Anti-Thyroid Peroxidase (Anti-TPO) antibodies are markers tested in the blood to detect autoimmune thyroid diseases. Thyroid peroxidase is an enzyme in the thyroid gland essential for the production of thyroid hormones. High Levels of Anti-TPO indicate the presence of antibodies that are attacking the thyroid. This is commonly seen in autoimmune thyroid disorders like Hashimoto’s thyroiditis (leading to hypothyroidism) and Graves’ disease (associated with hyperthyroidism).