Further Studies on Inhibitory Effect of Excess Iodide on Thyroidal Hormone Release in the Rat

To determine whether a decrease of thyroidal radioiodine release produced by excess iodide in animals receiving small daily doses of PTU reflects a change in the secretion rate of thyroid hormone, several mechanisms were investigated in the rat. In agreement with the previous study (Yaraada et al., Endocrinology72: 83, 1963), the present data indicated that thyroidal radioiodine release was inhibited by 50 or 200 μg KI in animals receiving small daily doses of PTU. Excess iodide apparently does inhibit the secretion of thyroid hormone. Furthermore, a decrease of plasma PBI in KI-treated animals was suggested by the measurements of muscle and resin sponge uptake of labeled thyroxine, processes which indicate indirectly the concentration of stable hormone in the blood. The inhibitory effect of excess iodide on thyroid hormone release was completely abolished by prior administration of KC1O4. The blocked thyroid failed to respond to exogenous TSH. In animals receiving large daily doses of PTU or in animals receiving no PTU, excess iodide did not inhibit thyroid hormone release. It is suggested that high concentration of intrathyroidal iodide inhibits the effect of TSH on thyroidal secretion in animals receiving small daily doses of PTU. (Endocrinology79:138,1966)