Re-evaluation of the Effect of Estrogen on Thyroid Activity in the Rat and Its Mechanism

Abstract

Estradiol benzoate (EB) was injected sc daily for 14 days into male and female rats. EB, over a wide dose range, produced an increase of thyroid weight and thyroidal radioiodine uptake. The increase in these indices was roughly proportional to the doses of EB used. By similar treatment, thyroidal radioiodine release was increased. In hypophysectomized animals, EB stimulated thyroidal radioiodine uptake slightly, but had no effect on thyroid weight. The EB-induced increase of thyroid weight and thyroidal radioiodine uptake of intact animals decreased progressively with the administration of increasing doses of thyroxine, but, at all levels of thyroxine dosage, thyroid weight and thyroidal radioiodine uptake were greater in EB-treated animals than in the controls. EB caused a statistically significant increase in the uptake of labeled thyroxine from plasma by human red blood cells and rat diaphragm. EB lowered plasma PBI in thyroidectomized, thyroxine-maintained animals. It is concluded that EB augments thyroid activity by directly stimulating the thyroid at all levels of circulating TSH, and by augmenting TSH release via the feedback mechanism. (Endocrinology79: 849, 1966)