T3 fails to mimic certain effects of T4 in munia birds: physiological implications for seasonal timing

The comparative effects of equimolar doses of thyroxine (T4) and tri-iodothyronine (T3) were studied on fattening, gonadal status and moulting in thyroidectomized female spotted munia. The effects of suppression of peripheral conversion of T4 to T3 were also studied on these parameters. T4 administration led within 10 days to a significant regression of gonads and mobilization of fat, the response being linear. T3 had no effect on these parameters at any dose level. However, both T4 and T3 were effective, the former to a greater extent in stimulating feather regeneration in the same birds. Inhibition of peripheral monodeiodination of exogenous T4 in thyroidectomized (TX) birds by iopanoic acid (IOP) did not suppress T4 response, but resulted in greater regression of gonads and fat mobilization; however, IOP alone was ineffective. Long-term suppression of peripheral monodeiodination of endogenous T4 in intact birds with IOP suppressed follicular growth and fat deposition but enhanced moult. Hormonal profiles of T4 and T3 indicated that seasonal moult-inducing, anti-gonadal and anti-fattening effects of the thyroid gland are reflected by circulating T4 but not T3 in the munia bird. It may be concluded that T4 may be more potent than T3 in eliciting certain biological actions and that T3 may not mimic all T4 effects. Contrary to general opinion T4-induced effects need not necessarily be mediated through its prior conversion to T3, giving it an independent hormonal role. This may be a mechanism evolved by passerine birds which are generally known to be iodine-deficient, to preclude the simultaneous occurrence of seasonal events requiring high energy and nutrition, such as moulting, reproduction and migration.