The thyroid synthesizes thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) from iodine and the amino acid tyrosine through tyrosyl iodination and coupling reactions, with production regulated by TSH from the anterior pituitary. Thyroid hormones increase metabolic rate and are essential for normal growth, development, and thermoregulation.
From the endocrine system overview, you know that hormones are chemical signals that regulate distant target cells, and from the hypothalamic-pituitary axis, you understand that the hypothalamus controls many endocrine glands through a two-step relay via the pituitary. Thyroid hormone regulation is one of the clearest examples of this hierarchical control system, and the thyroid gland itself has a unique synthetic mechanism — it is the only endocrine gland that stores large quantities of preformed hormone extracellularly, in a protein-rich colloid within follicles.
The thyroid gland is organized into spherical follicles, each lined by a single layer of follicular epithelial cells surrounding a lumen filled with thyroglobulin — a large glycoprotein that serves as the scaffold for hormone synthesis. The process begins with iodide trapping: the sodium-iodide symporter (NIS) on the basolateral membrane actively concentrates iodide from the blood into follicular cells (to 20–40 times plasma levels). Iodide is then transported across the apical membrane into the colloid, where the enzyme thyroid peroxidase (TPO) oxidizes it and attaches it to tyrosine residues on thyroglobulin. A single iodine attachment creates monoiodotyrosine (MIT); a second creates diiodotyrosine (DIT). TPO then couples these iodinated tyrosines: two DIT molecules couple to form T4 (thyroxine), while one MIT and one DIT couple to form T3 (triiodothyronine). The iodinated thyroglobulin remains stored in the colloid — the gland holds weeks' worth of hormone supply.
When thyroid hormones are needed, follicular cells endocytose colloid droplets, fuse them with lysosomes, and proteolyze thyroglobulin to liberate T4 and T3. The gland releases mostly T4 (about 90%), which is relatively inactive — it serves as a circulating reservoir. Peripheral tissues, especially the liver and kidneys, convert T4 to the more potent T3 using deiodinase enzymes that remove one iodine atom. This peripheral conversion means that target cells can locally regulate their own thyroid hormone exposure, adding a layer of fine-tuning beyond what the central axis provides.
The entire system is governed by a classic negative feedback loop. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH binds receptors on follicular cells and stimulates every step of hormone production — iodide uptake, thyroglobulin synthesis, TPO activity, colloid endocytosis, and hormone release. As circulating T3 and T4 levels rise, they inhibit both TRH and TSH secretion, reducing thyroid stimulation. This feedback keeps thyroid hormone levels remarkably stable. In hypothyroidism (insufficient hormone), TSH rises as the pituitary tries to whip an underperforming gland into action — elevated TSH is the most sensitive early marker. In hyperthyroidism (excess hormone, as in Graves disease where antibodies mimic TSH), TSH is suppressed because the feedback loop is intact but the gland is being driven by an autonomous stimulus.