A butterfly gland in the neck meters metabolism with iodine
The thyroid is a vertebrate endocrine organ that in humans forms a butterfly or H below the Adam's apple. Two lobes join by an isthmus; follicles lined by thyrocytes store colloid and release T3 and T4, while parafollicular cells add calcitonin for calcium balance.
Adult glands weigh about twenty-five grams, each lobe roughly five by three by two centimetres, often larger in women and enlarging in pregnancy. They wrap the front of larynx and trachea beneath thyroid and cricoid cartilages, moving with swallowing via Berry's ligament. Superior thyroid arteries from the external carotid and inferior branches from the thyrocervical trunk feed the tissue; a pyramidal lobe remnant of the thyroglossal duct appears in roughly one-fifth to nearly half of people.
Hypothalamic TRH steers pituitary TSH, which tells follicles to process thyroglobulin into the two iodinated hormones that set metabolic rate, protein synthesis, and childhood growth. Excess means hyperthyroidism—commonly Graves' disease—while shortage yields hypothyroidism. Globally iodine deficiency is the leading preventable cause of childhood intellectual disability; where iodine is adequate, Hashimoto's thyroiditis dominates.
Microscopic follicles zero-point-zero-two to zero-point-nine millimetres across store precursor protein until TSH prompts secretion. Disorders span goitre, nodules, inflammation, and cancer, so a small neck gland punches far above its weight as the body's metabolic thermostat.
The hormones' names are literal. Built from iodine and the amino acid tyrosine, T3 carries three iodine atoms per molecule and T4 carries four. Once released, almost all of it rides on carrier proteins: roughly 70 percent on thyroxine-binding globulin, 15 percent on albumin and 10 percent on transthyretin. Only about 0.03 percent of T4 and 0.3 percent of T3 circulate unbound, and that sliver does the work. Much of the active T3 is not made in the neck at all; up to 85 percent comes from T4 converted by deiodinase enzymes in other organs. A fetus starts early, with TSH measurable by week 11 and its own T4 output self-sufficient by weeks 18 to 20.
Source: Thyroid
