Tri-iodothyronine fails to mimic gonado-inhibitory action of thyroxine in spotted munia: Effects of injections at different times of the day

Abstract

In spotted munia an inverse relationship between thyroid status and reproduction has been demonstrated. Thyroidectomy induces unseasonal gonadal development and abolishes seasonal regression, and administration of 1-thyroxine causes regression at any time of the year. We present evidence indicating that unlike thyroxine, tri-iodothyronine, the other major circulating thyroid hormone, does not inhibit the gonadal function in this bird. Experiments with injections of small doses of T4 and T3 at different times of the day indicated a daily rhythmicity in the gonadal response to T4, T3 being ineffective at any time. The failure of T3 to elicit a response is thus not due to a possible phase difference in underlying circadian rhythms of sensitivity to T4 and T3. General consensus holds that T4 is a prohormone, T3 being the finally active deiodinated compound. In the present experiments the effects of T4 are not mimicked by T3. This argues for an intrinsic activity of T4 with respect to gonadal function in birds.