studies)

Principles of method if other than guideline:

A chronic toxicity study was conducted, in which male and female F344/DuCrj rats were given potassium iodide (KI) in the drinking water at concentrations of 0, 10, 100 or 1000 ppm for 104 weeks.

GLP compliance:

not specified

Limit test:

no

Specific details on test material used for the study:

-Purity: more than 99%
- Source: Nacalai Tesque (Kyoto, Japan).

Species:

rat

Strain:

Fischer 344/DuCrj

Sex:

male/female

Details on test animals or test system and environmental conditions:

An excess of 200 male and 200 female 5-wk old F344/ DuCrj rats were purchased from Charles River Japan (Atsugi, Japan) as specific pathogen free (SPF) animals. They were housed two or three to a polycarbonate cage with wood chips (Softchip, Sankyo Laboratory Service, Tokyo, Japan) for bedding, in an air-conditioned animal room (room temperature, 24±1℃; relative humidity, 55±5%; ventilation, 18 times/hr; 12-hr light/dark cycle). Cages were changed twice a week. Only animals showing no abnormalities during a 1-wk acclimatization period were used in the study. Distilled water alone or containing KI and CRF-1 diet (Oriental Yeast, Tokyo, Japan) were available ad lib. throughout.

Route of administration:

oral: drinking water

Dose / conc.:

10 ppm

Dose / conc.:

100 ppm

Dose / conc.:

1 000 ppm

Remarks:

1000 ppm was set as the highest dose, exceeding the 260 ppm which has been reported to induce colloid goiters.

No. of animals per sex per dose:

Both sexes were divided into four groups, each consisting of 60, 40, 40 and 60 rats after the acclimatization, and given KI in the drinking water at concentrations of 0, 10, 100 and 1000 ppm for 2 yr, respectively.

Control animals:

yes, concurrent no treatment

Observations and examinations performed and frequency:

During the treatment period, general condition was checked daily, and water consumption was recorded every 2 wk up to 12 wk, and every 4 wk thereafter. Body weights were recorded at the same times.

White blood cell count (WBC), red blood cell count (RBC), hemoglobin (Hb), hematocrit (Ht) and platelet count (PLT) were recorded with the aid of an automated hematology analyzer (Sysmex M-2000, Toa Medical Electronics Co., Ltd, Hyogo, Japan).

Sacrifice and pathology:

In the 0 and 1000 ppm groups, five rats each were necropsied 3, 6, 12 and 18 months after the initiation of the treatment. All survivors were fasted overnight prior to necropsy, which was performed after exsanguination from the aorta under ether anesthesia. Livers, kidneys, lungs, hearts, spleens, adrenals, brains, pituitaries and thyroids were fixed in 10% neutral buffered formalin, for the interim necropsy. For animals treated for 2 yr, in addition to the organs described above, testes, noses, tongues, tracheas, salivary glands, esophagus, stomachs, small and large intestines, pancreas, urinary bladders, prostates, seminal vesicles, ovaries, uteri, vaginae, mammary glands, lymph nodes, sternums, femurs, spinal cords, eye balls, skin and skeletal muscles were fixed in the same manner. Grossly visible lesions were also separately fixed. Histopathological examination was performed on hematoxylin-eosin (HE)-stained specimens processed routinely. Animals that died or were killed in a moribund condition were also necropsied and examined histopathologically.

Statistics:

Body weight, water consumption, organ weight and hematology data were analyzed for homogeneity of variance using Bartlett's test. When they proved to be homogeneous, one-way analysis of variance was applied. If significant heterogeneity of variance (P≤0.01) was apparent, the equivalent non-para-metric statistical method of Kruskal-Wallis was employed. Where significant differences between the groups were detected, a multiple comparison test (Dunnett's test or Scheffe's method) was used. Final survival rates and the incidences of tumors were compared with the Fisher's exact probability test.

Mortality:

mortality observed, treatment-related

Description (incidence):

Survival rates of male rats were decreased in the 100 and 1000 ppm groups as compared to the control from around 80 wk after the initiation of the treatment. The rates for females of any treated groups were similar to that of the controls.

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Body weights in the 1000 ppm groups of both sexes were depressed in the latter half of the treatment period

Water consumption and compound intake (if drinking water study):

no effects observed

Description (incidence and severity):

Water consumption in all treated groups was similar to the control level.

Haematological findings:

no effects observed

Description (incidence and severity):

Hematological examination did not reveal any effects of the treatments.

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

On histopathological examination, the incidence of thyroid follicular dilatation was found to be increased in the 10, 100 and 1000 ppm groups of both sexes. This lesion was composed of increased colloid in the lumen and flattened epithelia due to the compression. Thyroid proliferative lesions derived from follicular epithelia were not increased by KI-treatment, although examples were observed in each group.
No KI-related induction of any lesions was apparent in the other organs or tissues. Lesions with incidences more than 25% in one or more groups were, foci of cellular alteration in the liver and hyperplasias of the pituitary in both sexes, C-cell hyperplasias of the thyroid, medullary hyperplasias and pheochromocytomas of the adrenals, and inter- stitial cell tumors of the testis in males, and cystic endometrial hyperplasias and endometrial stromal polyps of the uterus in females. However, these are all known to spontaneously occur and neither increase in their incidence nor special types of lesions were observed in the treated groups.

Histopathological findings: neoplastic:

effects observed, treatment-related

Description (incidence and severity):

In the salivary gland, focal acinar atrophy, ductular proliferation and squamous metaplasias were frequently observed, and SCCs were noted in four males and three females of the 1000 ppm group. Lobular atrophies were well-circumscribed from the surrounding tissue and triangular in shape, suggesting focal lesions related to single ducts, and accompanied by ductular proliferation in most cases. Squamous metaplasias were observed in the epithelium of proliferating ductules, and the morphological continuum to squamous cell carcinomas were observed in these metaplasias.

Dose descriptor:

NOEL

Effect level:

< 10 ppm

Based on:

test mat.

Sex:

male/female

Basis for effect level:

histopathology: non-neoplastic

Critical effects observed:

not specified

Lowest effective dose / conc.:

10 ppm

System:

endocrine system

Organ:

thyroid gland

Both males and females showing follicular dilatation in the thyroid were increased in the 10, 100 and 1000 ppm group in the present long-term study. This change is not neoplastic, resembling the morphological alteration reported to be observed in humans under conditions of chronic exposure to excess iodine. The no-effect-level for these lesions can be concluded to be less than 10 ppm, since they were observed even at this low dose. However, neither focal hyperplasias, adenomas nor carcinomas derived from the follicular epithelium were increased, despite the fact that KI was administered for 2 yr. It was therefore concluded that long-term treatment of KI per se does not result in thyroid tumor induction in rats. In contrast, SCCs were observed in the submandibular gland in the 1000 ppm groups of both sexes, along with focal acinar atrophy and/or ductular proliferation, frequently accompanied by squamous metaplasia. Based on the fact that the cell proliferation of these proliferating ductules was higher in cases with metaplasia, and the evidence of a morphological continuum from metaplasias to SCCs, a histogenetic relationship is suspected.

Conclusions:

Follicular dilatation in the thyroid were increased in the 10, 100 and 1000 ppm group in the present long-term study, the no-effect-level for these lesions can be concluded to be less than 10 ppm.

Executive summary:

A chronic toxicity study was conducted, in which male and female F344/DuCrj rats were given potassium iodide (KI) in the drinking water at concentrations of 0, 10, 100 or 1000 ppm for 104 weeks.

Survival rates of male rats were decreased in the 100 and 1000 ppm groups as compared to the control from around 80 wk after the initiation of the treatment. The rates for females of any treated groups were similar to that of the controls. Body weights in the 1000 ppm groups of both sexes were depressed in the latter half of the treatment period. Water consumption in all treated groups was similar to the control level. Hematological examination did not reveal any effects of the treatments. On histopathological examination, the incidence of thyroid follicular dilatation was found to be increased in the 10, 100 and 1000 ppm groups of both sexes. Thyroid proliferative lesions derived from follicular epithelia were not increased by KI-treatment. In the salivary gland, focal acinar atrophy, ductular proliferation and squamous metaplasias were frequently observed, and SCCs were noted in four males and three females of the 1000 ppm group. No KI-related induction of any lesions was apparent in the other organs or tissues. The no-effect-level for these lesions can be concluded to be less than 10 ppm.

Reference 6

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

The effects of chronic excess iodine intake on thyroid functions and thyroid oxidative stress state in hypothyroid rats was investigated. Sixty rats were divided into euthyroid and hypothyroid (thiocyanate-induced) groups with or without administration of excess iodine (3000 or 6000 μg/L) for 8 weeks.

GLP compliance:

not specified

Species:

rat

Strain:

Sprague-Dawley

Sex:

male

Details on test animals or test system and environmental conditions:

Sixty male Sprague-Dawley rats weighing 220-240 g were used in the present study. They were purchased from the Vaccine and Immunization Authority (Helwan, Cairo, Egypt) and housed (animal house, Department of Medical Physiology, Faculty of Medicine, Mansoura University, Egypt) in standard cages in groups of 4-6 animals per cage under controlled conditions (temperature 23 ± 1 °C, and a 12 h light : 12 h dark cycle).The animals were fed ad libitum with standard rat chow and tap water.

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

After 1-week acclimation to the laboratory environment, animals were randomly assigned to 4 groups and given different doses of iodine at the levels of 3000 and 6000 μg/L by using sterile water as the vehicle. Group I (15 rats) included normal (euthyroid) rats that received vehicle (tap water) daily for 8 weeks. Group II (15 rats) included rats in which hypothyroidism was induced by administration of thiocyanate (1 g/L) in the drinking water for 4 weeks. Group III (30 rats) included rats in which hypothyroidism was induced by administration of thiocyanate (1 g/L) in the drinking water for 4 weeks. This group was then administered iodine in the drinking water in doses of 3000 mg/L (IIIa, 15 rats) or 6000 mg/L (IIIb, 15 rats) for another 8 weeks.

Analytical verification of doses or concentrations:

not specified

Control animals:

yes, concurrent vehicle

Observations and examinations performed and frequency:

- Sampling of blood and urine:
At the end of the experimental period, blood and urine samples were obtained. Blood samples were taken from the orbital sinus by using fine Pasteur pipette under halothane anesthesia. These blood samples were collected without anticoagulant, left for 10 min, then centrifuged for 10 min at 4000 r/min to obtain serum, which was stored at -20 °C until further analysis for determination of serum total thyroxine (T4), total triiodothyronine (T3), and thyrotropin (TSH). Also, rats were housed in metabolic cages for 24 h to collect urine samples for measurement of urinary iodine concentration.
- Thyroid gland sampling: Thyroid glands were removed from all rats; 14 samples from each group were kept at -80 °C until biochemical analysis of oxidative markers (7 samples) and reverse transcriptase (RT) - PCR for gene expression (7 samples) were performed. Six samples were randomly selected from all groups for light microscopy. They were placed in 10% buffered formalin, embedded in methacrylate, step sectioned, mounted on glass slides, and stained with hematoxylin and eosin.
- Biochemical investigations:
Estimation of urinary iodine and serum T4, T3, and TSH concentrations: Iodine concentration in urine was measured by a modified Cer-arsenit colorimetric method. Serum total thyroxine (tT4), total triiodothyronine (tT3), and TSH were measured in rat serum using enzyme-linked immunosorbent assay (ELISA) kits (GenWay Biotech Inc., San Diego, Calif., USA) according to the manufacturer’s instructions. Optical density was read at 450 nm using a plate reader (SunRise absorbance reader, Tecan Group Ltd.). Kits for measurement of TSH were species specific.
Estimation of thyroid oxidative and antioxidative parameters: Thyroid tissue was perfused with phosphate-buffered saline solution (pH 7.4) containing 0.16 mg/mL heparin to remove any red blood cells and clots. Then, thyroid was homogenized in 5–10 mL cold buffer (50 mmol/L potassium phosphate (pH 7.5), 1 mmol/L EDTA). Homogenates were centrifuged at 10 000 g for 15 min at 4 °C, and the supernatant was kept at –80 °C until used for analysis of lipid peroxides, total antioxidants, catalase, SOD, and glutathione Stransferase (GST), which were measured by using a colorimetric kit (Bio-Diagnostics, Dokki, Giza, Egypt) according to the manufacturer’s instructions.

Statistics:

The data were expressed as means ± standard deviation (SD). Data were processed and analyzed using SPSS version 10.0 (SPSS, Inc., Chicago, Ill., USA). One-way ANOVA was done followed by Tukey’s post hoc test. Results were considered statistically significant at p ≤ 0.05.

Details on results:

- Effects of excess iodine on urinary iodine and serum T3, T4, and thyroid-stimulating hormone concentrations in hypothyroid rats:
Urinary iodine concentration increased in rats with thiocyanate-induced hypothyroidism when compared with normal rats (p < 0.05). After exposure to high doses of iodine (3000 or 6000 μg/L) for 8 weeks in hypothyroid rat, the urinary iodine concentration increased in a dose-dependent manner (p < 0.05). Serum levels of T4 and T3 decreased significantly in hypothyroid rats (p < 0.05) when compared with normal rats. Excessive administration of iodine (3000 or 6000 μg/L) to hypothyroid rats caused a significant increase in plasma T4 and T3 levels (p < 0.05) relative to that of hypothyroid rats, although plasma T4 reached nearly normal levels (p > 0.05) and T3 was still below normal levels (p < 0.05). Plasma TSH level significantly increased in hypothyroid rats when compared with levels in normal rats; however, TSH decreased significantly (p < 0.05) with excess iodine administration (3000 or 6000 ug/L) and nearly reached normal levels (p > 0.05).

- Effects of excess iodine on markers of oxidative stress: thyroid catalase, superoxide dismutase, total antioxidants, and malondialdehyde
In the thyroid gland of rats with thiocyanate- induced hypothyroidism, a significant increase (p < 0.05) in lipid peroxide (MDA) concentration was observed relative to that of the control group. In hypothyroid rats, a high iodine dose caused a significant increase (p < 0.05) in MDA concentrations. There was a significant increase (p < 0.05) in catalase, SOD, and total antioxidants of thyroid gland in rats with thiocyanate-induced hypothyroidism and in hypothyroid rats given high doses of iodine (3000 or 6000 μg/L).

Effects of excess iodine on expression of Na+/I- symporter, D1 deiodinase, and thyroid peroxidase:
The relative gene expressions of NIS, D1, and TPO in rats expression of NIS increased significantly (p < 0.05) in rats with thiocyanate-induced hypothyroidism. Meanwhile, NIS expression decreased (p < 0.05) after high iodine doses (3000 or 6000 μg/L), but remained higher than that of the control group (p < 0.05). On the other hand, despite the significant increase in D1 gene expression in rats with thiocyanate-induced hypothyroidism, it significantly (p < 0.05) decreased after excess iodine intake by hypothyroid rats (p < 0.05). Gene expression of TPO increased significantly (p < 0.05) in hypothyroid rats, but decreased after high iodine administration (p < 0.05), although remained greater than that of the control group (p < 0.05).

- Effects of excess iodine on thyroid gland weight and histology:
Thyroid gland weight was significantly (p < 0.05) increased in rats with thiocyanate-induced hypothyroidism; however, after excess iodine (3000 or 6000 μg/L) administration, its weight was significantly decreased (p < 0.05) but still more than that of the control group (p < 0.05). Thyroids obtained from thiocyanate-induced hypothyroid rats show closed follicles, follicular cell hyperplasia, an increase in follicular number and vascularity, as well as a decrease in colloid volume. Signs of colloid goiter were found in the thyroids of rats exposed to high doses of iodine; the follicles were filled with colloid and the epithelial cells were flattened.

Conclusions:

Thiocyanate-hypothyroidism increases gene expression of NIS, TPO, and TPO and induces oxidative stress. High iodine intake decreases NIS and D1 deiodinase gene expression in hypothyroid rats. Moreover, excess iodine increase thyroid hormones, lipid peroxides, and antioxidants in hypothyroid rats.

Executive summary:

The effects of chronic excess iodine intake on thyroid functions and thyroid oxidative stress state in hypothyroid rats was investigated. Sixty rats were divided into euthyroid and hypothyroid (thiocyanate-induced) groups with or without administration of excess iodine (3000 or 6000 μg/L) for 8 weeks. Serum thyroxine (T4), triiodothyronine (T3), thyroid-stimulating hormone (TSH), thyroid antioxidants (catalase, superoxide dismutase enzymes, and total antioxidants), and lipid peroxide (malondialdehyde; MDA) were measured. Reverse transcription - PCR gene expression for thyroidal Na+/I– symporter (NIS), D1 deiodinase, and thyroid peroxidase (TPO) were performed.

Thiocyanate significantly decreased thyroid hormones (T3, T4), increased lipid peroxides and antioxidants, and increased gene expression of NIS, D1 deiodinase, and TPO. Excess iodine intake in hypothyroid rats increased T3 and T4. Also, high iodine intake by hypothyroid rats significantly decreased NIS, D1 deiodinase, and TPO genes expression. Excess iodine significantly increased MDA and antioxidants in euthyroid and hypothyroid rats. In conclusion, thiocyanate-hypothyroidism increases gene expression of NIS, TPO, and TPO and induces oxidative stress. High iodine intake decreases NIS and D1 deiodinase gene expression in hypothyroid rats. Moreover, excess iodine increase thyroid hormones, lipid peroxides, and antioxidants in hypothyroid rats.

Reference 7

Endpoint:

short-term repeated dose toxicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

GLP compliance:

not specified

Limit test:

no

Species:

other: Human

Details on test animals or test system and environmental conditions:

Thirty normal men, ages between 22 and 40 years (mean age 27)

Route of administration:

oral: unspecified

Details on route of administration:

The iodide solutions were prepared by dissolving sodium iodide, in deionized water, and adjusting the concentrations to 500, 1500 or 4500 μL/mL. Ascorbic acid (1 mg/mL) was added to each solution. The men took the iodide in 0.5 mL aliquots, twice daily, for 14 days.

Vehicle:

water

Analytical verification of doses or concentrations:

no

Duration of treatment / exposure:

14 day

Frequency of treatment:

twice daily

Dose / conc.:

500 other: μg iodide/day

Dose / conc.:

1 500 other: μg iodide/day

Dose / conc.:

4 500 other: μg iodide/day

Details on study design:

On Day 1 of the study, the men were admitted to the CRC at 0800 h, after an 8-h fast. All had collected a 24 h urine specimen for iodide and creatinine measurements during the day prior to admission. Baseline studies included serum T4, T3, T3-charcoal uptake, TSH, protein-bound iodide (PBI) and total iodide determinations. All then received an i.v. bolus dose of 500 μg. TRH, and blood samples were collected at 15 min intervals for 1 h for TSH determinations. After completion of the TRH test, the men received their initial dose of iodide. The men took the iodide in 0.5 mL aliquots, twice daily, for 14 days. On day 15, the protocol outlined for day 1 was repeated, completing the study. Subjects were instructed to continue their usual diets throughout the study period.

Observations and examinations performed and frequency:

Serum inorganic iodide was calculated by subtracting the serum PBI from the serum total iodide. Serum T4, T3 and TSH concentrations and T3-charcoal uptake ratios were measured in duplicate. The serum free thyroxine index (FTI) was calculated by multiplying the serum T4 concentration by the T3-charcoal uptake ratio. Normal ranges for adults are as follows: T4, 4.5-12.0 μg/dl (58-154 nmol/L); T3 80-180 ng/dl (1.2-2.8 nmol/L); T3-charcoal uptake ratio, 0.85-1.15; and TSH < 1.0-5.0 uU/mL (< 1.0-5-0 mU/L). For urine and serum iodide measurements, 1 μg iodide= 7.9 nmol iodide. All samples from an individual man were analysed in the same assay.

Statistics:

Paired Student’s t-tests were used to compare within-group changes before and after iodide administration. The results are expressed as the mean ± SEM.

The significant dose-related increases in urinary iodide excretion were proportionate to the iodide dose administered, confirming compliance with the prescribed regimen. Similar changes occurred in serum total iodide and inorganic iodide concentrations. None of the iodide doses caused significant changes in mean serum PBI concentrations.

There were no significant changes in the men who received 500 μg iodide/day. However, administration of 1500 and 4500 μg/day resulted in significant decreases in mean serum T4 concentrations and free T4 index values. Serum T3 concentrations and T3-charcoal uptake ratios did not change at any of the iodide doses.

The mean basal serum TSH concentration increased in the men who received 1500 and 4500 μg iodide/day, but not in those who received 500μg. However, a significant enhancement of the TSH response to TRH occurred in all three groups.

Conclusions:

Significant dose-related increases in serum total and inorganic iodide concentrations were found, as well as urinary iodide excretion. The mean serum T4 concentration and free T4 index values decreased significantly at the 1500 μg/day and 4500 μg/day doses. No changes in T3- charcoal uptake or serum T3 concentration occurred at any dose. Administration of 500 μg iodide/day resulted in a significant increase (P < 0.005) in the serum TSH response to TRH, and the two larger iodide doses resulted in increases in both basal and TRH-stimulated serum TSH concentrations.

Executive summary:

Thirty normal men aged 22-40 years were randomly assigned to receive 500, 1500, and 4500 μg iodide/day for 2 weeks. Blood was obtained on days 1 and 15 for measurement of serum T4, T3, T3-charcoal uptake, TSH, protein-bound iodide (PBI) and total iodide, and 24 h urine samples were collected on these days for measurement of urinary iodide excretion. TRH tests were performed before and at the end of the period of iodide administration. Serum inorganic iodide was calculated by subtracting the PBI from the serum total iodide. Significant dose-related increases in serum total and inorganic iodide concentrations were found, as well as urinary iodide excretion. The mean serum T4 concentration and free T4 index values decreased significantly at the 1500 μg/day and 4500 μg/day doses. No changes in T3- charcoal uptake or serum T3 concentration occurred at any dose. Administration of 500 μg iodide/day resulted in a significant increase (P< 0.005) in the serum TSH response to TRH, and the two larger iodide doses resulted in increases in both basal and TRH-stimulated serum TSH concentrations. Thus, iodide supplementation within the range of normal daily intake has a significant inhibitory effect on thyroid hormone secretion in normal men.

Reference 8

Endpoint:

short-term repeated dose toxicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

GLP compliance:

not specified

Limit test:

no

Species:

other: Human

Route of administration:

oral: capsule

Of the 446 women aged 25-54 selected from the Cardiff general practice, 50 were excluded according to the criteria and 171 declined to attend leaving 225 women who were screened for microsomal antibody. Twenty of the 31 antibody positive women and 30 of 48 antibody negative controls agreed to take part in the trial of iodide supplementation versus placebo. In the Dowlais practice, of the 142 subjects aged 60-75 selected there were 48 exclusions, 59 non-responders and 35 enrolled among whom the median length of residence in this previously iodide deficient area had been 66 years. Response rates for the group aged 60-75 in Cardiff were similar, where of 112 selected there were 35 exclusions, 48 non-responders and 29 subjects enrolled.

For each group, mean urinary iodide excretion was calculated rather than the iodide/creatinine ratio because of unexpectedly large variations in creatinine Ievels. BasaI geometric mean urinary iodide excretion was similar in all the groups ranging from 48μg/L in the antibody positive control group to 68 μg/L in the previously iodide deficient controls. Iodide excretion rose markedly in the iodide supplemented groups though the rise in the elderly subjects, especially those from the iodide-deficient area, was significantly greater than that observed in the younger women. In contrast, iodide excretion in the control groups varied Iittle over the study period.

No patients became clinically or biochemically hyperthyroid. All iodide supplemented groups showed similar small decreases in FT4 and compensatory rises in TSH which were statistically significant in the antibody negative and elderly groups. The overall fall in FT4 14 days after the start of supplementation was -1.22 (95% confidence interval - 0.59 to -1.84) pmol/L and at 28 days -0.86 (- 0.30 to -1.43) pmol/L and rise in TSH at 14 days 0.55 (0.19 to 0.92) mU/L and at 28 days 0.59 (0.12 to 1.07) mU/L. TSH levels rose above the laboratory reference range (≤5.0 mU/L) in two of the iodide supplemented elderly subjects (from 3.7 and 5.0 to 7.2 and 8.2 mU/L respectively at28 days). In a further elderly subject in Cardiff TSH increased from 7.3 to 11.1 mU/L, and in two women in the antibody positive group from 12.4 to 24.6 and from 5.3 to 6.8 mU/L. In contrast, there were no significant changes in thyroid function in the placebo treated controls and none developed biochemical evidence of hypothyroidism.

Conclusions:

Dietary Iodide Intake of 750 μg/day or more may adversely affect thyroid function, especially in Individuals with borderline hypothyroidism.

Executive summary:

The aim of this study was to evaluate the risk of exposure to an increase in dietary Iodine intake amongst potentially susceptible population groups in Britain.

All iodide supplemented groups showed similar small decreases in FT4 and compensatory rises in TSH which were statistically significant in the antibody negative and elderly groups. The overall fall in FT4 14 days after the start of supplementation was -1.22 (95% confidence interval - 0.59 to -1.84) pmol/L and at 28 days -0.86 (- 0.30 to -1.43) pmol/L and rise in TSH at 14 days 0.55 (0.19 to 0.92) mU/L and at 28 days 0.59 (0.12 to 1.07) mU/L. In two of the Iodide supplemented subjects thyrotrophin levels rose above the laboratory reference range and in a further three subjects initially elevated hyrotrophin values increased further. In contrast, there were no significant changes in thyroid function in the placebo treated controls and none developed biochemical evidence of hypothyroidism.

Dietary Iodide Intake of 750 μg/day or mor. may adversely affect thyroid function, especially in Individuals with borderline hypothyroidism.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

0.01 mg/kg bw/day

Study duration:

chronic

Species:

other: Human data

Quality of whole database:

Adequate animal and human data available. It should be noted that rats are much more sensitive to thyroid hormone imbalance than humans (McClain, 1992) and thus the human data are considered more relevant.

System:

endocrine system

Organ:

thyroid gland

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Mode of Action Analysis / Human Relevance Framework

The mechanism by which excess iodide produces hypothyroidism is not completely understood. It has been proposed that excess iodide inhibits the iodination of thyroglobulin in the thyroid gland, and inhibits the release of T4 and T3 from the gland. As a consequence, TSH release is stimulated, leading to increased serum levels. Hypertrophy of the thyroid gland is an additional potential effect that is known to accompany iodide-induced suppression of the thyroid gland as summarised in WHO (2020).

Additional information

Iodine is an essential micronutrient for the synthesis of thyroid hormones which are required for the normal development, growth, and function of numerous metabolic pathways. The synthesis of thyroid hormones requires a normally functioning thyroid gland, as well as an adequate intake of iodine.

 

An extensive set of literature is available on the effects of iodine on human physiology and on toxic effects after insufficient or excessive iodine intake (ATSDR, 2004; WHO, 2009; WHO, 2020). The principal systemic effects of repeated exposure to excess iodine through ingestion are on the thyroid gland, leading to effects on thyroid hormone production and secretion. Secondary effects on the endocrine system (pituitary and adrenal glands) and many other organs (including skin, cardiovascular system, pulmonary system, kidneys, GI tract, liver, blood, neuromuscular system, skeleton and reproductive systems) result from disorders of the thyroid gland. The principal direct effects of excessive iodine ingestion are on the thyroid gland and can be classified into three types: hypothyroidism, hyperthyroidism and thyroiditis. Iodine deficiency can result in goitre, hypothyroidism, mental retardation, reproductive impairment, cretinism, decreased child survival and varying degrees of other growth and developmental abnormalities.

The association between iodine intake and the risk of disease is U-shaped. The curve is nonsymmetrical with the most serious problems associated with iodine deficiency (Laurber et al., 2009).  Inorganic iodine is rapidly converted to iodide in the digestive and/or respiratory tracts (WHO, 2009). Therefore, all internal exposure is essentially related to iodide and this form the basis for the consideration of iodine and iodide salts as equivalent in toxicological terms. Although Sherer et al. (1991) reported that iodine and iodide may affect thyroid hormones status in different ways in rats, both treatments tend to a decrease in T3 levels (in line with the so-called “Wolff-Chaikoff effect”) which was higher in iodine treated group. Robison et al. (1998) failed to confirm the differential effect of iodine and iodide on thyroid hormone imbalance in humans.

Sherer et al. 1991 and de Raaf - Beekhuijzen (2010) reaffirmed the fact that the effects observed after repeated oral doses of iodine are referable to modifications on thyroid function almost exclusively, authenticating the basis for reading across data from iodine and iodide salts data. Based on that, the threshold chosen for safety assessment is based on the thyroid as main target organ. Because of differences in iodine metabolism between animals and humans, data from animals is of limited use, especially when human data are available (Hetzel and Maberly, 1986) as the case of iodine and iodide salts. For that reason, key studies for establishing No-observed- adverse-effect-levels (NOAELs) and subsequent Derived-no-effects-levels (DNELs) were selected from the vast epidemiological literature available which was reviewed by the United States Agency for Toxic Substances and Disease Registry (ATSDR) in 2004. A NOAEL of 0.01 mg/kg/day has been derived for chronic oral exposure to iodine. The NOAEL is based on subclinical hypothyroidism in healthy human children (Boyages et al., 1989; Li et al., 1987). Supporting studies indicate that the NOAEL would be applicable to elderly adults who may represent another sensitive subpopulation (Chow et al., 1991; Szabolcs et al., 1997). This means that both sensitive subpopulations have been considered. Key references are summarized under epidemiological data in the section 7.10.2.

 

Gardner 1988 studied the effects of 500-4500 μg iodide/day for 14 days on thyroid function in normal men. Iodide supplementation within the range of normal daily intake has a significant inhibitory effect on thyroid hormone secretion in normal men.

 

Paul 1988 evaluated the effects of much smaller quantities (250-1500 μg) of iodine on thyroid function in normal volunteers. A small increase in dietary iodine can induce subtle changes (all values remaining within the normal range) in pituitary-thyroid function, probably by inhibiting thyroid hormone release. The smaller iodine supplements of 500 and 250 μg daily did not affect thyroid function.

 

Fischer 1989 determined the effect of increasing amounts of dietary iodine on the incidence of lymphocytic thyroiditis, and the appearance of antibodies to triiodothyronine (T3), thyroxine (T4) and thyroglobulin (Tg), using the BB Wistar rat. High iodine intakes increase Tg antibodies, which may be associated with an increase in autoimmune thyroiditis in these animals.

 

Hussein 2012 investigated the effects of chronic excess iodine intake (3000 or 6000 μg/L) on thyroid functions and thyroid oxidative stress state in hypothyroid rats. Thiocyanate-hypothyroidism increases gene expression of NIS, TPO, and TPO and induces oxidative stress. High iodine intake decreases NIS and D1 deiodinase gene expression in hypothyroid rats. Moreover, excess iodine increases thyroid hormones, lipid peroxides, and antioxidants in hypothyroid rats.

Takegawa 2000 conducted a study to determine whether KI alone can cause thyroidal tumours with long-term application for 2 yr. Follicular dilatation in the thyroid were increased in the 10, 100 and 1000 ppm group in the present long-term study, the no-effect-level for these lesions can be concluded to be less than 10 ppm.

 

References (not summarized in the endpoint): 

- Agency for Toxic Substances and Disease Registry (ATSDR). 2004. Toxicological profile for iodine.

- World Health Organization (WHO). 2009. Iodine and inorganic iodides. In: Concise International Chemical Assessment Document 72.

- WHO, 2020: Iodin in drinking water, Background document for development of WHO guidelines for drinking-water quality.

- Hetzel, B.S., Maberly, B.F. 1986. Iodine. In: Mertz, W., ed. Trace Elements in Human and Animal Nutrition 2, Orlando: Academic Press, 139 -208.

- Laurberg, P., Pedersen, I.B., Carlé, A., Andersen, S., Knudsen, N., Ovesen, L., Rasmussen, L. 2009.The u-shaped curve of iodine intake and thyroid disorders. In: “Comprehensive handbook of iodine: nutritional, biochemical, pathological and therapeutic aspects”. USA.

- Szabolcs I, Podoba J, Feldkamp J, et al. 1997. Comparative screening for thyroid disorders in old age in areas of iodine deficiency, long-term iodine prophylaxis and abundant iodine intake. Clin Endocrinol 47:87-92.

 

Repeated dose toxicity: via oral route - systemic effects (target organ) glandular: thyroids

Justification for classification or non-classification

Key human data (Boyages et al., 1989; Li et al., 1987) are available from which the chronic NOAEL in humans can be set at 0.01 mg/kg bw/day for the most sensitive sub populations (based on subclinical hypothyroidism in healthy human children) The same data has been used by the ATSDR and WHO as the basis for setting a mimimum risk level (ATSDR) and a Tolerable Daily Intake (WHO) of 0.01 mg/kg bw/day which are values for acceptable oral chronic exposure to iodine.

In the studies by Boyages et al., 1989; Li et al., 1987, there was a 65% prevalence of goitre in a high iodine group compared with a prevalence og 15% in a low iodine group. Also, the high iodine group contained 15% of subjects that had a grade 2 goitre, compared with 0% having a grade 2 goitre in the low iodine group. Although the subjects were all euthyroid with normal values for serum thyroid hormones and TSH concentrations, TSH concentrations were significantly higher (33%) in the high iodine group.

Based on evidence in humans, iodine is to be classified with STOT RE 1 H372 (thyroid) according to Regulation (EC) No 1272/2008.