Effect of estrogens on thyroid function. I. Alterations in rhesus plasma thyrotropin and its kinetics

The circulating levels of TSH, its metabolism, and its response to synthetic TRH were studied in five euthyroid menstruating rhesus monkeys before and during treatment with estradiol monobenzoate (E2B, 50 microgram/kg BW/day sc). The pre-E2B treatment mean plasma TSH level was 1.4 +/- 0.12 (SE) microunit/ml. A significant increase in mean plasma TSH (P less than 0.01) to 1.54 +/- 0.29 microunit/ml was observed as early as 48 h after intiation of E2B treatment; it continued to rise progressively to day 28 when it plateaued around a mean concentration of 3 microunit/ml. It normalized within 10 days after cessation of E2B therapy. After iv TRH (5 microgram/kg BW), a consistent rise in plasma TSH was observed before and on days 11 and 56 of E2B therapy. The peak TSH level and maximum rise over the basal level (deltaTSH) during the three tests were not significantly different. During E2B therapy there were remarkable changes in TSH kinetics. These alterations included a significant decrease (P less than 0.01) in metabolic clearance rate, contraction of the distribution space, and expansion of the extrapituitary TSH pool, but there was no appreciable change in TSH production rate. Although a definite trend towards the above alterations was discernible on day 17 of treatment, they were well established by day 66. These data suggest that the estrogen-induced rise in circulating TSH was caused mainly by decreased degradation and not by increased production.