Thyroid‑specific peroxidase (TPO) is present on the microsomes of thyrocytes and is expressed at its apical cell surface. In synergy with thyroglobulin (Tg) this enzyme has an essential function in the iodination of L‑tyrosine and the chemical coupling of the resulting mono‑ and di‑iodotyrosine to form the thyroid hormones T4, T3, and rT3.
TPO is a potential autoantigen. Elevated serum titers of antibodies to TPO are found in several forms of thyroiditis caused by autoimmunity. The still frequently found term “microsomal antibody” originates from the time when TPO had not yet been identified as an antigen in autoimmunity caused by microsomes. In the clinical sense the two terms anti‑TPO and microsomal antibody can be used synonymously; there are differences, however, with regard to the test methods.
High anti‑TPO titers are found in up to 90 % of patients with chronic Hashimoto's thyroiditis. In Graves' disease, 70 % of the patients have an elevated titer. Although the sensitivity of the procedure can be increased by simultaneously determining other thyroid antibodies (anti‑Tg, TSH‑receptor‑antibody - TRAb), a negative finding does not rule out the possibility of an autoimmune disease. The magnitude of the antibody titer does not correlate with the clinical activity of the disease. Initially elevated titers can become negative after lengthy periods of illness or during remission. If antibodies reappear following remission, then a relapse is probable.