Abstract
The administration of estrogen for 3 days to castrate and hypophysectomized rats markedly increased thyroidal clearance. The administration of estrogen for 15 and 30 days doubled thyroidal clearance in hypophysectomized rats but was without effect in castrate rats. Renal clearance was slightly decreased by the injection of estrogen. I131 space and serum I131 levels were not significantly influenced by estrogen. A direct action of estrogen on the thyroid is indicated.