Novel MCT8 mutation: diagnostic value of T3/T4 ratio

This case highlights the diagnostic value of the T3/T4 ratio (>0.75 pmol/pmol) as an essential biochemical marker of AHDS in any male infant presenting with unexplained developmental delay and hypotonia. A systematic diagnostic approach including early T3 measurement and T3/T4 ratio calculation should be applied in the initial evaluation of severe developmental delays, even in the presence of normal brain MRI findings, to avoid diagnostic delays in AHDS.