Allan-Herndon-Dudley Syndrome

X-linked MCT 8 mutations cause Allan-Herndon-Dudley syndrome (AHDS), characterized by severe developmental delay and specific thyroid function abnormality. The report describes a 2-y-old boy who presented with severe developmental delay, generalized hypotonia and thyroid function abnormality (high FT3, low FT4 and normal TSH) suggesting a form of impaired thyroid hormone sensitivity. Whole exome sequencing (WES) analysis revealed mutation in exon 3 of MCT 8 gene. Improvement of neurodevelopmental delay and clinical and biochemical features of thyrotoxicosis occurs with use of tri iodoacetic acid.