EFFECT OF THYROIDECTOMY UPON SEASONALITY IN RAMS

Experiments were conducted to examine whether seasonal breeding patter ns of male sheep are abrogated by thyroidectomy. In Expt I, Welsh Moun tain rams were thyroidectomized in early autumn (September) and then m aintained on either 16 h light:8 h dark (long days; n = 6) or 8 h ligh t:16 h dark (short days; n = 6) for 8 months. Intact rams (n = 6 per g roup) were also housed in long or short days, or in natural photoperio ds. Results were similar in animals housed on long or short days. In t hyroidectomized rams, plasma FSH concentrations and scrotal circumfere nce were maintained at values typical of the breeding season throughou t the investigation, whereas in intact animals both reached a nadir in December and January. In Expt 2, a further II rams were thyroidectomi zed in March and, together with 23 intact animals, were maintained the reafter in natural photoperiods. In control rams, scrotal circumferenc e increased slowly between May and September, whereas in thyroidectomi zed animals the circumference increased rapidly in the first 4 weeks f ollowing thyroidectomy (3.7 +/- 0.7 cm), with a further increase (5.9 +/- 1.0 cm) in the next 4 weeks. The scrotal circumference of thyroide ctomized rams was therefore significantly (P < 0.01) greater than that in intact animals between April and August. Plasma FSH concentrations were significantly (P < 0.01) higher in thyroidectomized than in cont rol rams by two weeks after surgery. These results indicate that thyro idectomy overcomes the seasonal (or photorefractory) inhibition of rep roductive activity in rams and supports a key role for thyroid hormone s in the expression of seasonal patterns of breeding activity.