Effects of Estrogen Upon Thyroidal I131Release and Excretion of Thyroxine in Ovariectomized Rats

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Abstract
Estradiol benzoate (1.5 μg/100 g) increased the amount of exogenous l-thyroxine necessary to prevent thyroidal I131 release in ovariectomized rats. This effect was not mediated by a direct action on the thyroid gland since estradiol failed to evoke discharge of thyroidal I131 when thyrotropin was totally suppressed by exogenous thyroxine, whereas marked acceleration of thyroidal I131 release occurred during partial thyrotropin suppression. Daily administration of the same level of estradiol for 4 weeks to ovariectomized rats significantly increased 24- and 48- hour urinary and fecal excretions of radioactivity following a single administration of I131l-thyroxine, without significantly altering serum PBI or thyroid weight. These data suggest that estrogen, in the proper dosage, may stimulate the rate of thyroxine secretion by the thyroid gland while also increasing its rate of degradation by the organism.

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