The Role of Hypothalamic Tri-Iodothyronine Availability in Seasonal Regulation of Energy Balance and Body Weight

Figure 1

Schematic representation of the influence of photoperiod on deiodinase enzyme gene expression in the hypothalamus. In mammals, photoperiodic information detected by the retina is relayed via the suprachiasmatic nucleus (SCN) to the pineal gland, where the nocturnal secretion of melatonin signals directly to the pars tuberalis. In birds, photoperiodic signalling is not dependent on this pineal-melatonin pathway. Across both vertebrate groups, long photoperiods promote TSHβ production by the pars tuberalis which maintains DIO2 expression in tanycytes. T4 is taken up from the circulation via MCT8 transporters and is converted by type 2 deiodinase (DIO2) to T3 which exerts local anabolic actions within the mediobasal hypothalamus. In short photoperiods (SD) the long nocturnal duration of melatonin reduces TSHβ production, expression of type 3 deiodinase (DIO3) is upregulated, thus, inactive metabolites of T4 are produced resulting in a catabolic state.