hyroid antibodies are proteins produced by the body's immune system that mistakenly attack the thyroid gland. Testing for these antibodies helps diagnose autoimmune thyroid diseases. The most common thyroid antibody tests are:   Anti-thyroid peroxidase (Anti-TPO) antibodies: These attack thyroid peroxidase, an enzyme important for thyroid hormone production.   Anti-thyroglobulin (Anti-Tg) antibodies: These attack thyroglobulin, a protein used to produce thyroid hormones.   Sometimes, both tests are done as a panel. Why are thyroid antibody tests done? Diagnose autoimmune thyroid diseases: Such as Hashimoto's thyroiditis (hypothyroidism) and Graves' disease (hyperthyroidism). Determine the cause of thyroid problems: If TSH, Free T3, and Free T4 levels are abnormal, antibody tests can help pinpoint the cause. Monitor autoimmune thyroid conditions: To track the progression of the disease.