Thyroglobulin (TGB) is a glycoprotein, synthesized in the thyroid gland, which is a precursor of the thyroid hormones thyroxine and triiodothyronine. Thyroglobulin consists of about 5,000 amino acids, some of which are tyrosine residues. Iodine binds to these residues in thyroglobin, which is then hydrolyzed into iodotyrosines that combine to form thyroxine or triiodothyronine.