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Experiments were conducted to examine whether seasonal breeding patterns of male sheep are abrogated by thyroidectomy. In Expt 1, Welsh Mountain rams were thyroidectomized in early autumn (September) and then maintained on either 16 h light:8 h dark (long days; n = 6) or 8 h light:16 h dark (short days; n = 6) for 8 months. Intact rams (n = 6 per group) were also housed in long or short days, or in natural photoperiods. Results were similar in animals housed on long or short days. In thyroidectomized rams, plasma FSH concentrations and scrotal circumference were maintained at values typical of the breeding season throughout the investigation, whereas in intact animals both reached a nadir in December and January. In Expt 2, a further 11 rams were thyroidectomized in March and, together with 23 intact animals, were maintained thereafter in natural photoperiods. In control rams, scrotal circumference increased slowly between May and September, whereas in thyroidectomized animals the circumference increased rapidly in the first 4 weeks following thyroidectomy (3.7 ± 0.7 cm), with a further increase (5.9 ± 1.0 cm) in the next 4 weeks. The scrotal circumference of thyroidectomized 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 control rams by two weeks after surgery. These results indicate that thyroidectomy overcomes the seasonal (or photorefractory) inhibition of reproductive activity in rams and supports a key role for thyroid hormones in the expression of seasonal patterns of breeding activity.
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