Effect of Propylthiouracil on Degradation of I131-Labeled Thyroxine and Triiodothyronine

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Abstract
Effects of antithyroid drugs on certain aspects of thyroxine and triiodothyronine metabolism in the rat were studied by measuring the tissue distribution and the excretion pattern of radioactivity following the injection of the I131-labeled hormones. Both TCA-soluble and TCA-precipitable fractions of radioactivity were assayed. In the presence of propylthiouracil, the rate of degradation of both labeled hormones was apparently slowed, as manifested by decreased urinary excretion and lower blood levels of TCA-soluble (nonhormonal) radioactivity. An increased fecal excretion of radioactivity occurred and was a result of a more rapid release of radioactivity into the bile. Potassium perchlorate, administered for 2 weeks, produced thyroidal enlargement, but had no measurable effect on the distribution or excretion of radioactivity, following labeled thyroxine administration.