Thyroglobulin, or Tg, is a glycoprotein of high molecular weight produced by the thyroid epithelial cells in the thyroid gland. It is used by the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). The iodinated tyrosine moieties contained in thyroglobulin form the active hormones thyroxine and tri-iodothyronine.
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.
Thyroglobulin is the protein precursor of thyroid hormone should not be confused with Thyroxine-binding globulin, which is a carrier protein responsible for carrying the thyroid hormones in the blood. Tg is secreted and stored in the follicular lumen.