Thyroid peroxidase (TPO) is an enzyme normally found in the thyroid gland. TPO plays an important role in the production of thyroid hormones. A TPO test detects antibodies against TPO in the blood. If you've been diagnosed with thyroid disease, your doctor may suggest a TPO antibody test and other thyroid tests to help find the cause.

The presence of TPO antibodies in your blood suggests that the cause of thyroid disease is an autoimmune disorder, such as Hashimoto's disease or Graves' disease. In autoimmune disorders, your immune system makes antibodies that mistakenly attack normal tissue. Antibodies that attack the thyroid gland cause swelling, rarely tenderness and reduced function of the thyroid.