TPO is an enzyme expressed mainly in the thyroid where it is secreted into colloid. It is involved in the production of thyroxine (T4) or triiodothyronine (T3), the thyroid hormones. TPO is stimulated by TSH, which upregulates gene expression, and is inhibited by the thioamide drugs. In diagnostic IHC, the expression of thyroid peroxidase is lost in papillary thyroid carcinoma.