Thyroid hormones are essential for brain development. The active thyroid hormone, T3, binds to several products of two genes, the nuclear thyroid hormone receptors alpha and beta, and thus regulates gene expression. Mutations in a thyroid hormone transmembrane transport protein, monocarboxylate transporter 8 (MCT8), underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome. This discovery sparked great interest in the process of thyroid hormone ...