Anti-thyroid peroxidase antibodies (Anti-TPO) are the most common thyroid autoantibodies, found in approximately 90–95% of people with Hashimoto's thyroiditis and 70–80% of those with Graves' disease. Thyroid peroxidase is an enzyme essential for the production of thyroid hormones, and antibodies against it can progressively damage thyroid tissue over time.

Anti-TPO antibodies can be elevated for years or even decades before TSH becomes abnormal, making them a valuable early marker of autoimmune thyroid disease. People with elevated Anti-TPO who have normal thyroid function are at increased risk of developing overt hypothyroidism over time — particularly during pregnancy, when thyroid autoimmunity is associated with miscarriage, postpartum thyroiditis, and foetal complications.