Thyroglobulin (Tg) serves as the building block for the iodinated thyroid hormones thyroxine (T4) and triiodothyronine (T3).1 Tg is a large glycoprotein located in healthy thyroid follicular cells. Thyroglobulin is beneficial for detecting papillary and follicular thyroid cancer and for recognizing thyroid-origin tumors when dealing with adenocarcinoma of an unknown source.