– Thyroid peroxidase, also called thyroperoxidase (TPO) or iodide peroxidase that encodes a 933 amino-acid residue (aa) molecule with a single membrane-spanning region. Thyroid peroxidase plays a key role in thyroid hormone synthesis by catalyzing both the iodination of tyrosine residues to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues and the coupling of iodotyrosine residues in Tg, resulting in the formation of T3 and T4. It is a frequent epitope of autoantibodies in autoimmune thyroid disease, for example, the expression of thyroid peroxidase is lost in papillary thyroid carcinoma.