Thyroglobulin is a large protein used by the thyroid gland to make thyroid hormones (T4 and T3) and store iodine in the body, made in the thyroid by the so-called follicular cells. When the thyroid is stimulated by thyroid-stimulating hormone (TSH), it combines iodine with thyroglobulin to create the hormones T4 and T3. While most thyroglobulin stays in the thyroid, a small amount leaks out into the bloodstream. Levels in the blood are directly proportional to the size of the thyroid. For example, thyroid cancer cells enlarge the gland and make thyroglobulin in high amounts. Any disorder that increases the size of the thyroid (cancer, autoimmune disease, nodules, etc.) can raise thyroglobulin levels.