Effects of thyroxine on avian moulting may not involve prior conversion to tri-iodothyronine

Abstract

The comparative effects of equimolar doses of thyroxine (T₄) and tri-iodothyronine (T₃) and of the inhibition of peripheral monodeiodination were studied on feather regeneration and moult in thyroidectomized and intact adult male spotted munia (Lonchura punctulata). T₄ was significantly more effective than T₃ at all dose levels in inducing feather regeneration in thyroidectomized birds. Suppression of mono-deiodination by iopanoic acid (IOP) was not accompanied by a suppression of T₄-induced feather regeneration in thyroidectomized birds and IOP alone was without effect in thyroidectomized birds. Long-term treatment of intact birds with IOP accelerated the process of moulting by about 3 months. These findings question the prevailing concept that T₃ is the finally active biological compound and T₄ a mere precursor.