TPO and congenital hypothyroidism: From a clinical genetic standpoint, distinction between the two is important because goitrous congenital hypothyroidism is mostly due to autosomal recessive Mendelian diseases such as the thyroglobulin defect3, the thyroid peroxidase defect4 and dual oxidase 2 defect5, whereas only small fraction (<10%) of nongoitrous congenital hypothyroidism is explained by Mendelian diseases, including the thyroid-stimulating hormone (TSH) receptor defect and the PAX8 defect6,7.