THE METABOLISM OF RING-LABELED L-THYROXINE-C14IN VIVO

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Abstract
The metabolism of L-thyroxine-C14 with the phenolic ring uniformly labeled was investigated in rats, dogs, and humans. In rats approximately 67% of the administered radioactivity was excreted via the feces as free thyroxine. Biliary excretion was substantial (83% of dose) and rapid (T 1/2 = 10 hours). Thyroxine glucuronide was the principal biliary metabolite. Of the administered radioactivity, 7 to 30% was eliminated in the urine by all 3 species. A single C14-labeled metabolite, accounting for nearly all of the urinary radioactivity, was isolated and partially characterized but not completely identified. Less than 0.1% of the dose was eliminated as C14O2 in rats. Experiments in which I131- and C14-ring-labeled thyroxine was administered simultaneously revealed species differences in the metabolism of thyroxine and in the in vivo fate of the 2 radio-active labels.