Monocarboxylate transporter 8 (MCT8) deficiency is an X-linked disorder resulting from an impairment of the transcellular transportation of thyroid hormones. Within the central nervous system thyroid hormone transport is normally mediated by MCT8. Patients are described as affected by a static or slowly progressive clinical picture which consists of variable degrees of mental retardation, hypotonia, spasticity, ataxia and involuntary movements, occasionally paroxysmal. The authors describe the clinical and neuroradiological picture of 3 males patients with marked delayed brain myelination and in which the clinical picture was dominated by early onset nonparoxysmal extrapyramidal symptoms. In one subject a novel mutation is described.