This chapter is a general introduction to this book and contains basic
concepts of thyroid hormone signaling for a better understanding of the book’s subject.
It begins with an introduction that offers a simplified view of thyroid hormones as
iodine-containing compounds and the regulatory function of the hypothalamuspituitary-thyroid axis, followed by a description of the thyroid gland and thyroid
hormone synthesis. Iodide transporters concentrate iodide in the gland and after
oxidation, it is incorporated into thyroglobulin tyrosyl residues. The coupling of
iodotyrosyl residues forms T4 and T3, which are released after thyroglobulin
hydrolysis. Thyroid hormones act via nuclear receptors, which are ligand-regulated
transcription factors, and T3 is the primary active thyroid hormone that binds to the
receptors. T3 is produced primarily in extrathyroidal tissues by the action of deiodinase
enzymes catalyzing the removal of an iodine atom from T4. Thyroid hormones are
ancient signaling molecules with critical actions on growth and metabolism that
regulate many developmental transitions, with evolutionary roots at the base of the
chordate species.
Keywords: Amphioxus, Colloid, DUOX2, Dehalogenase, Deiodinases, DIO, Evolution, Endostyle, Follicles, Genomic actions, Iodine, lamprey, Metamorphosis, Nuclear receptors, Non-genomic, NIS, Peroxidase, Selenium, Thyroglobulin.
