Thyroxine biosynthesis

Production and secretion of the thyroid hormone T3 (3,5,3'-triiodothyronine) and its inactive precursor T4 (3,5,3',5'-tetraiodothyronine) by thyroid follicular cells is triggered by Thyroid-stimulating hormone (TSH, thyrotropin). T3 and T4 is taken up by target cells where more T3 is produced by deiodinating T4. Ultimately T3 binds to thyroid hormone receptors in the nucleus and regulates the target cell's gene expression (reviewed in Carvalho & Dupuy, 2017; Jing & Zhang, 2022). In the thyroid, follicular cells form a compartment that traps iodide imported from the blood plasma. T3/T4 synthesis proceeds on thyroglobulin (TG) as a platform; each monomer provides about 30 tyrosine (Tyr) residues for iodination. TG carrying the iodinated Tyr residues is then imported into follicular cells and cleaved in lysosomes, releasing T3, T4 and other iodinated tyrosines. Released iodide is recycled. Finally T3 and T4 are exported to the blood (reviewed in Carvalho & Dupuy, 2017; Jing & Zhang, 2022).

Summary from Reactome (R-HSA-209968), read 2026-09-23.