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

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 monodeiodination 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.