THYROID I131METABOLISM IN THE ABSENCE OF THE PITUITARY: THE UNTREATED, HYPOPHYSECTOMIZED RAT11

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Abstract
Hypophysectomized rats and their controls were injected with 5 [mu]g of iodide labeled with I131. At intervals ranging from 45 minutes to 8 days, measurements were made of the total thyroid uptake of I131 and of the chemical distribution of I131 in the thyroid gland and in the plasma. T/S ratios were also measured. Total thyroid uptake of I131 was reduced by hypophysectomy to only a few per cent of control values, and T/S ratios were also greatly reduced. Previous iodine intake, however, exerted a marked influence on the values obtained. Thyroglobulin formation in the absence of the pituitary was reduced to very low levels. The rate was estimated in two experiments to be about one per cent that of the controls. Thyroglobulin that was formed by the thyroids of hypophysectomized rats, moreover, contained only very low percentages of I131-thyroxine and other iodothyronines, but a normal percentage of I131 diiotyrosine and a greater than normal fraction of I131-monoiodotyrosine. The plasma of hypophysectomized rats contained no detectable amounts of I131-thyroxine, even as late as 8 days after I131 injection. The striking reduction in all phases of thyroid iodine metabolism observed in rats without pituitaries leads us to emphasize the importance of pituitary, rather than autonomous, control in the regulation of thyroid iodine metabolism.