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).
- Thyroxine is produced by combining two moieties of DIT.
- Triiodothyronine is produced by combining one molecule of MIT and one molecule of 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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