The Anti-Thyroglobulin (ATG) test measures the level of antibodies against thyroglobulin, a protein produced by the thyroid gland. Elevated levels of these antibodies can be indicative of autoimmune thyroid diseases, such as Hashimoto’s thyroiditis or Graves’ disease. This test is often used to help diagnose thyroid disorders, especially when other thyroid function tests are inconclusive. It can also be used to monitor patients with thyroid cancer or those who have had thyroid surgery. A blood sample is required for this test.