Thyroid hormone and seasonal regulation of reproduction

• Animals use changes in photoperiod as a calendar.
• In mammals, nocturnal melatonin secretion provides photoperiodic information.
• In birds, light is perceived by deep brain photoreceptors to regulate the seasonality.
• The pars tuberalis thyrotropin is the key hormone regulating seasonal reproduction.
• Hypothalamic thyroid hormone activation regulates seasonal GnRH secretion.

Organisms living outside the tropics use changes in photoperiod to adapt to seasonal changes in the environment. Several models have contributed to an understanding of this mechanism at the molecular and endocrine levels. Subtropical birds are excellent models for the study of these mechanisms because of their rapid and dramatic response to changes in photoperiod. Studies of birds have demonstrated that light is perceived by a deep brain photoreceptor and long day-induced thyrotropin (TSH) from the pars tuberalis (PT) of the pituitary gland causes local thyroid hormone activation within the mediobasal hypothalamus (MBH). The locally generated bioactive thyroid hormone, T3, regulates seasonal gonadotropin-releasing hormone (GnRH) secretion, and hence gonadotropin secretion. In mammals, the eyes are the only photoreceptor involved in photoperiodic time perception and nocturnal melatonin secretion provides an endocrine signal of photoperiod to the PT to regulate TSH. Here, I review the current understanding of the hypothalamic mechanisms controlling seasonal reproduction in mammals and birds.