Involvement of thyroid hormones in seasonal reproduction.

This article reviews experiments performed to investigate the importance of thyroid hormones to the expression of the seasonal reproductive cycle of ewes. Thyroidectomy was found to block the transition from the breeding season to anoestrus and to cause ewes to exhibit oestrous cycles all year round. Mechanistically, thyroidectomy produced this effect by preventing the seasonal increase in responsiveness to the negative feedback action of oestradiol on episodic GnRH secretion, thus interfering with a key neuroendocrine process necessary for anoestrus to develop. This response to thyroidectomy was fully prevented by replacement with physiological concentrations of thyroxine. Furthermore, the reproductive response to thyroidectomy was specific to the mechanisms that lead to anoestrus; other aspects of reproductive neuroendocrine function and seasonal and photoperiodic mechanisms were not affected. Evidence is presented to indicate that thyroid hormones have a permissive mode of action on seasonal reproduction, and that their effect is exerted during a restricted 'window' of time during the year. It is concluded that thyroid hormones play a crucial role of physiological significance to generation of the seasonal reproductive cycle of ewes.