Introduction: The prevalence of goiter, especially in iodine deficient regions, is higher in women than men. This investigation was conducted to determine the effect of testosterone on thyroid weight and function in iodine deficient normal and castrated rats. Materials and Methods: Male Wistar rats were divided into 6 groups, of 7 animals each: Castrated, hormone treated (C+T), castrated, non-hormone treated (C+NT), normal (N), iodine deficient diet, castrated, hormone treated (ID+C+T), iodine deficient diet, castrated, non-hormone treated (ID+C+NT), iodine deficient diet, normal (ID+N). Three weeks after castration, the C+T and ID+C+T groups received daily intraperitoneal injections of 1 mg/kg testosterone enontate, for 9 weeks. At the end, of the study, we measured thyroid weight and serum testosterone, T4, free T4, T3 and TSH and urinary iodine concentrations. Results: Serum testosterone levels significantly decreased in the C+NT and ID+C+NT groups (p<0.001). In ID groups, serum TSH , T3 and thyroid weight levels significantly increased and serum T4 and free T4 levels significantly decreased as compared with the iodine sufficient groups (p<0.001). The ID+C+NT group, had higher serum TSH and thyroid weight and lower serum freeT4 than the ID+C+T and ID+N groups (p<0.01). The C+NT group had higher serum TSH and lower serum free T4 than C+T and N groups (p<0.01). Conclusion: These results suggest that testosterone decreases thyroid enlargement and serum free T4 levels in ID castrated rats, which may explain the lower incidence of goiter in men than women in iodine deficient regions.