Bio-synthesis of thyroid hormone

Synthesis of thyroid hormone occurs in five stages:

1) Thyroglobulin synthesis: Endoplasmic reticulum and Golgi apparatus in the follicular cells of thyroid gland synthesize and secrete thyroglobulin continuously. Thyroglobulin molecule is a large glycoprotein containing 140 molecules of amino acid tyrosine. After synthesis, thyroglobulin is stored in the follicle.

2) Iodide trapping: iodide is actively transported from blood into follicular cell, against electrochemical gradient. This process is called iodide trapping. Iodide is transported into the follicular cell along with sodium by sodium-iodide Symport pump.

3) Oxidation of iodide: iodide must be oxidized to elementary iodine, because only iodine is capable of combining with tyrosine to form occurs inside the follicular cells in the present of thyroid peroxidase. Absence or inactivity of this enzyme stops the synthesis of thyroid hormone.

4)Transport of iodine into follicular cavity: From the follicular cells, iodine is transport into the follicular cavity by an iodide-chloride pump called Pendrin.

5) Iodination of tyrosine: combination of iodine with tyrosine is known as iodination. It takes place in thyroglboin. First, iodine is transported from follicular cell into the follicular cavity, where it binds with thyroglobulin. Then, iodine combines with tyrosine, which is already present in thyroglobulin. Iodination process accelerated by the enzyme iodinase, which is secreted by follicular cell. Tyrosine is iodized first into monoiodotyrosine (MIT) and later into di-iodotyrosine(DIT). MIT and DIT are call the iodotyrosine residues.

Coupling reactions:

Tyrosine + I =  Monoiodotyrosine (MIT)

MIT + I         =  Di-iodotyrosine(DIT).

MIT + DIT    =  Reverse T3

DIT +DIT      =  Tetraiodotyrosine or Thyroxine (T4)