Thyroglobulin (Tg) is a 660 kDa, dimeric protein produced by and used entirely within the thyroid gland. Tg is used by the thyroid gland to produce the thyroid hormones thyroxine (T4) and triiodothyronine (T3). The active form of thyroxine, triiodothyronine, is produced both within the thyroid gland and on the periphery by 5’-deiodinase, which has been referred to as Tetraiodothyronine-5-deiodinase.