Thyroid gonadal interactions in the male domestic duck in relationship with the sexual cycle

Abstract

The annual variations of the plasma testosterone concentrations (PT), and the metabolic clearance rate (MCR) of 3H-testosterone, that permits evaluation of the secretion rate (SR = PT × MCR) of the hormone, have been measured in Pekin ducks reared outdoors. Three salient features were observed: (1) during the progressive phase of the reproductive cycle, there is no close parallelism between the testis weight (spermatogenetic activity) and the SR of testosterone; in 1971–1972 both rhythms followed a particularly asynchronous course; (2) in late May the PT falls down to almost autumnal values when the testis weight is still high; (3) this breakdown of the PT is concomitant with a 80% rise in the MCR of testosterone, which contributes, together with a depressed SR to lower the PT. The two latter events (lowered PT in association with an enhanced MCR) can be reproduced by thyroxine administration (0.1 mg or 1 mg/day over 9 days) either during the vernal progressive phase, or during autumn in ducks photo-stimulated by “long days.” The recently evidenced increase in thyroid function in May–June in ducks reared under similar conditions led to the assumption that a seasonal hyperthyroidism may, at least partially, account for the characteristic pattern of testosterone secretion and metabolism at the onset of the regressive phase of the sexual cycle. The postnuptial molt occurs once this peculiar endocrine balance (hyperthyroidism associated to hypoandrogenism) is fulfilled.