Effect of repeated administration of excess iodide on thyroid hormone secretion in rats, with special reference to the escape from the iodide effect.

Two hundred μg to 2 mg KI were injected every 12 or 24 hr ip for up to 7 days in rats, and thyroidal radioiodine release and the increase of intracellular colloid droplet formation induced by TSH were measured to assess the effect of repeated injections of excess iodide. A single injection of excess iodide blocked thyroidal radioiodine release, but this effect continued for about 34 hr in spite of repeated injections of excess iodide. An increase of colloid droplet formation induced by TSH was blocked by excess iodide, but 2 to 3 injections of excess iodide were required to produce maximal inhibition. This iodide effect on colloid droplet formation disappeared almost completely by the Sth day of iodide administration. However, a huge dose of KI again depressed the increase of TSH-induced colloid droplet formation in the thyroid which had escaped from the inhibitory effect of excess iodide. It is suggested that excess iodide blocks the release of thyroid hormone in rats, but only for a short time. (Endocri...