SLC16A2 and thyrotoxicosis: MCT8 deficiency (OMIM 300523: Allan–Herndon–Dudley syndrome or MCT8 deficiency; ORPHA:59) is characterized by a varying neurodevelopmental delay due to cerebral hypothyroidism, and a wide range of clinical sequelae secondary to chronic peripheral tissue thyrotoxicosis caused by elevated serum T3 concentrations (104) (summarized in Table 4).