The effect of acute administration of small doses of iodide on the activity of the thyroid iodide transport mechanism was studied in intact and hypophysectomized iodine-deficient rats and correlations were sought between alterations in iodide transport and the thyroidal iodinations so induced. When organification of the administered iodide was permitted for 2 hr, values for thyroid/serum iodide concentration ratios (T/S) were depressed significantly below control values. Administration of these same doses of iodide in the presence of PTU-blockade resulted in no change from control values. The demonstration of acute lowering of T/S by iodide in the hypophysectomized rat indicated the autoregulatory nature of the response, which was also shown to be dependent upon organification of some of the administered iodide load. To further assess the relationship between lowering of T/S and the amount and nature of newly formed organic iodine, double isotope experiments were performed so as to permit measurement of organic iodinations and T/S in the same animal. In response to widely varying doses of iodide, T/S values were depressed to a similar extent, despite wide variations in both total iodinations and in the amounts of newly synthesized mono-and diiodotyrosine and iodothyronines induced. Thus, while the acute depression of T/S following iodide administration is clearly dependent in some manner on organification of some of the dose, the extent of the response is not directly dependent upon the newly synthesized quantities of the major products of organic iodinations. (Endocrinology83: 737, 1968)
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