Afterwards, in 2004, the first mutations in MCT8 gene (SLC16A2) were discovered by two distinct investigation groups (Dumitrescu et al. and Friesema et al.), and neurological findings of AHDS were explained by the resistance of T3 on entering the neuronal target cells, that leads to a classical thyroid profile found in all affected patients [8, 9]. Here, SLC16A2 is linked to Allan-Herndon-Dudley syndrome.