Thyroid peroxidase (TPO) is a thyroid membrane-bound glycoprotein that functions in the iodination of tyrosine residues in thyroglobulin and pehnoxy-ester formation between iodinated tyrosines to generate thyroxine and triiodothyronine. Mutations in this gene are associated with autoimmune hypothyroidism, congenital goiter, and thyroid hormone organification defect IIA. The TPO antibody test is used to detect antibodies against TPO in patients with thyroid autoimmune disorders such as Graves or Hashimoto’s disease. TPO is expressed in thyroid follicular epithelium. References: Endocr Rev 1992 13(2):192; ISRN Endocrinol 2013:509746