Thyroid peroxidase

Thyroid peroxidase or Thyroperoxidase (TPO) is an enzyme mainly expressed in the thyroid that liberates iodine for addition onto tyrosine residues on thyroglobulin for the production of thyroxine (T4) or triiodothyronine (T3) (thyroid hormones).

Inorganic iodine enters the body primarily as iodide, I-. After entering the thyroid follicle(or thyroid follicular cell) via a Na+/I- symporter (NIS) on the basolateral side, iodide is shuttled across the apical membrane into the colloid via pendrin, after which thyroid peroxidase oxidizes iodide to atomic iodine (I) or iodinium (I+). The "organification of iodine," the incorporation of iodine to thyroglobulin for the production of thyroid hormone is inseparable from oxidation and is catalyzed by TPO. The chemical reactions catalyzed by thyroid peroxidase occur on the outer apical membrane surface and are mediated by hydrogen peroxide.

Stimulation and inhibition
TPO is stimulated by TSH, which upregulates gene expression. It is inhibited by the thioamide drugs, such as propylthiouracil and methimazole.

Clinical significance
It is a frequent epitope of autoantibodies in autoimmune thyroid disease so the antibody titer can be used to assess disease activity in patients who have developed such antibodies.