thiourea; thiocarbamide

Endpoint summary

Description of key information

Several repeated dose studies are available. Key study is Hartzell (1945/1942), a chronic (1y/3y) study in mice and rats. The effects level is supported by a 90day study in rats (Hazelton, 1987). In addition, data on short term exposure (Astwood, 1943; 1945), a 28-day dose range feeding study conducted by TNO (1979), a 28-day oral toxicity study by Korte and Greim (1981) and two sub-chronic drinking water studies conducted by TNO (1984a and b) are available.

 

The NOAEL for oral repeated dose toxicity was concluded to be 6.88 mg/kg bw/d, based on the key chronic toxicity study.

 

Further investigation will be conducted according to ECHA decision No. CCH-D-2114539794-36-01/F (19th January 2021).

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:

chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key 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:

In a chronic toxicity study, thiourea was administered daily in drinking-water at concentrations of 1.72, 6.88, or 27.5 mg/kg body weight to mice for 2 years and to rats for the duration of their lifetimes or a maximum of 3 years.

GLP compliance:

not specified

Limit test:

no

Species:

other: rat and mice

Strain:

not specified

Sex:

not specified

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

Administration of thiourea in drinking water at concentrations of In a chronic toxicity study, thiourea was administered daily in drinking-water at concentrations of 1.72, 6.88, or 27.5 mg/kg body weight to mice for 2 years and to rats for the duration of their lifetimes or a maximum of 3 years.

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

no data

Duration of treatment / exposure:

Duration of exposure:
mice for 2 years
rats for the duration of their lifetimes or a maximum of 3 years

Frequency of treatment:

daily

Dose / conc.:

1.72 mg/kg bw/day (nominal)

Remarks:

Basis: nominal in water

Dose / conc.:

6.88 mg/kg bw/day (nominal)

Remarks:

Basis: nominal in water

Dose / conc.:

27.5 mg/kg bw/day (nominal)

Remarks:

Basis: nominal in water

No. of animals per sex per dose:

no data

Control animals:

not specified

Details on study design:

no data

Positive control:

no data

Observations and examinations performed and frequency:

no data

Sacrifice and pathology:

no data

Other examinations:

no data

Statistics:

no data

Clinical signs:

effects observed, treatment-related

Mortality:

mortality observed, treatment-related

Body weight and weight changes:

effects observed, treatment-related

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

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

not specified

Ophthalmological findings:

not specified

Haematological findings:

not specified

Clinical biochemistry findings:

not specified

Urinalysis findings:

not specified

Behaviour (functional findings):

not specified

Organ weight findings including organ / body weight ratios:

not specified

Gross pathological findings:

not specified

Histopathological findings: non-neoplastic:

not specified

Histopathological findings: neoplastic:

not specified

Details on results:

A reduction in body weight gain and an enlargement of the thyroid gland were observed only in the rats in the highest dose group, and no other changes were detected, either macroscopically or microscopically.

Key result

Dose descriptor:

LOAEL

Effect level:

27.5 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

not specified

Basis for effect level:

organ weights and organ / body weight ratios

water consumption and compound intake

Key result

Dose descriptor:

NOAEL

Effect level:

6.88 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

not specified

Basis for effect level:

organ weights and organ / body weight ratios

water consumption and compound intake

Critical effects observed:

not specified

Conclusions:

In a chronic toxicity study a LOAEL of 27.5 mg/kg bw and a NOAEL of 6.88 mg/kg bw was established for thiourea in rats.

Executive summary:

In a chronic toxicity study, thiourea was administered daily in drinking-water at concentrations of 1.72, 6.88, or 27.5 mg/kg body weight to mice for 2 years and to rats for the duration of their lifetimes or a maximum of 3 years. A reduction in body weight gain and an enlargement of the thyroid gland were observed only in the rats in the highest dose group, and no other changes were detected, either macroscopically or microscopically. A LOAEL of 27.5 mg/kg body weight per day (reduction of body weight and enlargement of thyroid gland) and a no-observed-adverse-effect level (NOAEL) of 6.88 mg/kg body weight per day for rats was established. The results of the publication by Hartzell (1945) are also cited in the "Concise International Chemical Assessment Document 49" (WHO, 2003).

Reference 2

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1987-01-12 to 1987-05-14

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

GLP/OECD 408 Guideline Study; only one dose tested

Qualifier:

according to guideline

Guideline:

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

GLP compliance:

yes

Limit test:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Laboratories
- Age at study initiation: 49 days
- Weight at study initiation: 232.1 g to 298.7 g (males) and 165.4 g to 199.2 g (females)
- Fasting period before study: Fasting on days immediately prior to urine and blood collection or terminal sacrifice
- Housing: Individually
- Diet: Ad libitum
- Water: Ad libitum
- Acclimation period: 14 days

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 72 ± 6 °F
- Humidity (%): 50 ± 20 %
- Air changes (per hr): No data
- Photoperiod (hrs dark/hrs light): 12/12

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

Test animals received thiourea in the drinking water continuously from study initiation, until the day before their terminal sacrifice or start of the recovery phase of the study in week 14. Twice a week a thiourea solution was prepared with deionised water to yield a 2.5 ppm solution. This solution served as test and stock solution.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Analyses of thiourea concentration in the test solution samples were performed. Routine analyses were performed weekly on test solutions prepared for the first four weeks of the study then in weeks 6, 10 and 12 (initial mix) test solutions. In addition, analyses were performed on the week 11, group 2 dosing solution.
Thiourea in deionised water was determined by reversed phase high performance liquid chromatography (HPLC) and quantified against external standards using peak area integration.

Duration of treatment / exposure:

13 weeks (91 days)

Frequency of treatment:

Continuously in drinking water

Dose / conc.:

2.5 ppm

Remarks:

equivalent to 1.73 mg/kg/d (males)
Basis:nominal in water

Dose / conc.:

2.5 ppm

Remarks:

equivalent to 2.65 mg/kg/d (females)
Basis:nominal in water

Dose / conc.:

0.5 ppm

Remarks:

Basis:nominal in water

Dose / conc.:

0.1 ppm

Remarks:

Basis:nominal in water

Dose / conc.:

0.02 ppm

Remarks:

Basis:nominal in water

Dose / conc.:

0 ppm

Remarks:

Basis:nominal in water

No. of animals per sex per dose:

10 (dose groups 1-4); 20 (dose group 5 (2.5 ppm))

Control animals:

yes, concurrent vehicle

Details on study design:

- Dose selection rationale: Test concentratrions were selected to establish a no effect level
- Rationale for animal assignment (if not random): Random
- Rationale for selecting satellite groups: No data
- Post-exposure recovery period in satellite groups: Yes; a recovery group consisting of 9 male and 10 rats was incorporated into the study for an additional 4 weeks
- Section schedule rationale (if not random): No data

Positive control:

no

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Once daily
- Cage side observations included: Signs of overt toxicity as evidenced by changes in general appearance, behaviour, excretion, respiration and skin

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule:

BODY WEIGHT: Yes
- Time schedule for examinations: prior the initiation of the study and weekly thereafter

FOOD CONSUMPTION AND COMPOUND INTAKE:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: Yes
- Compound intake calculated as time-weighted averages from the consumption and body weight gain data: Yes

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
- Time schedule for examinations: The quantitiy of water consumed during the interval between two weighings

OPHTHALMOSCOPIC EXAMINATION: Yes
- Time schedule for examinations: before initiation of the treatment and during week 13
- Dose groups that were examined: each animal was subjected to indirect opthalmoscopic examinations

HAEMATOLOGY: Yes
- Time schedule for collection of blood: During weeks 7 and 14 all surviving non-recovery animals were fasted overnight and blood was drawn via orbital sinus puncture.
- Anaesthetic used for blood collection: Yes (Ketamine HCl)
- How many animals: 5 animals of groups 1-4 and 10 animals of group 5
- Parameters that were examined: Hematocrit, erythrocyte count, leukocyte count, cellular morphology, hemoglobin, platelet count, differential leukocyte count.

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: During weeks 7 and 14 all surviving non-recovery animals were fasted overnight and blood was drawn via orbital sinus puncture.
- Animals fasted: Yes
- How many animals: 5 animals of groups 1-4 and 10 animals of group 5
- Parameters examined: sodium, glucose, aspartate aminotransferase, total protein, calcium, phosphorus, creatinine, gamma glutamyltranspeptidase, TSH, potassium, alanine aminotransferase, urea nitrogen, albumin, chloride, total bilirubin, globulin, T4, T3.

URINALYSIS: Yes
- Time schedule for collection of urine: during week 7
- Metabolism cages used for collection of urine: Yes
- Animals fasted: Yes
- Parameters examined: volume, occult blood, urobilinogen, glucose, pH, sediment, specific gravity, bilirubin, ketones, protein, appearance

Sacrifice and pathology:

GROSS PATHOLOGY: Yes
Following thirteen weeks of thiourea administration, each non-recovery animal was fasted overnight, weighed, then sacrificed by exsanguination while under sodium pentobarbital anesthesia. All surving animals from the recovery group were similarly sacrificed during week 18. The following examinations were performed postmortem:
- The external surface
- All orifices
- Cranial cavity
-Carcass
-External surfaces of the brain and spinal cord. Cut surfaces examined at the time of tissue trimming.
-The nasal cavity
-The thoracic, abdominal and pelvic cavities and their viscera
-The cervical tissues and organs
The following organs were weighed: liver, adrenals, kidney, thyroid (with parathyroid) and testes with epididymides.
HISTOPATHOLOGY: Yes
Tissues were preserved on 10% neutral buffered formalin.

Other examinations:

none

Statistics:

The criterion for significance of group comparisons was routinely at the 5% two-tailed probability level

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

alopecia

Mortality:

mortality observed, treatment-related

Description (incidence):

alopecia

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):

no effects observed

Ophthalmological findings:

effects observed, treatment-related

Description (incidence and severity):

Persistent hyaloid remnants; not compund related

Haematological findings:

no effects observed

Clinical biochemistry findings:

no effects observed

Urinalysis findings:

no effects observed

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

no effects observed

Gross pathological findings:

effects observed, treatment-related

Description (incidence and severity):

Findings were noted infrequently in the lung, liver, kidney, stomach, seminal vesicle, urinary bladder, eye, lymph node, thymus, uterus and skin

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

All other histopathological findings were observed either in isolated individuals or their incidences did not correlate with the dose of test material received.

Histopathological findings: neoplastic:

not examined

Details on results:

CLINICAL SIGNS AND MORTALITY: All animals survived until their scheduled sacrifice except for one group 5 recovery animal which was accidently killed during week 13 of the study. Alopecia was the only other clinical observation made during the course of the study. This sign was observed in approximately 24 % of the test animals, with the highest incidences in group 5 males and females.

BODY WEIGHT AND WEIGHT GAIN: Analysis of the group mean absolute body weight data indicated that the mean body weight values of all groups within each sex were statistically equivalent at week 13.

FOOD CONSUMPTION AND COMPOUND INTAKE: Statistical analysis of total food consumption through 13 weeks of administration suggested that comparable amounts of food were consumed by all test groupsand sexes.

FOOD EFFICIENCY: Not determined

WATER CONSUMPTION AND COMPOUND INTAKE (if drinking water study): The results presented suggest that the quantity of thiourea consumed each week by the animals of each dose group was fairly constant. However, because body weights increased substantially during the course of the study, the dose of thiourea per unit of body weight continually declined as the study progressed.

OPHTHALMOSCOPIC EXAMINATION: All animals examined had no visible lesions, except for some animals in which persistent hyaloid remnants were identified. There is no evidence to suggest that the abnormalities noted were compound related.

HAEMATOLOGY: No significant differences among the treatment groups were found in any of the haematopathology parameters evaluated in blood collected either at week 7 or prior to sacrifice in week 14.

CLINICAL CHEMISTRY: Statistical analysis revealed a significant increase in serum alanine aminotransferase of group 2 of males in comparision with the control at week 7. All other clinical chemistry data were considered to be unremarkable.

URINALYSIS: Results were considered to be unremarkable.

NEUROBEHAVIOUR: Not examined

ORGAN WEIGHTS: All organ weights and organ to body weight ratios for the test groups dosed with thiourea (excluding recovery animals) were statistically equivalent to appropriate control organ weights and ratios.

GROSS PATHOLOGY: Findings were noted infrequently in the lung, liver, kidney, stomach, seminal vesicle, urinary bladder, eye, lymph node, thymus, uterus and skin. The incidences of these findings were distributed throughout the test groups. All other gross observations were considered to be unremarkable.

HISTOPATHOLOGY: NON-NEOPLASTIC
Non-treatment related findings were noted in the adrenal cortex, lung, heart, liver, kidney, glandular and non-glandular stomach, pancreas, testes, urinary bladder, thymus, skin, lymph node, seminal vesicle and eye. All other tissues examined were considered to be unremarkable. Peribronchial/perivascular lymphoid infiltration was observed in the lungs of 11 animals. Chronic progressive nephropathy and degenerative cardiomyopathy were the only other lesions observed in more than one or two of the animals per test groups examined. The nephropathy was observed in three to five animals in each of the male test groups, and the cardiomyopathy was observed in four of the group 1 males. All other histopathological findings were observed either in isolated individuals or their incidences did not correlate with the dose of test material received.

Key result

Dose descriptor:

NOEL

Effect level:

>= 1.73 mg/kg bw/day (actual dose received)

Based on:

test mat.

Sex:

male

Basis for effect level:

other: no adverse effect observed

Key result

Dose descriptor:

NOEL

Effect level:

>= 2.65 mg/kg bw/day (actual dose received)

Based on:

test mat.

Sex:

female

Basis for effect level:

other: no adverse effect observed

Key result

Critical effects observed:

no

Conclusions:

Thiourea administered in drinking water to rats for 13 weeks did not cause adverse effects up the highest concentration of 2.5 ppm (= 1.73 mg/kg b.w./d for males and 2.65 mg/kg b.w./d for females).

Executive summary:

In a subchronic toxicity study thiourea was administered daily in drinking water to 60 male and 60 female Sprague-Dawley rats (10 rats/sex/dose) in the dose groups 1–4 (0, 0.02 ppm, 0.1 ppm, 0.5 ppm) and 20 rats/sex/dose in group 5 (2.5 ppm). Nine males and 10 females of group 5 served as recovery group over 4 weeks. The objective of this study was to determine a no-observed effect level of thiourea when administered daily via drinking water to male and female Sprague-Dawley rats for at least 13 weeks. A full complement of qualitative and quantitative parameters was evaluated after 7 and 13 weeks, and data generated from this study indicated that levels up to 2.5 ppm of thiourea in the drinking water failed to elicit a demonstrable biological effect. It may therefore be concluded that the no-observed effect level of thiourea administered via drinking water to rats for 13 weeks is at least 2.5 ppm, or approximately 1.73 mg/kg b.w./d for males and 2.65 mg/kg b.w./d for females.

Reference 3

Endpoint:

short-term repeated dose toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Study period:

no data

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Remarks:

28-day dose range finding study (preceding the 90-day drinking water study)

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 407 (Repeated Dose 28-Day Oral Toxicity Study in Rodents)

Deviations:

yes

Remarks:

exposure period in one group is 8 weeks; diet with high level of iodine (2.2 ppm)

GLP compliance:

no

Limit test:

no

Species:

rat

Strain:

Wistar

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: CIVO
- Age at study initiation: Weanling
- Weight at study initiation: Males mean weight 53-56 g; females mean weight 49-50 g
- Fasting period before study: No
- Housing: 5 per cage
- Diet: Ad libitum
- Water: Ad libitum
- Acclimation period: No

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 23 ± 1 °C
- Humidity (%): 50 %
- Air changes (per hr): Air conditioned

Route of administration:

oral: feed

Vehicle:

other: Thiourea was incorporated into the CIVO stock diet.

Details on oral exposure:

PREPARATION OF DOSING SOLUTIONS: Thiourea was incorporated into the CIVO stock diet at levels of 0, 300, 1000 and 3000 ppm.

DIET PREPARATION
- Rate of preparation of diet (frequency): Not reported
- Mixing appropriate amounts with: CIVO stock diet: soybean-oil meal 11.0 %, ground whole maize 29.7 %, ground whole wheat 36.0 %, fish meal 7.0 %, meat scraps 4.0 %, brewer's yeast 3.0 %, grass meal 3.0 %, whey powder 2.0 %, steamed bone meal 0.4 %, trace mineralized salt 0.5 %, vitamins 0.4 %, soybean oil 3.0 %; level of iodine 2.2 ppm
- Storage temperature of food: Not reported

VEHICLE
- Justification for use and choice of vehicle: Thiourea was incorporated in the stock diet
- Concentration in vehicle: 0, 300, 1000, 3000 ppm

Analytical verification of doses or concentrations:

no

Duration of treatment / exposure:

4 or 8 weeks

Frequency of treatment:

With stock diet daily; only the day before autopsy the test diets were replaced by the unsupplemented CIVO stock diet

Dose / conc.:

0 ppm

Dose / conc.:

300 ppm

Remarks:

Doses / Concentrations: 300 ppm = 30 mg/kg/d
Basis: actual ingested

Dose / conc.:

1 000 ppm

Remarks:

Basis: nominal in diet

Dose / conc.:

3 000 ppm

Remarks:

Basis: nominal in diet

No. of animals per sex per dose:

10

Control animals:

yes, plain diet

Details on study design:

- Dose selection rationale: Range finding study

Positive control:

No positive control

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: Condition and behaviour were inspected regularly
- Cage side observations checked in table: No

DETAILED CLINICAL OBSERVATIONS: Yes
- Time schedule: After 4 and 8 weeks

BODY WEIGHT: Yes
- Time schedule for examinations: Weekly during the within 4 weeks; after week 6 and week 8

FOOD CONSUMPTION AND COMPOUND INTAKE:
- Food consumption for each animal determined and mean daily diet consumption calculated as g food/kg body weight/day: No; the food intake of each group was recorded weekly
- Compound intake calculated as time-weighted averages from the consumption and body weight gain data: Yes

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: Yes

WATER CONSUMPTION AND COMPOUND INTAKE: No
- Time schedule for examinations: n.a

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: Yes
- Time schedule for collection of blood: After four and eight weeks
- Anaesthetic used for blood collection: No data
- Animals fasted: No
- How many animals: 5 per sex and dose group
- Parameters checked in table [No.3] were examined.

CLINICAL CHEMISTRY: Yes
- Time schedule for collection of blood: After four and eight weeks
- Animals fasted: No
- How many animals: 5 per sex and dose group
- Parameters checked in table [No.4] were examined.

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

GROSS PATHOLOGY: Yes (see table 5)
HISTOPATHOLOGY: Yes

Statistics:

yes, but not reported in detail

Clinical signs:

not specified

Mortality:

not specified

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

Dose related decrease in males and females of all test groups

Food consumption and compound intake (if feeding study):

effects observed, treatment-related

Description (incidence and severity):

Dose related decrease in males and females of all test groups

Food efficiency:

effects observed, treatment-related

Description (incidence and severity):

Slightly decreased at 300 and 1000 ppm and distinctly decreased at 3000 ppm, both in males and females (see Table 2).

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

not examined

Ophthalmological findings:

not examined

Haematological findings:

effects observed, treatment-related

Description (incidence and severity):

Haemoglobin and PCV decrease, prothrombin time was slightly increased in males and females at 3000 ppm after 4 weeks

Clinical biochemistry findings:

effects observed, treatment-related

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

effects observed, treatment-related

Description (incidence and severity):

Significant increase for liver and thyroid (Table 5)

Gross pathological findings:

no effects observed

Description (incidence and severity):

No treatment related gross lesions

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

Treatment-related changes in thyroid and liver, but not in kidneys

Histopathological findings: neoplastic:

not examined

Details on results:

BODY WEIGHT AND WEIGHT GAIN: Dose-related decrease

FOOD CONSUMPTION AND COMPOUND INTAKE (if feeding study): Dose-related decrease

FOOD EFFICIENCY: Week 1-4: slightly decreased at 300 and 1000 ppm; distinctly decreased at 3000ppm, both in males and females

HAEMATOLOGY: Haemoglobin and PCV decrease in males and females in all test groups after 4 weeks and in males of the two higher test groups after 8 weeks; slight increase of prothrombin time in males and females at 3000 ppm after 4 weeks and in males at 3000 ppm after eight weeks.

CLINICAL CHEMISTRY: Thyroid function test showed a dose-related decrease of T3 uptake in males and females in all test groups; T4 concentrations in the plasma were generally lower than those of the controls;

ORGAN WEIGHTS: Toxicologically significant increases for the liver after 4 and 8 weeks both in males and females and for the thyroid at the two higher test levels after 4 and 8 weeks both in males and females.

GROSS PATHOLOGY: No treatment related gross lesions

HISTOPATHOLOGY: NON-NEOPLASTIC: Treatment related changes in the thyroid and the liver of both males and females, but not in the kidneys; thyroid of males and females in the mid-and high-dose group was activated; increased number of small follicles, lined by cuboidal epithelium; liver: lobular pattern formed by centrolobular foamy cytoplasm;

Dose descriptor:

LOAEL

Effect level:

300 ppm

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: LOAEL of thiourea is lower than 300 mg/kg diet, which is equivalent to 30 mg/kg bw/d

Key result

Dose descriptor:

LOAEL

Effect level:

30 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

body weight and weight gain

food efficiency

Critical effects observed:

not specified

Table 1: Mean body weights in week 1-4 of 10 animals per group; in week 6 and 8 of 5 animals/group

Thiourea (ppm) in the diet	Mean body (in g) at end of week
	0	1	2	3	4	6	8
males
0	55	81	107	138	172	234	267
300	56	71**	93**	118**	142***	176***	205***
1000	56	70**	90***	111***	132***	178***	213***
3000	53	65***	81***	93***	102***	112***	119***
females
0	49	70	95	116	132	156	177
300	50	64*	84*	99**	112**	142	162
1000	49	61***	76***	90***	102***	116***	135***
3000	49	60***	71***	84***	90***	98***	103***

* P<0.05; ** 0.001<P<0.01; ***P<0.001

 

 

Table 2: Mean food intake and food efficiency

Thiourea (ppm) in the diet	Food intake in g/rat/day in week				Food efficiency during week 1-4
	1	2	3	4	gain	food	gain/food
males
0	9.6	11.9	14.6	16.0	117	364	0.32
300	7.4	10.3	12.0	13.6	86	303	0.29
1000	7.1	9.3	10.4	11.3	76	267	0.28
3000	6.8	8.3	7.7	7.8	49	214	0.23
females
0	8.3	11.0	12.4	12.1	83	307	0.27
300	9.1	9.5	10.6	10.6	62	264	0.23
1000	6.1	8.2	9.0	9.3	54	228	0.24
3000	6.0	7.2	7.7	7.3	41	197	0.21

 

 

Table 3: Mean hematological values in 5 rats/group

Thiourea (ppm) in the diet	At week 4					At week 8
	Hb mmol/L	PCV L/L	RBC x10^12	WBC x 10^9/L	PT sec	Hb mmol/L	PCV L/L	RBC x10^12	WBC x 10^9/L	PT sec
males
0	9.0	0.452	6.1	19.2	43	10.1	0.519	7.5	20.8	56
300	8.5	0.421**	6.1	18.4	42	9.8	0.516	7.7	17.2	53
1000	8.3*	0.424**	6.1	16.4	44	9.1**	0.490	7.3	17.2	54
3000	7.9**	0.421	6.1	16.5	46**	9.4*	0.477*	7.7	19.2	66**
Females
0	9.6	0.478	6.6	19.2	37	9.1	0.479	6.9	16.6	52
300	8.9	0.453	6.5	16.4	38	9.2	0.475	6.9	16.4	51
1000	8.8**	0.437***	6.2	19.4	41	8.9	0.462	6.7	14.3	51
3000	7.9***	0.379***	5.8*	15.3	43	8.7	0.449	6.8	16.0	52

*0.01<P<0.05; **0.001<P<0.01; *** P<0.001

Hb= hemoglobin

PCV= packed cell volume

RBC= red blood cells

WBC= white blood cells

PT= prothrombin time

 

 

Table 4: Mean values of T3-uptake and T4-concentration in blood plasma of 5 rats/group, unless otherwise indicated by figure in parentheses

Thiourea (ppm) in the diet	At week 4		At week 8
	T3-uptake (% of Calibration standard)	T4-concentration µg/100mL	T3-uptake (% of Calibration standard)	T4-concentration µg/100mL
Males
0	46.3	7.3	51.2 (4)	5.3 (4)
300	42.7**	5.2***	50.0 (4)	5.3 (4)
1000	38.2**	4.5***	49.5*	3.4**
3000	31.6***	2.4***	42.9***	4.1
Females
0	45.1 (4)	4.5 (4)	47.4	3.9
300	44.2	4.0	47.0 (4)	3.0 (4)
1000	43.0*	5.0	46.5	3.1
3000	30.5***	1.9***	40.3*** (3)	1.3** (3)

*0.01<P<0.05; **0.001<P<0.01; *** P<0.001

 

Table 5: Mean relative organ weights (in g/100 g body weight) of 5 rats/sex/group

Thiourea (ppm) in the diet	After 4 weeks				After 8 weeks
	BW^1)	Liver	Kidneys	Thyroid	BW^1)	Liver	Kidneys	Thyroid
males
0	172	4.59	0.88	0.015	267	3.70	0.74	0.011
300	142***	5.75**	0.98*	0.019*	205***	4.62**	0.87**	0.012
1000	132***	5.48*	0.92	0.037*	213***	4.91***	0.83**	0.017*
3000	102***	6.39*	0.84	0.046***	119***	4.34***	0.80	0.053***
Females
0	132	4.26	0.90	0.018	177	3.36	0.71	0.012
300	112**	4.71	0.92	0.019	162	3.74**	0.77	0.015
1000	102***	5.02	0.94	0.033**	135***	3.99**	0.78*	0.023*
3000	90***	5.21*	0.85	0.062***	103***	4.23***	0.76	0.060***

 1)     BW = body weight (in g)

*0.01<P<0.05; **0.001<P<0.01; *** P<0.001

 

 

Conclusions:

The LOAEL was determined to be 300 ppm thiourea in the diet. This is equivalent to 30 mg/kg bw/d.

Executive summary:

Groups of 10 male and 10 female weanling rats were fed stock diet supplemented with 0 (control), 300, 1000 or 3000 ppm thiourea. After 4 weeks five rats/sex/ group were killed for interim observations of haematology, thyroid function, organ weights and histopathology. The remaining rats were killed and examined after 8 weeks. Growth, food intake, and food efficiency were decreased in all dose groups. Haemoglobin levels and packed cell volume were decreased in males and females of all dose groups after 4 weeks and in males of the two higher dose groups after 8 weeks. The relative weight of the liver was increased at all dose levels both after 4 and 8 weeks. The relative weight of the thyroid was increased only at the two high dose levels both after 4 and 8 weeks. In the mid- and low-dose group the T3-uptake and T4-concentration were generally somehow lower than in controls, especially in males. No relevant changes were seen upon gross examination. Microscopically, treatment-related changes occurred in the thyroid of rats in the mid­ and high-dose group and in the liver of rats in all dose groups. It was concluded that the no-toxic-effect level of thiourea in the present study was lower than 300 mg/kg diet, which is equivalent to 30 mg/kg bw/d.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

6.88 mg/kg bw/day

Study duration:

chronic

Species:

rat

Quality of whole database:

good

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

Additional information

In a chronic toxicity study, thiourea was administered in drinking-water at concentrations of 1.72, 6.88, or 27.5 mg/kg/d to mice for 2 years and to rats for the duration of their lifetimes or a maximum of 3 years. A reduction in body weight gain and an enlargement of the thyroid gland were observed in the highest dose group. No other changes were reported, either macroscopically or microscopically (Hartzell, 1942, 1945; WHO, 2003).Thus, a NOAEL of 6.88 mg/kg bw/day was concluded. This result was supported by a subchronic (90d) drinking water study (Oserhof/Hazelton, 1987). Thiourea was administered to 10 Sprague-Dawley rats/sex/dose at 0, 0.02 ppm, 0.1 ppm, 0.5 ppm, and to 20 rats/sex/dose at 2.5 ppm. Nine males and 10 females of the 2.5 ppm group served as recovery group over 4 weeks. A full complement of qualitative and quantitative parameters was evaluated after 7 and 13 weeks. Animals were observed for mortality and moribundity and for overt signs of toxicity. Detailed physical examinations and individual body weight and food consumption measurements were performed. Clinical pathology parameters (haematology, clinical chemistry, urinalysis, triiodothyronine [T3], T4, and TSH levels in blood) were evaluated. There was no evidence of substance-related clinical or histopathological effects at levels up to 2.5 ppm of thiourea. Thus, the NOAEL in this study is considered to be at least 2.5 ppm, or approximately 1.73 mg/kg bw/d for males, and 2.65 mg/kg b.w./d for females. A LOAEL was not determined.

In a 28d dose range finding study groups of 10 albino rats/sex/dose were fed stock diet supplemented with 0, 300, 1000 or 3000 ppm thiourea (Beek/TNO 1979). After 4 weeks five rats/sex/dose were killed for interim observations of haematology, thyroid function, organ weights and histopathology. The remaining rats were killed and examined after 8 weeks. Growth, food intake, and food efficiency were decreased in all dose groups. Haemoglobin levels and packed cell volume were decreased in males and females of all dose groups after 4 weeks and in males of the two higher dose groups after 8 weeks. The relative weight of the liver was increased at all dose levels both after 4 and 8 weeks. The relative weight of the thyroid was increased only at the two high dose levels both after 4 and 8 weeks. In the mid- and low-dose group the T3-uptake and T4-concentration were generally somehow lower than in controls, especially in males. No relevant changes were seen upon gross examination. Microscopically, treatment-related changes occurred in the thyroid of rats in the mid­ and high-dose group and in the liver of rats in all dose groups. It was concluded that the LOAEL in the present study was 300 mg/kg diet, which is equivalent to 30 mg/kg bw/d. A NOAEL has not been determined.

In two subsequent 90 day studies thiourea was administered to 10 Wistar rats/sex/dose at concentrations of 0, 10, 25 and 250 ppm (Lina/TNO 1984(a)), and 10 female rats/dose at 0, 0.02, 0.1, 0.5, 2.5 ppm (Lina/TNO, 1984 (b)), respectively, in drinking water. Growth reduction, a decrease in the number of neutrophilic granulocytes and a decrease in the relative weight of the thymus was observed in the females of all test groups. In males similar changes were observed in the mid- and top-dose group, but not in the low-dose group. It was concluded that the NOAEL of thiourea when administered in drinking water to rats for 13 weeks, is 10 ppm (0.9 mg/kg/d) for males and below 10 ppm (1.3 mg/kg/d) for females (Lina/TNO 1984(a)). In the corresponding study the same author concluded an effect level in females below 0.2 ppm (0.02 mg/kg/d) (Lina, TNA 1984 (b)). However, these effect level are based on missing dose-response relationship or transient effects. Only the change in absolute/relative thymus weight are considered to be substance related. This may be attributed to two factors. First, very young rats (3.5 w ) were tested, which is not recommended in OECD 408 and could result in a pronounced effect of the developing immune system. Second, the concentration of folic acid in the diet was very low, especially in the second study diet contains 0.5 ppm folic acid, compared to 3.5 ppm in the standard diet. Thioles are known to increase folic acid demands. As folic acid is needed to maintain the immune system, a deficiency is a known cause of increased thymus weight. Thus, it is questionable that this effect is relevant for human risk assessment.

Supporting data is provided in a couple of other studies from 1940ies. Daily ingestion of 131 mg thiourea/kg body weight in drinking-water by female rats for 10 consecutive days led to hyperplasia of the thyroid, which could be demonstrated both macroscopically and microscopically. No such effect resulted from treatment with 12 mg thiourea/kg body weight (Astwood, 1943). In addition, the iodine level of the thyroid gland was reduced from 73 to 13 mg/100 g tissue upon the oral administration of thiourea at 70 mg/kg body weight for 10 days (Astwood et al., 1945). McKenzie 1943 reported a slight increase in thyroid weight in rats administered 25 mg/kg bw/day, however, no statistical analyses were presented. Mice appear to be less sensitive to thiourea than rats, in that daily subcutaneous administration at 500 mg/kg body weight for 10 days resulted in only a slight reduction in the colloid content of the thyroid (Jones, 1946).

Some non-rodent data provide effect levels of 50 mg/kg/d, but no NOAELs. Twenty-seven female, 2 -3 month old lambs were orally administered 0 or 50 mg/kg bw/day thiourea over a time period of 2, 4, or 6 month (6 animals per exposure group, 3 animals per control group) (Nasser, 1987). The lambs suffered from slight to moderate facial oedema, significant reduction in weight gain, stunted growth, weakness, profound depression, loss of appetite, alopecia, a moderately to severly enlarged thyroid gland, muscular weakness, hypoglycaemia, hyperlipidaemia/hypercholesterolaemia, and a significant fall in serum T4 were related to length of treatment. In another study on sheep, eight male lambs aged 3–3.5 months were orally administered 50 mg /kg/d for 3.5 months together with four control lambs (Sokkar et al., 2000; see section 8.7.2). The dosed animals became weak, emaciated, anaemic, and significantly reduced in body weight, with facial oedema and alopecia at thigh, legs, and abdomen. Clinical analysis showed significant reduction in erythrocyte and leukocyte numbers and in levels of T3 and testosterone at the end of the experiment. Histopathology of the thyroid gland revealed hyperplasia of the follicle-lining epithelial cells that project into the lumen. The lumina were devoid of colloid. The testes showed ill developed, small, empty seminiferous tubules. Hepatocytes in the liver showed degeneration and vacuolation with proliferation of Kupffer cells. The kidney showed glomerular lipidosis with accumulation of haemosiderin pigment in the cytoplasm of the renal tubules. Hyperkeratosis of the epidermis was associated with excessive keratin formation within the hair follicles.

Human Data: Several case studies have been reported from the therapeutic treatment of hyperthyroidism. For example, a female patient was treated for hyperthyroidism with 85g Thiourea for 5 weeks (1-3g/person/d; 20-50 mg/kg/d) without adverse effects. After 5 weeks significant side effects were reported (blood count, hemorrhage, granulocytopenia), which showed a gradual return to normal within 12 days after the end of the treatment (Newcomb, 1944). From these data it can be concluded that there is a risk of cumulative exposure, most likely due to a saturable biotransformation/elimination process.

Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
Study meets generally accepted scientific standards, provides a dose response relationship and the NOAEL/LOAEL is supported by several other repeat dose studies.

 

Further investigations

Dose Range Finding

In the first Dose Range Finding study, the toxicity of thiourea, when administered daily via drinking water to rats, was investigated over a period of 2 consecutive weeks. Three groups of 4 male and 4 female Sprague Dawley SD rats each received the test item at nominal dose levels of 1, 5 and 15 mg/kg/day. A fourth similarly constituted group received untreated drinking water and acted as a control. No mortality occurred and no clinical signs were seen during the study.A slight but treatment-related decrease in body weight was observed in high dose treated males (statistically significant when compared to controls) and in high dose females from Day 5 of treatment. Food consumption did not show relevant changes through the study. Water consumption was not affected by treatment. The calculated mean daily achieved dosages for the 2 week period of treatment were 1, 6, 21 mg/kg/day for the males and 1, 9 and 20 mg/kg/day for the females, in terms of test item as supplied. At the end of the treatment period a statistically significant decrease in terminal bodyweight was observed in treated males when compared to the controls.
A statistically significant decrease in absolute and relative mean thymus weight was observed in high and mid-dose treated males and females and in low dose males. The decreased terminal body weight and thymus weight of treated animals of both sexes were considered to be most likely treatment related. No treatment-related changes were noted following gross pathology examination at the end of treatment period.

 

In a second dose range finding study, the toxicity of thiourea, when administered daily via drinking water, was investigated in Sprague Dawley rats after daily oral administration for 2 weeks, in order to select dose levels for subsequent toxicological studies.
The study was performed in 3 phases:
During the first phase of the study (Phase 1), three groups of 4 male and 4 female rats were treated for 1 day only with the test item at nominal doses of 60, 180 and 240 mg/kg/day. A fourth similarly constituted group received untreated drinking water (sterile water) and acted as a control. Due to the mortality observed on Day 1 (1 male of Group 2 and 1 male of Group 4) the treatment was suspended on the same day. A one-week wash-out period was allowed before restarting treatment at different nominal doses. In the second phase of the study (Phase 2), 3 groups of 7 or 8 rats each were treated for a period of two weeks at 3 nominal ascending dose levels (30, 60 and 180 mg/kg/day). A 1-week interval was allowed between the start of each treatment. One additional group was treated with untreated drinking water and acted as a control. Due to a severe reduction in bodyweight in both male and female rats in Group 4, treatment
was discontinued after 8 days. During the third phase of the study (Phase 3), Group 3 (4 males and 4 females) was treated again after a 1-week wash-out period for 2 consecutive weeks at nominal dose level of 120 mg/kg/day.

Mortality

Phase 1
On Day 1 of Phase 1 two cases of premature death occurred in one male treated at nominal dose level of 60 mg/kg/day (low dose) and in one male treated at nominal dose level of 240 mg/kg/day (high dose). The findings observed at post mortem examination were pale areas in lungs in both animals,
whereas red mucosa of duodenum, jejunum and ileum, dark red areas in the adrenal glands were also noted in the high dose male. The observed findings are associated with the death of the animals and therefore are considered treatment-related.

Phase 2
No mortality occurred in Groups 1, 2 and 3 during Phase 2 of study. For Group 4 due to the marked reduction in body weight it was decided to discontinue
treatment on Day 8, therefore they were prematurely sacrificed on Day 9.

Phase 3
No mortality occurred in Group 3 during Phase 3 of the study.

Clinical signs
Phase 1
Dyspnoea was the main treatment-related clinical sign observed on Day 1 in the majority of treated males.

Phase 2
No clinical signs were observed for animals in Groups 2 and 3. Decreased activity, decreased muscle tone, hunched posture and semiclosed eyes were
observed during the study in a single male animal of Group 4.

Phase 3
No clinical signs were observed in male and female rats in Group 3.

Body weight

Phase 2
Dose-related body weight losses were observed in males from all treated groups from Day 1
to Day 5, but a progressive recovery of body weight was observed from Day 8.

Phase 3
Animals of both sexes inGroup 3 showed a bodyweight losses fromDay 1 toDay 5. Recovery was observed from Day 8 onwards.

Body weight

Phase 2
Dose-related body weight losses were observed in males from all treated groups from Day 1 to Day 5, but a progressive recovery of body weight was observed from Day 8.

Phase 3
Animals of both sexes inGroup 3 showed a bodyweight losses fromDay 1 toDay 5. Recovery was observed from Day 8 onwards.

Food consumption
Phase 2
Food consumption inGroup 2, in both males and females, was not affected by the treatment. A slight decrease in food consumption was observed in Group 3 males from Day 1 to Day 5 with a recovery from Day 8. Treatment-related reduction in food consumption was observed in Group 4, which was more pronounced in males than in females.

Phase 3
Group 3 showed a decrease in food consumption in both sexes, with a recovery in males from Day 12 and in females from Day 8.

Water consumption
Phase 2
Males in Group 2 and males and females in Group 3 showed very low water intake in the first 3 or 4 days of treatment, gradually increasing until the end of treatment period. Water consumption of the animals in Group 4 was highly compromised by the treatment throughout the administration period.

Phase 3
Reductions in water consumption were observed in both sex animals during the first week of treatment, achieving normal values during the second week of administration.

The calculated mean daily achieved dosages for Groups 2 and 3 over a 2-week treatment period were 36 and 59 mg/kg/day for the males and 44 and 55 mg/kg/day for the females, in terms of test item as supplied. The calculated mean daily achieved dosages for Group 3 over a 2-week treatment period were 152 mg/kg/day for the males and 143 mg/kg/day for the females, in terms of test item as supplied. Any organ weight changes were within the range of occasionally observed and expected spontaneous changes in rats of the same age and considered unrelated to treatment. No treatment-related changes were noted following gross pathology examination at the end of treatment period for animals of Groups 2 and 3. In all males and one female of Group 4, only a small thymus was noted.

 

A subchronic oral toxicity study in the rat according to OECD No. 408 is on-going. After finalization of this test, this section will be updated accordingly.

Justification for classification or non-classification

Thiourea does not elicit specific target organ toxicity. The impairment of the thyroid function results in a disturbance of the hormonal balance which is accounted for by classifying thiourea as toxic to reproduction (Repro Cat 2, CLP), but further investigations are on-going (see above).