Effect of thyroidectomy on maintenance of seasonal reproductive suppression in the ewe

Thyroid hormones are essential to end the breeding season in sheep; however, it is not clear whether thyroid hormone action is limited to initiation of seasonal reproductive suppression in the ewe. The purpose of this study was to determine the influence of thyroid hormones on maintenance of anestrus and onset of the subsequent breeding season in the ewe. In both experiments, ewes were thyroidectomized (THX) either soon after they had completed the transition from the breeding season to anestrus or just before the transition into the breeding season (late anestrus). All ewes were ovariectomized and received constant-release silicone elastomer implants of estradiol. Circulating levels of LH were monitored as an index of seasonal changes in reproductive neuroendocrine activity. After thyroidectomy early in anestrus, timing of the subsequent LH rise, indicative of the next neuroendocrine breeding season, was the same among THX and thyroid-intact ewes. As observed previously, LH remained elevated in ewes THX late in anestrus beyond the time associated with development of anestrus. We conclude therefore that thyroid hormones are not needed to maintain suppression of the reproductive neuroendocrine axis once anestrus has been established, nor do they influence the onset of the subsequent breeding season in the ewe. Rather, thyroid hormone action on seasonal alterations of the reproductive neuroendocrine axis in the ewe is restricted to the changes that cause development of anestrus.