The metabolism of ring-labeled L-thyroxine-C14 in vivo.

In the past, investigators have used thyroxine labeled either with JIP" in the 3',5'-position or with C14 in the carboxyl position to study the metabolism of thyroxine in vivo. Although these studies have yielded much valuable information, our knowledge about the metabolic fate of the ring structure in thyroxine is incomplete because both the 3'5'-IJ13 and the carboxyl-C14 are rapidly removed from thyroxine in vivo by deiodination and .decarboxylation. To investigate the metabolic fate of the thyroxine nucleus, L-thyroxine-C14 labeled uniformly in the phenolic ring has been synthesized in our laboratory. Studies on the metabolism of ring-labeled L-thyroxine-C14 following iv administration to rats, dogs, and human subjects are reported in this communication.