<i>Background:</i> Cellular uptake and release of thyroid hormone are mediated by transporters. Among these, monocarboxylate transporter 8 (MCT8) shows particularly high activity towards the active thyroid hormone 3,3′,5-triiodothyronine (T₃). MCT8 is expressed in different tissues, including the brain where it is predominantly localized in neurons. The <i>MCT8</i> gene is located on the X chromosome. <i>Conclusions:</i> Mutations in <i>MCT8</i> have been identified in boys with severe psychomotor retardation who also have very high serum T₃ levels. Thyroid hormone is crucial for brain development, and mutations in <i>MCT8</i> prevent the access of T₃ to its main target cells. Thus, mutations in <i>MCT8</i> represent a novel mechanism for the pathogenesis of thyroid hormone resistance.