Thyroglobulin (Tg) is a 660 kDa, dimeric protein produced by and used entirely within the thyroid gland. In earlier literature, Tg was referred to as "colloid".Thyroglobulin should not be confused with Thyroxine-binding globulin, a carrier protein responsible for carrying the thyroid hormones in the blood.Tg is used by the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). The active form of triiodothyronine, 3, 5, 3' triiodothyronine, is produced both within the thyroid gland and periphery by 5'-deiodinase (which has been referred to as tetraiodothyronine 5' deiodinase.)Thyroglobulin levels in the blood can be used as a tumor marker for certain kinds of thyroid cancer. Thyroglobulin levels in the blood can also be elevated in cases of Graves' disease.