Paradoxical Response of the Unblocked Hyperthyroid Gland to Iodide

Abstract

Studies were carried out on 16 patients with hyperthyroidism in order to assess the effect of large doses of potassium iodide on the rate of reduction of radioactivity in the unblocked thyroid gland. In 8 patients, glandular depletion of radioactivity was enhanced by potassium iodide, while in the remaining half the appearance of the secretion rate remained unchanged. At no time was inhibition of the biological half-life observed; in all patients, however, the serum thyroxine concentration and resin sponge I131-T3 uptake values declined, indicating that thyroid function had been depressed. It is conceivable that iodide may have inhibited the rate of thyroxine release, but the net loss of radioactivity suffered from failure of glandular reincorporation of radioiodide in some patients may have exceeded the fraction of retained I131 and thereby resulted in the pattern of an accelerated release of radioactivity. These data suggest that, in the unblocked hyperthyroid gland, impairment of thyroxine synthesis is an important consequence of iodide administration and may, to some extent, be responsible for the therapeutic properties of the ion.