AbstractDefective thyroid hormone transport due to deficiency in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) results in severe neurodevelopmental delay due to cerebral hypothyroidism and in clinical negative sequelae following a chronic thyrotoxic state in peripheral tissues. The life expectancy of patients with MCT8 deficiency is severely impaired. Increased mortality is associated with lack of head control and being underweight at young age. Treatment options are available to alleviate the thyrotoxic state; particularly, treatment with the thyroid hormone analogue triiodothyroacetic acid seems a promising therapy. This review provides an overview of key clinical features and treatment options available and under development for this rare disorder.