Effect of Propylthiouracil on thein VivoDeiodination of Thyroxine Labeled with I131in Different Positions

Abstract

It has been found, using thyroidectomized, thyroxine-maintained rats, that the administration of propylthiouracil depresses the urinary excretions of iodide I¹³¹ when labeled L-thyroxine is injected previously. This decrease was found whether the hormone was labeled with I¹³¹ on the atoms of the phenolic ring, on those of the inner ring, or had been biosynthetically labeled. It is concluded that this thiouracil partially inhibits in vivo deiodination of thyroxine on both the phenolic and the inner ring.