STUDIES ON THE INHIBITORY ACTION OF EXCESS IODIDE ON THYROIDAL RADIOIODINE RELEASE IN THE RAT

Abstract

In an attempt to study a thyroid state in which excess iodide would inhibit thyroidal radioiodine release, normal and hemithyroidectomized animals were fed a low and high iodine diet for 7 days. During the measurement of thyroidal radioiodine release, animals were injected with or without graded doses of propylthiouracil.
In normal rats fed a low iodine diet, thyroidal radioiodine release was inhibited by excess iodide when small amounts of propylthiouracil (5mg) were injected, while in high iodine diet fed animals, thyroidal radioiodine release was not inhibited by excess iodide in any of the experimental conditions studied.
In hemithyroidectomized and low iodine diet fed animals, thyroidal radioiodine release was inhibited significantly by excess iodide regardless of the dosesof propylthiouracil used, but the inhibition was most marked when 5mg of propylthiouracil were given. However, in the animals similarly treated but fed a high iodine diet, thyroidal radioiodine release was inhibited only when small amounts of propylthiouracil were given.
It is suggested that excess iodide inhibits thyroidal radioiodine release when the thyroid gland is overfunctioning.