EFFECT OF THYROTROPIN UPON RELEASE OF IODIDE FROM HUMAN AND RAT THYROID*†

Abstract

The early effects of thyrotropin (TSH) upon urinary and thyroidal iodide were studied in man and in the rat. Human subjects. (a) TSH given intramuscularly to euthyroid and hyperthroid persons whose thyroidal iodine stores had been labeled with I131 produced within two to four hours an increase in the urinary excretion of iodine -I131 which was maximal in twelve to twenty-four hours. Myxedematous subjects failed to respond. (b) When organic binding was prevented (by methima-zole) and iodide-trapping was partially impaired (by thiocyanate), the injection of TSH led, in most cases, to a prompt decrease in glandular radioiodide content. Rats. (a) The urinary excretion of I131 increased significantly within two and a half hours after administration of TSH to intact rats given I131 several days earlier; in thyroidectomized rats there was a smaller but distinct, increment in urinary I131. (b) When organic binding of injected I131 was blocked (by propylthiouracil) and thyroidal iodine stores were unlabeled, the glandular concentration of I131 was distinctly lower fifteen to one hundred and twenty minutes after intravenous TSH than it had been before injection. (c) The percentage of iodide-I131 in glands in which the organic iodine stores had been labeled and in which organic binding was not blocked, rose promptly after injection of TSH. The results are compatible with the hypothesis that in these two species the administration of TSH leads to an increase in the intrathyroidal concentration of iodide derived from deiodination reactions, and the iodide so formed may be released from the gland.