One highly specific transporter is monocarboxylate transporter 8 (MCT8); its inactivating mutations could be the cause of diseases characterized by local TH shortage, such as Allan-Herndon-Dudley syndrome, a disorder characterized by normal TSH level and elevated T3 and decreased T4 serum levels [7]. The gene discussed is SLC16A2; the disease is Allan-Herndon-Dudley syndrome.