Effect of testosterone on thyroid weight and function in iodine deficient castrated rats

The prevalence of goiter, especially in iodine deficient regions, is higher in women than in men. This investigation was conducted to determine the effect of testosterone on thyroid weight and function in both normal iodine deficient and castrated rats. Male Wistar rats were divided into 6 groups of 7 animals each: castrated hormone treated (C+T); castrated nonhormone treated (C+NT); normal (N), iodine deficient diet, castrated hormone treated (ID+C+T); castrated iodine deficient diet, nonhormone treated (ID+C+NT); and normal iodine deficient diet (ID+N). Three weeks after castration, C+T and ID+C+T groups received daily intraperitoneal injections of 1 mg/kg testosterone enanthate, for 9 weeks. At the end, we measured thyroid weight and serum testosterone, T(4), free T(4), T3, and TSH, and urinary iodine concentrations. Serum testosterone level significantly decreased in the C+NT and ID+C+NT groups (p<0.001). In ID groups, serum TSH, T(3) and thyroid weight levels increased significantly and serum T4 and free T (4) levels decreased significantly as compared to iodine sufficient groups (p<0.001). ID+C+NT group, had higher serum TSH and thyroid weight and lower serum-free T4 than the ID+C+T and ID+N groups (p<0.01). The C+NT group had higher serum TSH and lower serum-free T(4) than the C+T and N groups (p<0.01). These results suggest that testosterone decreases thyroid enlargement and prevents the fall in free T(4) levels in ID castrated rats, which may explain the lower incidence of goiter in men than women in iodine deficient regions.