SLC16A2 and Allan-Herndon-Dudley syndrome: Human MTC8 transporter mediates transport of thyroid hormones and the importance of transport for thyroid hormone signaling was revealed by the discovery that inactivating mutations in the human monocarboxylate transporter-8 (MCT8) cause Allan-Herndon-Dudley syndrome, an X-linked developmental disorder characterized by hypotonia, spasticity, muscle weakness, neurological problems, and cognitive impairment due to thyroid hormone deficiency in the CNS [127].