Anti-TPO antibodies are antibodies that attack the thyroid peroxidase (TPO) enzyme, which produces thyroid hormones1. They are associated with Hashimoto's disease and are a sign that the immune system is mistakenly attacking the thyroid gland. The presence of anti-TPO antibodies in your bloodstream is an indication that you may have an autoimmune condition targeted to your thyroid gland. Elevated antibodies could be an early sign of developing autoimmune thyroid disease3. Anti-TPO antibodies should be investigated even if other thyroid biomarkers are normal or even optimal.