X-linked MCT8 mutations cause Allan-Herndon-Dudley syndrome (AHDS) characterized by severe developmental delay and specific thyroid function abnormality. The report describes a 1 year-old boy with severe developmental delay, truncal hypotonia, quadriparesis (spastic), dystonia, and thyroid function abnormality (high free T3, low reverse T3, low free T4, and normal TSH) suggesting a form of impaired thyroid hormone sensitivity, namely, AHDS. WES analysis yielded novel hemizygous single-base pair duplication in exon 1 of MCT8 gene, validated by Sanger sequencing. MRI revealed generalized demyelination with poor gray and white matter differentiation and loss of brain volume. Indirect markers revealed thyrotoxicosis in some peripheral tissues. Having high index of suspicion for this rare disorder and plausible benefit of inclusion of free T3 assay in initial evaluation of infants and children with idiopathic developmental delay are emphasized. Amelioration of thyrotoxic features with tri-iodothyroacetic acid likely benefits patients at all ages. Neurodevelopmental improvement has been observed in those who received tri-iodoacetic acid early, which is being currently investigated.