Sex hormone modulation of serum TSH levels

This study was undertaken to determine the effects of estrogen and testosterone on baseline and thyrotropin-releasing hormone (TRH)-stimulated serum thyroid-stimulating hormone (TSH) levels in rats. Seven groups of 20 rats each were studied: intact males (group I), intact females (group II), castrated males (group III), castrated females (group IV), castrated males with testosterone replacement (group V), castrated females with testosterone supplement (group VI), and intact females with testosterone supplement (group VII). Two weeks after initiation of these hormone treatments, blood samples for TSH levels were obtained before and 10 minutes after TRH injection. Testosterone levels were also determined to verify hormone release. The results of these studies indicated that (1) testosterone has a primary, stimulatory role in the control of TSH concentration in that it causes elevation of both baseline and TRH-stimulated TSH levels, (2) endogenous estrogens have no such direct effect on TSH levels, and (3) in the presence of testosterone, endogenous estrogens partially inhibit the TSH-elevating effect of the androgen. As TSH stimulation is considered requisite for thyroid carcinogenesis, the different effects of testosterone and estrogen on TSH levels, demonstrated in this study, may in part explain differences in incidence and prognosis of thyroid carcinoma observed between the sexes in both rats and human beings.