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Thyroid hormone synthesis

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Synthesis of the thyroid hormones, as seen in an individual thyroid follicular cell. Thyroglobulin is synthesized in the rough endoplasmic reticulum and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis. A sodium-iodide (Na/I) symporter pumps iodide (I−) actively into the cell, which previously has crossed the endothelium (A symporter is an integral membrane protein that transports many different types of molecules across the cell membrane at the same time, and is, therefore, a type of cotransporter). Iodide then enters the follicular lumen from the cytoplasm by the action of the transporter pendrin. In the colloid, iodide (I−) is oxidized to iodine (I0) by the enzyme thyroid peroxidase. Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues in its protein chain (which contains approximately 120 tyrosyl residues). In conjugation, adjacent tyrosyl residues are paired together. Thyroglobulin then reenters the follicular cell by endocytosis. Proteolysis by various proteases liberates thyroxine and triiodothyronine molecules. Efflux of thyroxine and triiodothyronine from follicular cells into the blood occurs largely through the action of monocarboxylate transporters (MCT) 8 and 10. (Source: Häggström, Mikael. Medical gallery of Mikael Häggström 2014. Wikiversity Journal of Medicine 1 (2).)