The Anti-Thyroglobulin Antibody (ATG) Test measures autoantibodies against thyroglobulin, a thyroid-produced protein. Elevated ATG levels indicate an autoimmune attack on the thyroid, often found in Hashimoto's thyroiditis and Graves' disease. The test is crucial for diagnosing autoimmune thyroiditis, distinguishing it from other thyroid conditions, and is especially significant in thyroid cancer cases. Elevated ATG in cancer follow-up can suggest recurrence or metastasis, affecting thyroglobulin (Tg) reliability as a tumor marker. Thus, ATG testing plays a key role in diagnosing autoimmune thyroid disease and monitoring thyroid cancer.