Thyroid hormone is essential for proper brain development and function. As a prerequisite for its action, transporters must exist to mediate its cellular entry. As impaired uptake of thyroid hormone into the CNS causes severe neurological symptoms, it is of utmost importance to identify these carriers. The monocarboxylate transporter 8 (MCT8) was recently characterized as a very specific thyroid hormone transporter. Inactivating mutations in the MCT8 gene are associated with a severe syndrome of psychomotor retardation and abnormal thyroid hormone parameters. To elucidate the underlying pathogenic mechanisms, MCT8-deficient mice that replicate the human thyroid phenotype, despite the absence of overt neurological symptoms, have been generated. Here, we summarize recent findings obtained by analyzing these animals and discuss their potential impact for the treatment of affected patients.