Approximately 10% of patients with congenital hypothyroidism harbor inborn errors of metabolism in one of the steps for thyroid hormone synthesis in thyrocytes. The most prevalent cause of thyroid dyshormonogenesis is TPO deficiency. Defects in TPO cause a severe form of congenital hypothyroidism characterized by a complete and immediate release of accumulated radioiodide from the thyroid after sodium perchlorate administration. This release of radioiodide represents total iodine organification defect (TIOD), a disruption of the process by which iodide present in the thyroid is oxidized by hydrogen peroxide and bound to tyrosine residues in thyroglobulin to form iodotyrosine. [From OMIM]