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. Author manuscript; available in PMC: 2020 Jun 11.
Published in final edited form as: Nat Rev Endocrinol. 2019 Aug 12;15(10):590–600. doi: 10.1038/s41574-019-0237-z

Figure 6. Pars tuberalis-derived TSH regulates seasonal thyroid hormone function.

Figure 6.

Light information received by the eyes is transmitted to the pineal gland via the suprachiasmatic nuclei (SCN). Pineal melatonin secretion pattern reflects the length of nights and suppresses pars tuberalis-derived TSH expression. Long days increase production of pars tuberalis-derived TSH, which acts on ependymal cells in the hypothalamus to induce type 2 iodothyronine deiodinase (DIO2) expression through the TSH receptor–Gsα–cAMP signaling pathway. DIO2-induced thyroid hormone activation, through the generation of T3 from T4, transmits the springtime signal128,132. Pars distalis-derived TSH stimulates the thyroid gland. Pars tuberalis-derived TSH has tissue-specific N-glycans and forms macro-TSH complexes with immunoglobulin (IgG) and albumin in the circulation. The macro-TSH complexes are unable to stimulate the thyroid gland, and this feature prevents functional crosstalk between the two TSHs, thus preventing the production of seasonal thyroid gland overactivity73.