CHANGES IN THYROIDAL FUNCTION DURING ADAPTATION TO LARGE DOSES OF IODIDE*

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Abstract
The thyroid function of animals adapted to large doses of iodide in vivo was compared with that of control thyroids by incubating glands in media containing I131 and varying concentrations of inorganic I131. In control thyroids increasing concentrating of medium iodide were associated with an increase followed by a decrease in the quantity of iodide organified, and MIT/DIT ratios rose, thus duplicating the effects of large doses of iodide in vivo (Wolff-Chaikoff effect). In adapted thyroids, total organification rose progressively and never declined. MIT/DIT ratios did not rise. Thus, in vitro escape from the acute Wolff-Chaikoff effect occurred in adapted thyroids. Qualitatively similar results were obtained in control and adapted hypophysectomized rats. The data suggest that escape from the Wolff-Chaikoff effect is due to intrinsic thyroid mechanisms which decrease iodide transport capacity and reduce intrathyroidal iodide sufficiently to allow organification to resume.