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The relative estrogenic potency of Octylphenol (PTOP) was reported in in vitro studies to be 10^3 – 10^5 fold lower compared to estradiol (Routledger & Sumpter, 1997; Laws, 2006). 
From the data of two uterotrophic assays it cannot be concluded that octylphenol acts as estrogen agonist or androgen antagonist (Yamasaki, 2003; ICI Surfactant, 1996).

Additional information

Estrogen-like effect:

The relative estrogenic potency of Octylphenol (PTOP) was reported in in vitro studies to be 10^3 – 10^5 fold lower compared to estradiol (Routledger & Sumpter, 1997; Laws, 2006). Receptor binding alone is not sufficient to postulate any estrogenic effect.

Two in vivo uterotrophic assays were conducted. ICI Surfactant (1996) reported a small but significant increases in relative (but not absolute) uterine weight at all dose levels (100-400 mg/kg bw). These changes were within historical control values and considered to be of no biological significance. Yamasaki (2003) conducted a similar test at dose levels of 100, 200 and 600 mg/kg bw. All animals of the high dose group died.Treatment with 200 mg/kg bw resulted in significant body weight loss. The accessory sex organ weight values were within the control ranges in all dose groups. The relative seminal vesicle weights increased significantly but not dose-related in the low-dose group only (50 mg/kg bw 4-tert-ocylphenol). The relative Cowper’s gland weight increased in rats co-administered with TP at 200 mg/kg bw but an apparent dose dependency was not observed. The seminal vesicle weight in this group was within the control ranges of other studies. From the data provided it cannot be concluded that octylphenol acts as an estrogen agonist or androgen antagonist.