Thyroid dyshormonogenesis is caused by mutations in genes encoding proteins involved in thyroid hormone synthesis: the SCL5A5/NIS (iodide transport defect; OMIM No. 274400); pendrin, SCL26A4/PDS (Pendred syndrome; OMIM No. 274600); thyroglobulin, TG (OMIM No. 274700); thyroid peroxidase, TPO (OMIM No. 274500); dual oxidase 2, DUOX2 (OMIM No. 607200); dual oxidase maturation factor 2, DUOX2A (OMIM No. 274900); or iodotyrosine deiodinase, IYD/DEHAL1 (OMIM No. 274800). The gene discussed is TPO; the disease is Pendred syndrome.