Evidence for the role of thyroxine as a hormone in the physiology of a lizard

Abstract

In vivo effects of thyroxine (T₄) and triiodothyronine (T₃) were studied in male lizards. T₄ or T₃ (0.5 to 2 nmol) was administered per day over 10 days to surgically thyroidectomized male lizards (35 ± 2.0 g) and the responses were measured in terms of scale shedding, whole body oxygen consumption (OC), and testicular weight. T₃ was more effective in stimulating OC as compared to T₄. T₄ accelerated scale shedding to a greater extent as compared to T₃. T₄ was more effective than T₃ in restoring the decline in the gonadal weight of thyroidectomized animals. Effect of inhibition of peripheral conversion of T₄→ T₃ by iopanoic acid (IOP) was studied on the above response parameters. IOP at both dose levels inhibited extrathyroidal conversion of T₄→T₃. The T₄-stimulated increase in OC of IOP-treated animals was suppressed, clearly indicating that the T₄ effect could be attributed to its conversion to T₃. But in these same animals, IOP failed to inhibit T₄-stimulated scale shedding and gonadal weight. As the proportion of T₄ converted to T₃ decreased as a result of IOP treatment, the effectiveness of T₄ increased in terms of scale shedding and restoration of gonadal weight. From these studies it appears that all effects of T₄ in lizards need not necessarily be mediated via conversion to T₃.