EFFECT OF THYROXINE ON TESTICULAR FUNCTION, CIRCULATING LUTEINIZING-HORMONE AND PITUITARY SENSITIVITY TO LUTEINIZING-HORMONE-RELEASING HORMONE IN THE COCKEREL (GALLUS-DOMESTICUS)

1. The effect of thyroxine (T4) on reproductive function in the adult cockerel was followed for 11 weeks. Broiler cockerels aged 96 weeks we re fed on diets containing either 0, 2 or 5 mg T4/kg for 4 weeks. 2. D aily sperm output (DSO) was significantly reduced (P < 0.0 1) in the T 4-treated groups compared with that of controls at weeks 5 and 7. In t he group given 5 mg T4/kg, plasma testosterone concentration was signi ficantly reduced (P < 0.0 1) compared with that in controls during the T4 treatment, in spite of the fact that there was a decrease in conce ntration in both control and experimental birds. Plasma luteinising ho rmone (LH) concentration was significantly decreased (P< 0.05) in both of the groups given T4 treatments after 3 weeks. 3. Plasma testostero ne concentrations and DSO had returned to control values at weeks 5 an d 11 respectively, while plasma LH showed a transient but significant (P < 0.00 1) rebound after removal of thyroxine from the food. 4. In c ontrast to other variables, the pituitary responsivity to cLHRH-I inje ctions, was not decreased during the feeding of the T4 diet but was, o n the contrary, significantly increased (P < 0.05) during treatment wi th 5 mg T4/kg diet, and after the end of the treatment with 2 mg T4/kg diet. 5. These results provide some evidence for an inhibitory effect of large doses of T4 on the reproductive function in the adult cocker el. Although the possibility of a direct effect of T4 on the testes ca nnot be excluded, T4 is likely to act, at least in part, at the hypoth alamo-pituitary level, and not through a reduction in the pituitary se nsitivity to LHRH.