Objective[|]To investigate the effects of di-n-hexyl phthalate (DHP) and dicyclohexyl phthalate (DCHP) on hypophysis/thyroid axis in utero, pregnant rats were exposed to DHP or DCHP at doses of 0, 20, 100, and 500 mg/kg bw/d, by gavage, on gestational days (GD) 6–19.[¤]Materials and Methods[|]The rats were allowed to grow up until adult (PD 90) stages. Body weights were recorded weekly. After treatment period, hormone analysis was determined in serum samples. Histopathological examinations revealed histopathological changes in thyroid gland on rats that received DHP or DCHP.[¤]Results[|]There were no significant differences in body weights of male adult rats among groups. However, in female rats, final body weights were statistically decreased in 100 mg/kg/d DHP group but increased in DCHP treated rats at dose of 100 and 500 mg/kg/d in dose–response experiment. There was a decrease of TSH level in DHP 100 mg/kg/d and DCHP 500 mg/kg/d groups in male and female rats. T4 levels were increased in DCHP 20 and 100 mg/kg/d groups. In male rats, an increase of TSH level was found in DCHP 20 and 100 mg/kg/d treatment groups. Similarly, an increase of T3 level was found in DCHP 100 and 500 mg/kg/d groups. Also, in thyroid tissue, increase of adipose tissue, colloidal degeneration, and follicular degeneration was observed in all treatment groups.[¤]Conclusion[|]The results of this study suggest that DHP and DCHP, which was applied in pregnancy period, cause changes to T3, T4, and TSH hormone levels and thyroid histology in adult rats.[¤]