THYROID I131METABOLISM IN THE ABSENCE OF THE PITUITARY: THE HYPOPHYSECTOMIZED RAT TREATED WITH THYROTROPIC HORMONE1

Hypophysectomized rats that had been treated with thyrotropic hormone (TSH) for varying periods were injected with 5 µg. of iodide labeled with I131. Treatment with TSH readily reversed the abnormal distribution of I131 (low thyroxine I131, high monoiodotyrosine-I131, which occurs in the thyroids of hypophysectomized rats. Thyroids removed from hypophysectomized rats that had received injections of TSH showed greater ability to concentrate iodide in vitro than did thyroids removed from saline injected hypophysectomized rats. The mechanism of action of TSH on the thyroid is discussed. Evidence is presented which indicates that the effects of TSH on both the thyroid iodide pump and on thyroid cellular growth are independent of any effect on iodine discharge. It is suggested that the various steps of thyroid iodine metabolism are affected by TSH independently. The effects of TSH on iodine metabolism are probably secondary to its effect on some basic cellular process in the thyroid.