Effets de la photopériode et du taux d'androgène circulant sur la fonction thyroïdienne du Canard

Abstract

Thyroid function has been studied in male ducks under various experimental conditions, including: (1) controls reared in a natural light environment (december); (2) natural light + 14 days i.m. testosterone (10 mg/day); (3) 14 days exposure to an 18L-6D light regimen; (4) 14 days at 18L-6D + testosterone (10 mg/day); (5) castration; (6) castration + testosterone (10 mg/day); (7) castration + 14 days at 18L-6D. Castration was followed: (a) at the thyroid level by an increased radioiodine uptake, and, among the labeled iodoamino acids, an increased radiothyroxine ratio; (b) at the peripheral level, by an increased ratio of unbound radiothyroxine (Sephadex); and by an increased biological half-life ( t 1 2 ) of labeled thyroxine. Testosterone injections led to opposite effects, and reversed the situation in castrates. Testosterone seems therefore to inhibit thyroid function, while it increases the peripheral metabolism of thyroid hormones. Testosterone-thyroid interrelationships must be kept in mind when considering the effects of “long days” photoperiods on thyroid function. However, “long days” per se may act on thyroid function, as evidenced in the castrates, by decreasing (1) the radioiodine uptake and (2) the PB 125I (Sephadex). On the other hand, exposure to increased lighting regimens inhibits the stimulating effect of testosterone on the peripheral metabolism of thyroxine. In the present experiments, “long days” photoperiods seem thus to exert an inhibitory effect on thyroid function, although this environmental factor has been claimed to stimulate the pituitary thyrotropic activity in birds.