Allan-Herndon-Dudley syndrome (AHDS), an X linked condition, is characterized by 
severe intellectual disability, dysarthria, athetoid movements, muscle 
hypoplasia and spastic paraplegia in combination with altered TH levels, in 
particular, high serum T3 levels. Mutations in the MCT8 gene coding for the 
monocarboxylate thyroid hormone transporter 8 have been associated with AHDS. 
Here we describe a family with the presence of a MCT8 gene mutation, p.A224T, in 
three consecutive generations. In two generations its presence was detected in 
the hemizygous state in two males with neurological abnormalities including 
mental retardation, axial hypotonia, hypertonia of arms and legs and athetoid 
movements. One of them presented normal thyroid hormone levels. Mutation was 
also detected, although in the heterozygous state, in three females showing 
thyroid hormone levels in the normal range. Our results show the difficulty of 
distinguishing AHDS from patients with X-linked intellectual disability solely 
on the basis of clinical features and biochemical tests, and we advise screening 
for MCT8 mutations in either young or older patients with severe intellectual 
disability, axial hypotonia/dystonia, poor head control, spastic paraplegia, and 
athetoid movements even when they have normal thyroid hormone profiles.