Effect of estrogens on thyroid function. II. Alterations in plasma thyroid hormone levels and their metabolism

The circulating levels of total triiodothyronine (TT 3), thyroxine (TT 4, and T 4-bbinding globulin (TBG) and the kinetics of T 3 and T 4 were studied in five menstruating rhesus monkeys before, during, and after prolonged treatment with estradiol monobenzoate (E 2B, 50 μg/kg body weight/day subcutaneously). A significant increase over pretreatment ( p < 0.01) plasma TT 3, TT 4, and TBG was recorded on day 6 of E 2B therapy. A further significant stepwise increase in these parameters was noted up to day 19 of E 2B, when the levels plateaued for the rest of the period of E 2B treatment. Two weeks after discontinuation of E 2B, plasma TT 3, TT 4, and TBG had returned to the pretreatment range and remained so up to 40 days of observation. Although the percent free T 3 and percent free T 4 were significantly decreased ( p < 0.01) during E 2B therapy, the absolute concentrations of free T 3 and free T 4 were not altered. After prolonged E 2B treatment the metabolic clearance rate, distribution space, and production rate (PR) of both T 3 and T 4 were decreased ( p < 0.01). The extrathyroidal T 4 pool (ETT 4P) was significantly increased ( p < 0.01), whereas ETT 3P did not show any significant alterations ( p > 0.05). The decreased PR of T 4 might have been due to a direct inhibitory effect of E 2B on the thyroid, whereas the decrease in PR of T 3 might have been due to either decreased conversion of T 4 to T 3, to decreased secretion by the thyroid, or both.