BIOSYNTHESIS STORAGE AND TRANSPORT OF THYROID HORMONES ::
The lining cells of thyroid follicles have a remarkable ability to extract iodide from the blood This process is known as iodine trapping This is an active transport mechanism and it is stimulated by thyrotropic hormone from anterior pituitary gland
The trapped iodide is converted into iodine in the thyroid follicles This iodine combines with the aminoacid tyrosine to form monoiodotyrosine (MIT) and di-iodotyrosine (DIT) Iodination takes place at 3rd and 5th positions of tyrosine The next step is coupling or condensation of MIT and DIT Two molecules of DIT from tetraiodothyronine or thyroxine (T4) whereas one molecule of DIT and other molecule of MIT combines to form tri-iodothyronine (T3) The intermediate compounds T3 and T4 and are stored in the colloid of the follicles They are bound to thyroglobulin which is a high molecule protein
T3 and T4 are released in the blood from thyroglobulin by lysozymes The released T3 and T4 are bound to other proteins in plasma namely thyroxine binding alpha globulin and thyroxine binding pre-albumin T4 is bound much more firmly than T3 and hence T3 is quickly acting and short lasting whereas T4 is slow but long acting
Since T3 is more active than T4 it is also derived from T4 by removal of one iodine molecule through the enzyme deiodinase In fact it is the major source of T3 in the body Both T3 and T4 are metabolised in the liver and peripheral tissues by deamination and decarboxylation to form triodothyroacetic acid and tetraiodothyroacetic acid and are excreted in urine
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