Influence of Estrogen and Progesterone on Radioactive Iodine Uptake by Rat Thyroid

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Abstract
Physiological doses of estrogen and progesterone were injected into ovariectomized rats, alone or in combination, in the sequence believed to occur during the estrous cycle. Effect on thyroid function was measured, 48 hours after injection of the steroid hormones, by determining the 6-hour uptake of I131 by the thyroid. A single injection of either 6 or 300 R.U. estradiol benzoate significantly increased Il31 uptake. The concurrent administration of 0.4 or 0.8 mg progesterone reduced I131 uptake of estrogen-treated rats down to or slightly below the level observed in ovariectomized controls. Progesterone alone reduced I131 uptake slightly when given in doses of 0.2 or 0.4 mg while uptake was increased slightly with 0.8 mg; however, these differences were of doubtful significance. In ovariectomized-hypophysectomized rats estrogen had no effect on I131 uptake, indicating that a functional pituitary is necessary for its effect on the thyroid to be expressed. Similar animals given progesterone showed no significant change in I131 uptake. It is suggested that the previously observed variations in thyroid function during the estrous cycle of the rat may be explained by changes in balance of the ovarian hormones - estrogen and progesterone.

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