Responses of prepubertal and mature rams to thyroidectomy.

Thyroidectomy of seasonally breeding birds and mammals prevents the return to a state of sexual quiescence at the end of the breeding season. In starlings, thyroidectomy also causes premature sexual maturity. In this study, the effect of thyroidectomy upon the time of sexual maturity of prepubertal (8 week-old) ram lambs was examined. Thyroidectomy of four prepubertal and six mature rams was performed early in the spring. These and sham-operated controls were maintained in ambient photoperiods (south-west England). Scrotal circumference and serum LH, FSH, prolactin and thyroxine were measured every 2 weeks. In both the prepubertal lambs and the mature rams, scrotal circumference increased significantly within 5 weeks of thyroidectomy. FSH concentrations increased significantly in the mature rams after thyroidectomy. The relatively high FSH concentrations of thyroidectomized animals at the start of the experiment were maintained, but the FSH concentrations of intact lambs decreased during the late spring. These results provide the first indication that the timing of puberty in seasonally breeding mammals is a thyroid-dependent phenomenon.