Thyroglobulin (Tg) is a large, iodinated glycoprotein that serves as the precursor for the thyroid hormones thyroxine (T4) and triiodothyronine (T3). It is exclusively synthesized in the thyroid follicular cells and stored in the colloid of the thyroid follicles. Thyroglobulin contains numerous tyrosine residues that are iodinated and coupled to form T4 and T3. The hormones are then released into the bloodstream through a process involving endocytosis of thyroglobulin and its proteolysis within the lysosomes of thyroid cells. Tg is a significant biomarker for thyroid status and is used clinically to monitor patients with thyroid cancer. A subsequent elevation of the thyroglobulin level is an indication of recurrence of papillary or follicular thyroid carcinoma. In other words, a rise in thyroglobulin levels in the blood may be a sign that thyroid cancer cells are growing and/or the cancer is spreading. Hence, thyroglobulin levels in the blood are mainly used as a tumor marker for certain kinds of thyroid cancer (particularly papillary or follicular thyroid cancer). Thyroglobulin is not produced by medullary or anaplastic thyroid carcinoma. Thyroglobulin levels are tested via a simple blood test. Tests are often ordered after thyroid cancer treatment.