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| Names | |
|---|---|
| Other names O-(4-hydroxy-3,5-diiodophenyl)-3,5-diiodo-L-tyrosine, (-)-thyroxine, 3,3′,5,5′-tetraiodo-L-thyronine | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChEMBL | |
| ChemSpider | |
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| UNII | |
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| Properties | |
| C15H11I4NO4 | |
| Molar mass | 776.874 g·mol−1 |
| Appearance | white solid |
| Melting point | 235–236 °C (455–457 °F; 508–509 K) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Thyroxine, also known asT4, is ahormone produced by thethyroid gland. It is the primary form ofthyroid hormone found in the blood and acts as aprohormone of the more active thyroid hormone,triiodothyronine (T3).[1] Thyroxine and its active metabolites are essential for regulatingmetabolic rate, supporting heart andmuscle function, promotingbrain development, and maintainingbone health.[2][3]
Thyroxine has ahalf-life of approximately one week and hence maintains relatively stable blood levels. Its production and release are controlled through a complex feedback loop involving thehypothalamus,pituitary gland, and thyroid gland. This regulatory system ensures that optimal hormone levels are maintained.[4]
Thyroxine biosynthesis is a multi-step process that occurs infollicular cell within the thyroid gland. The synthesis of thyroxine requires adequateiodine supply and appropriate hormonal control.[5][6][4]
The process begins with the active uptake of iodide from the bloodstream by thyroid follicular cells through thesodium/iodide symporter (NIS) located in thebasolateral membrane. Once inside the cell, iodide is transported to the follicular lumen, where it undergoes oxidation by the enzymethyroid peroxidase (TPO) in the presence ofhydrogen peroxide generated by theNADPH oxidaseDUOX2.[6] The oxidized iodine then iodinates tyrosyl residues ofthyroglobulin (Tg), aglycoprotein synthesized by thyroid cells and stored in the follicular lumen.[5] This process, known asorganification, results in the formation ofmonoiodotyrosine (MIT) anddiiodotyrosine (DIT) residues within the Tg molecule.[6][4]
The final step in thyroxine synthesis involves thefree radical mediated coupling of two DIT residues, catalyzed by TPO, to form T4 while still attached to the Tg backbone.[5][6] When thyroid hormone is needed, Tg is internalized by thyrocytes, and proteolytic enzymes in lysosomes cleave the T4 from Tg, allowing for its release into the bloodstream.[4] This intricate biosynthetic pathway is tightly regulated bythyroid-stimulating hormone (TSH) from the pituitary gland, which influences virtually every stage of thyroid hormone production.[4]
