Time-course of thyroid hormone involvement in the development of anestrus in the ewe

It is well established that the thyroid gland is essential for termination of seasonal reproductive activity in a variety of birds and mammals. In the present study, we examined when during the breeding season the thyroid exerts this effect in female sheep. Previous results suggest that the presence of thyroid hormones during the first 4-6 wk (20-25%) of the breeding season is not sufficient for the neuroendocrine changes that lead to anestrus. We therefore hypothesized that thyroid hormone action is exerted at some point during the latter 75-80% of the breeding season. To test this hypothesis, ewes thyroidectomized early in the breeding season received replacement of thyroxine at various times to create gaps during the mid- to late breeding season when thyroid hormones were absent. We then examined the effect, if any, of this absence on development of seasonal neuroendocrine anestrus. Each ewe was ovariectomized and treated with a constant-release Silastic capsule containing estradiol. Serum concentrations of LH were used as an index of seasonal changes in reproductive neuroendocrine activity. We found that when thyroid hormones were removed for a 60-day period in mid- to late breeding season (from mid-Oct. to late Dec., which is approximately 40% of the entire breeding season), anestrus still developed at the normal time. We conclude, therefore, that thyroid hormones need not be present for much of the breeding season (mid-Sept. through late Dec.) for anestrus to develop in the ewe. Rather, we postulate that thyroid hormones need to be present for only a brief period of time near the end of the breeding season for the neuroendocrine changes that lead to anestrus.