Synthesis of thyroid hormone
Synthesis and secretion of thyroid hormone
Thyroid hormone is produced in the epithelial cells of the thyroid gland. The synthesis of the hormone is a rather complicated process, which involves three major steps:
- Production and accumulation of the basic components, tyrosine and iodide. Iodide ions are taken up from the blood by the sodium-iodide symporter (NIS). Once inside the cell, iodide is transported to the apical membrane and released in the follicle lumen by the pendrin transporter. The thyroid epithelial cell produces the protein thyroglobulin (TG), which contains tyrosines (tyr).
- Fabrication of the thyroid hormones. The enzyme thyroid peroxidase (TPO) conducts the reactions. Firstly, the tyrosines on the thyroglobulin are iodinated and secondly, the hormone thyroxine (T4) or triiodotyrosine (T3) are synthesized. The hormone is now still tied up in molecules of thyroglobulin.
- Secretion of T3 and T4. The thyroglobulin with T3/T4 is endocytosed by the epithelial cell. Lysosomes fuse with the endosome and TG is proteolysed by lysosomal enzymes, liberating the thyroid hormones. T3 and T4 are now ready for secretion and released in the bloodstream, where they bind carrier proteins for transport to target cells.
I. Iodide is required for normal thyroid function.
II. Iodide deficiency results in hypothyroidism.
Extra info: Iodide is part of thyroxine: three (T3) and four (T4) iodide atoms per molecule. Thus a shortage of iodide in the diet results in decreased thyroxine production.