Key Points

• The thyroid hormones thyroxine (T4) and triiodothyronine (T3) are a-amino acids derived from the condensation of two tyrosine molecules and contain two atoms of iodine on the inner ring and one (T3) or two (T4) iodine atoms on the outer ring.

• Biosynthetic reactions include uptake, peroxidation, and incorporation of iodine into tyrosine residues that are in peptide linkage within the large glycoprotein, thyroglobulin, followed by peroxidase-catalyzed coupling of nearby tyro-sine residues.

• Secretion involves endocytosis of thyroglobulin from the follicular lumen followed by proteoly-sis in lysosomes to release the T4 and T3. Synthesis and secretion are regulated by the thyroid-stimulating hormone (TSH) of the pituitary gland.

• Thyroid hormones circulate bound to plasma proteins and have long half-lives.

• T3 and T4 promote growth largely through interactions with growth hormone. Both are required for normal development of the central nervous system (CNS) during the perinatal period, and both increase sensitivity to sympathetic stimulation.

• T3 and T4 increase oxygen consumption and the basal metabolic rate and accelerate virtually all aspects of carbohydrate, lipid, and protein metabolism.

• In the absence of T3 and T4, defective cholesterol excretion leads to hypercholesterolemia, and some defect in protein metabolism leads

Essential Medical Physiology, Third Edition

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