Studies on the mechanism of the inhibitory action of excess iodide on the release of radioiodine from the rat thyroid gland.

Abstract

Thyroid release of radioiodine was inhibited by excess iodide in rats receiving small daily dose of propylthiouracil (PTU). This inhibition by excess iodide is not due to the synthesis and release of unlabeled thyroid hormone in amounts adequate to inhibit the release of thyrotropin, for the dilution of 131I within the thyroid by newly formed organic iodine after excess iodide is the same in the groups which received either a small (5 rag) or a large (30 mg) dose of PTU. Furthermore, the amount of plasma PBI newly secreted is negligible at both levels of PTU. Demonstration of the iodide effect in thyroxine and TSH treated animals indicated further that the iodide effect was not due to inhibition of TSH secretion. Administration ofsmall dose of PTU immediately augmented release of radioiodine from the thyroid, increased blood TSH within 24 hr and produced a large goiter after 2 weeks. Large doses of PTU are less effective in these actions. Administration of a small dose of methimazole similarly augments th...