{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"Thyroid peroxidase (TPO) is an enzyme expressed mainly in the apical plasma membrane of thyroid epithelial cells. Thyroid peroxidase catalyzes two reactions: 1) iodination of tyrosine on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT); 2) coupling of MIT and DIT to form triiodothyronine (T3) and coupling of 2 molecules of DIT to form thyroxine (T4). Thyroid peroxidase gene expression is under the regulation of thyroid stimulating hormone (TSH). In normal thyroid, expression of TPO described immunohistochemically is reported to produce a diffuse, fine, granular cytoplasmic stain in all follicular cells. The expression level in thyroid carcinomas is lower than that of normal and benign thyroid tumors. Decreased TPO expression is an early event in follicular tumorigenesis, taking place before development of invasiveness and in parallel with an acceleration of cell growth and appearance of cell atypia."}]}]}