Testosterone secretion and plasma luteinizing hormone concentration during a sexual cycle in the Pekin duck, and after thyroxine treatment

Abstract

Plasma testosterone and luteinizing hormone concentrations, together with the metabolic elearance rate for testosterone, were measured in Pekin ducks exposed to a natural cycle of day length, to artificial long day lengths, and to treatment with thyroxine. Long day lengths led to increases in testicular size and in plasma testosterone and LH levels. Under natural photoperiods a dissociation was observed between these parameters with the plasma testosterone concentration reaching a peak some months before the time of maximal testicular size and LH concentration. It is suggested that the fall in testosterone concentration, in the absence of a concomitant fall in luteinizing hormone, may reflect (a) an increase in testosterone metabolic clearance rate due to thyroxine, and (b) a decrease in sensitivity of the interstitial cells to LH through the breeding season. Exogenous thyroxine increases the testosterone MCR but has no consistent effect on plasma luteinizing hormone.