Thyroglobulin
Thyroglobulin (Tg) is a protein produced by and used entirely within the thyroid gland. Thyroglobulin is used by the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3).
Thyroglobulin is stored in the thyroid in follicle cysts surrounded by colloid cells.
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 linking two moieties of DIT. Combining one particle of MIT and one particle of DIT produces triiodothyronine.
Proteases in lysosomes digest iodinated thyroglobulin, releasing T3 and T4.
Thyroglobulin levels in the blood can be used as a tumour marker for certain kinds of thyroid cancer. Thyroglobulin levels in the blood can also be elevated in cases of Graves Disease.
Thyroglobulin should not be confused with Thyroxine-binding globulin, a protein responsible for carrying the thyroid hormones in the blood.
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