In the last few years, many studies have pinpointed the crucial role of thyroid hormone (TH) transporters for TH action in human target cells. The importance was better documented by the phenotype observed in patients harboring mutations of the monocarboxylate transporter 8 (MCT8) gene immediately linked to Allan-Herndon-Dudley syndrome, in which severe neurological findings are associated with abnormal TH levels. The hereditary pattern of MCT8 mutations is X chromosome linked, with males presenting a homogeneous neurological psychomotor phenotype and mental retardation associated with low serum thyroxine and elevated triiodothyronine levels. The mechanism of disease is still obscure, and the physiopathology as well as the existent therapeutic options need to be discussed in order to improve the clinical management.