Thyroid peroxidase (TPO) is an important enzyme in the thyroid, a gland which plays a crucial role in the production of thyroid hormones. In autoimmune thyroid diseases, the immune system produces autoantibodies against TPO, which damages the thyroid and impairs its ability to produce hormones. Detecting anti-TPO antibodies in the blood is important because they are present in most patients with Hashimoto’s thyroiditis and Grave’s disease.