Seasonal Rhythms: The Role of Thyrotropin and Thyroid Hormones

Seasonal changes in various physiological events have been reported in humans, including metabolism, immune function, and mood. However, the molecular and endocrine basis of these seasonal changes remains unclear.

Animals that breed seasonally, such as Japanese quail and the Siberian hamster, have sophisticated seasonal mechanisms, and hence they provide excellent opportunities to understand the underlying processes. Functional genomic analysis in quail uncovered the photoperiodic signal transduction pathway, which regulates avian seasonal reproduction: a long-day stimulus induces secretion of thyrotropin (TSH) from the pars tuberalis (PT) of the anterior pituitary gland. This PT-derived TSH locally activates thyroid hormone within the hypothalamus, which in turn induces gonadotropin-releasing hormone and then gonadotropin secretion, leading to gonadal growth.

Studies using TSH receptor-null mice confirmed the involvement of PT-derived TSH in mammalian seasonal reproduction. The pars distalis of the anterior pituitary gland is the major source of circulating TSH. Although the pars distalis and PT are in close proximity, tissue-specific glycosylation of circulating TSH alters its function to avoid cross talk.