An Introduction to Thyroid Physiology

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.