<h4>Background</h4>Allan-Herndon-Dudley syndrome (AHDS)/monocarboxylate transporter 8 (MCT8) deficiency is a rare X-linked encephalopathy caused by SLC16A2 variants, impairing thyroid hormone (TH) transport into the brain. The gene discussed is SLC16A2; the disease is Allan-Herndon-Dudley syndrome.