Monocarboxylate transporter 8 (MCT8) deficiency, also known as Allan-Herndon-Dudley syndrome, is a rare, severely debilitating, and life-limiting genetic disorder caused by variants in the SLC16A2 gene that render the MCT8 thyroid hormone transporter partially or completely dysfunctional. This evidence concerns the gene SLC16A2 and Allan-Herndon-Dudley syndrome.