Tuesday, 15 April 2014

THYROID GLAND- Biosynthesis of thyroid hormone



Tg is used by the thyroid gland to produce the thyroid hormonesthyroxine (T4) andtriiodothyronine (T3). The active form of triiodothyronine, 3, 5, 3' triiodothyronine, is produced both within the thyroid gland and in the periphery by 5'-deiodinase (which has been referred to as tetraiodothyronine 5' deiodinase). It is presumed that Tg and thyroid are also an important storage of iodine for all body needs, in particular, for many iodine-concentrating organs such as breast, stomach, salivary glands, thymus, choroid plexus and cerebrospinal fluid, etc.[2] (seeiodine in biology).
In fact, the Tg molecule, which contains approximately 120 tyrosyl residues, is able to form only very small amounts of thyroid hormone (5-6 molecules of T4 and T3).
Tg is produced by the thyroid epithelial cells, called thyrocytes, which form spherical follicles. Tg is secreted and stored in the follicular lumen.
Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine(DIT).

Small globules of the follicular colloid (Tg) are endocytosed (hormone (TSH)-mediated) and proteases in lysosomes digest iodinated thyroglobulin, releasing T3 and T4 within the thyrocyte cytoplasm. The T3 and T4 are then transported across (TSH-mediated) the basolateral thyrocyte membrane, into the bloodstream, by an unknown mechanism while the lysosome is recycled back to the follicular lumen.

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