Paradoxical effect of excess iodide on thyroidal release of organic iodine in thyroxine-treated animals.

When excess iodide was administered to rats and mice with prelabeled thyroid glands, a significant difference in the patterns of blood PB131I was found between control animals and animals injected with thyroxine (T4). When T4 was not administered, excess iodide lowered blood PB131I in rats and mice and depressed plasma T4 concentration in rats. In contrast, excess iodide increased blood PB131I in rats and mice treated with T4. This paradoxical increase of blood PB131I did not continue for more than 48 hr in spite of repeated injections of excess iodide. (Endocrinology 92: 946, 1973)