Metabolism of D-Thyroxine

Abstract

Deiodination of D-thyroxine from the 3 or 5 position of the alpha ring was more rapid than that of L-thyroxine in the dog, as shown by larger amounts of conjugated D-3,3 ′,5 ′-triiodothyronine and D-3,3 ′-diiodothyronine which were excreted in bile and urine, compared to similar metabolites of L-thyroxine. Thus, D-thyroxine was inactivated more rapidly than L-thyroxine by the dog. Deiodination from the beta ring of Dthyroxine like that of L-thyroxine was greatly diminished after total hepatectomy, and little radioactive iodide was excreted in the urine. D-Thyroxine was metabolized more rapidly by the rat than L-thyroxine, as shown by the greater excretion of radioactive iodide in urine. Metabolites excreted in the bile were similar after the injection of D-thyroxine and Lthyroxine, but there were great differences between rats and dogs in the amounts of these metabolites.