Inhibitory Effect of Excess Iodide on Thyroidal Radioiodine Release in the Rat1

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Abstract
The inhibitory effect of excess iodide on thyroidal radioiodine release was studied in more than 300 rats. In animals receiving 30 mg of PTU daily, excess iodide had no effect on thyroidal radioiodine release. However, excess iodide (50 or 200 μg KI) did suppress significantly the release of thyroidal radioiodine in all animals receiving small doses (0.1–5 mg) of PTU. Although the inhibition continued for more than 40 hr after a single injection of excess KI, this inhibition eventually was followed by resumption of normal thyroidal radioiodine release even after repeated injections of KI. Five μg injections of KI were without effect under the same experimental conditions. The possibility that appreciable quantities of the newly administered I127 were synthesized into thyroid hormone under the influence of small doses of PTU and that the decrease of specific activity resulted in the slowing down of thyroidal radioiodine release is not supported for the following 4 reasons: 1) Small doses of PTU were as effective as large doses in blocking the re-accumulation in the thyroid of I131 liberated from the degradation of endogenously labeled compounds; 2) small doses of PTU almost completely inhibited the formation of T3 +T4; 3) the dilution of thyroidal I131 by stable iodine did not alter the thyroidal radioiodine release in unblocked animals; 4) in animals fed a low iodine diet for a long period and depleted of thyroidal iodine, excess iodide did not suppress the release of thyroidal radioiodine. It is suggested that excess iodide inhibits the thyroidal radioiodine release of normal animals under some selected conditions.