High thyroid peroxidase is most often caused by damage to the thyroid gland due to thyroid immunity. Thyroid peroxidase antibodies are antibodies to the thyroid gland itself, a key enzyme in the synthesis of thyroid hormones, and are associated with immune damage to thyroid tissue. High thyroid peroxidase is commonly seen in hyperthyroidism, toxic diffuse goiter, Hashimoto’s thyroiditis, and hypothyroidism. 1. If high thyroid peroxidase antibodies are caused by Hashimoto’s thyroiditis, the condition mostly indicates the presence of chronic lymphocytic thyroiditis in the body. 2. High thyroid peroxidase, when combined with thyroid stimulating hormone values, can lead to early detection of hypothyroidism. 3. Toxic diffuse goiter may also present with high thyroid peroxidase. 4. Further tests are needed to rule out thyroid abnormalities caused by other diseases. This may also cause high thyroid peroxidase. 5. If there is a single high thyroid peroxidase, there is usually no diagnostic significance. Further tests such as thyroglobulin antibodies, antithyroid antibodies, thyroid ultrasound, and thyroid function are needed. A high thyroid peroxidase requires further tests to determine the cause and then treat it symptomatically.