Beneficial Effects of Propylthiouracil plus l-Thyroxine Treatment in a Patient with a Mutation in MCT8

CONTEXT: Mutations of the monocarboxylate transporter 8 (MCT8) gene determine a distinct X-linked phenotype of severe psychomotor retardation and consistently elevated T(3) levels. Lack of MCT8 transport of T(3) in neurons could explain the neurological phenotype. OBJECTIVE: Our objective was to determine whether the high T(3) levels could also contribute to some critical features observed in these patients.