INVOLVEMENT OF THYROID-HORMONES IN SEASONAL REPRODUCTION

This article reviews experiments performed to investigate the importan ce of thyroid hormones to the expression of the seasonal reproductive cycle of ewes. Thyroidectomy was found to block the transition from th e breeding season to anoestrus and to cause ewes to exhibit oestrous c ycles all year round. Mechanistically, thyroidectomy produced this eff ect by preventing the seasonal increase in responsiveness to the negat ive feedback action of oestradiol on episodic GnRH secretion, thus int erfering with a key neuroendocrine process necessary for anoestrus to develop. This response to thyroidectomy was fully prevented by replace ment 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 f unction and seasonal and photoperiodic mechanisms were not affected. E vidence is presented to indicate that thyroid hormones have a permissi ve mode of action on seasonal reproduction, and that their effect is e xerted during a restricted 'window' of time during the year. It is con cluded that thyroid hormones play a crucial role of physiological sign ificance to generation of the seasonal reproductive cycle of ewes.