The NADPH oxidases, DUOX1 and DUOX2, are critical for the production of extracellular hydrogen peroxide that is required for thyroperoxidase-mediated thyroid hormone synthesis in the thyroid gland; as a result, mutations in DUOX2 and/or DUOXA2 result in thyroid dyshormonogenesis and congenital hypothyroidism (De Deken and Miot 2019). Here, FMO5 is linked to congenital hypothyroidism.