effect of exogenous testosterone, finasteride, and castration on serum level of thyroxin

background: the secretion of thyroxin (t4) as the main hormone of thyroid gland is regulated by androgens. the present study aimed to evaluate the effect of testosterone and finasteride administration and castration on serum levels of t4 and to show the effect of this regulation on total body weight, weight of testis, and the weight of prostate. methods: male adult rats (n = 32) were divided into 4 groups (n = 8): group 1 (control), group 2 (castration), group 3 (finasteride: 20 mg/kg/day) and group 4 (testosterone: 5 mg/kg/day). at the end of the study (35 days), serum level of thyroxin, body weight, weight of testis, and prostate were determined. results: the data showed that the body weight increased in castrated (p = 0.04) and decreased in testosterone (p = 0.00) groups but did not differ in finasteride (p>0.05) group. there were not any differences in the weight of testis among control, finasteride, and testosterone groups but the weight of prostate increased in testosterone group (p = 0.00) and decreased in castrated (p = 0.03) and finasteride groups (p = 0.04). in addition, the serum level of t4 (nmo/ml) decreased in the three groups: finasteride (p = 0.03), testosterone (p = 0.04), and castrated (p = 0.00). conclusion: testosterone in both high and low levels decreased the amount of t4 with a time-dependent manner.