1. Iodide trapping: The first step in thyroid hormone synthesis is the transport of iodide ions into the thyroid follicular cells. This is mediated by an iodide transporter, known as the sodium iodide symporter (NIS), which actively transports iodide from the blood into the follicular cells.
2. Thyroglobulin synthesis: Thyroglobulin (TG) is a large glycoprotein that is synthesized within the follicular cells. It serves as a scaffold for thyroid hormone synthesis and storage. The synthesis of TG occurs in the endoplasmic reticulum of the follicular cells.
3. Iodination of tyrosine residues: Within the colloid-filled follicular lumen, iodide ions are oxidized to iodine, facilitated by an enzyme called thyroid peroxidase (TPO). The iodine atoms are then covalently attached to tyrosine residues on TG, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
4. Coupling of iodotyrosines: MIT and DIT residues on TG can undergo an enzyme-mediated reaction to form either T3 (triiodothyronine) or T4 (thyroxine). One MIT and one DIT residue combine to form T3, while two DIT residues combine to form T4.
5. Endocytosis and proteolysis: Once sufficient amounts of T3 and T4 are synthesized, the thyroid follicular cells engulf the colloid via endocytosis. The TG-filled vesicles then fuse with lysosomes, where proteases break down TG to release T3 and T4.
6. Secretion of thyroid hormones: T3 and T4 are transported out of the follicular cells and into the bloodstream. They bind to carrier proteins, such as thyroxine-binding globulin (TBG), which assist in their transport to target tissues throughout the body.
7. Peripheral activation of T4: Once in the target tissues, T4 can be enzymatically converted to T3 by the removal of one iodine atom. This conversion primarily occurs in the liver and kidneys and is facilitated by the enzyme deiodinase.
These steps collectively contribute to the synthesis, storage, release, and activation of thyroid hormones within the body.
step of thyroid hormone synthesis
1 answer
1 answer