THE ANTITHYROID ACTIVITY OF 5-IODO-2-THIOURACIL1

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Abstract
In tests with mature rats, thiouracil and 5-iodothiouracil (ITU) proved equally effective in suppressing thyroid uptake of iodide, and in suppressing organic binding of iodide by thyroid tissue. Intact ITU, synthesized with I131, entered the thyroid both in vitro and in vivo, but no concentration gradient between thyroid and blood serum was found. The ability of a single thyroid to deiodinate ITU was negligible, but deiodination was appreciable in the whole animal. A mixture of thiouracil-KI increased the thyrotrophic activity of pituitaries of rats, while ITU did not. However, both thiouracil and ITU produced a small augmentation of exogenous thyrotrophin action, and both increased the biologic decay rate of thyroidal iodine. Measurement of urinary products after injection of radioactive ITU permitted an estimate of the contribution of released thiouracil and iodide to the antithyroid effect of ITU. It was concluded that ITU is an active antithyroid compound similar to thiouracil, but that the antithyroid effect is considerably modified by in vivo release of iodide from a portion of the administered drug.