Effect of Thyroxine Analogues on the Peripheral Metabolism of Thyroxine: The Half-life and Pattern of Elimination

Abstract

The effects of thyroxine analogues on the metabolism of ¹³¹I radiothyroxine were investigated in thyroidectomized rats maintained in euthyroid state by exogenous L-thyroxine. The biological half-life of radiothyroxine and the radioactivity in urine and feces were compared in control and experimental animals. It was found that sodium iodide had no effect. D-Thyroxine, 3,3′,5′- triiodo-DL-thyronine, 3,3′,5′-triiodothyropropionic acid and 3,5-diiodothyroacetic acid lengthened the biological half-life of radiothyroxine, increased the radioactivity recovered in feces, and decreased the radioactivity recovered in urine. Carrier L-thyroxine showed similar effects in much smaller doses. The results suggest that the manner in which these thyroxine analogues antagonize the calorigenic action of thyroxine in vivo is related to competition for mechanisms by which thyroxine is degraded. (Endocrinology74: 611, 1964)