The Anti TPO Test, also known as the Anti-Thyroid Peroxidase Antibody Test, is a blood test used to detect antibodies against thyroid peroxidase, an enzyme in the thyroid gland essential for the production of thyroid hormones. This test helps in diagnosing autoimmune thyroid diseases such as Hashimoto's thyroiditis and Graves' disease, indicating an immune response that may lead to thyroid dysfunction.