Metabolism of Thyroid Hormones in Frogs and Toads

Abstract

The rate of turnover of ¹³¹I-labeled thyroxine and triiodothyronine in adult frogs and toads is temperature-dependent. The similar turnover rates for the 2 hormones and studies of plasma binding by equilibrium dialysis suggest that thyroxinebinding activity in amphibian plasma is low. Moreover, turnover of the hormones is slowed by injection of human plasma into the animal. A search for deiodinating systems in vitro revealed a heat-labile one only in frog skin from certain loci. Heat-stable deiodination proceeds rapidly in muscle and may account for the whole-body turnover of the hormones. Tetraiodothyroacetic acid appears in gut contents and in liver, particularly in toads, and its origin may be the skin which rapidly deaminates and decarboxylates the parent molecule. (Endocrinology75: 157, 1964)