Novel MCT8 mutation: diagnostic value of T3/T4 ratio

Objectives To highlight the diagnostic value of the T3/T4 ratio in Allan–Herndon–Dudley syndrome (AHDS) through a case report of a novel SLC16A2 mutation. Case presentation We report a 36-month-old boy with severe neurodevelopmental delay and axial hypotonia. Initial thyroid function tests at 10 months showed TSH at 4.77 μIU/mL and T3 at 8.9 pmol/L. Brain MRI was normal. At 28 months, genetic analysis identified a novel hemizygous c.1343_1344dup mutation in the SLC16A2 gene. Follow-up thyroid profiling at 36 months revealed the characteristic AHDS pattern: elevated free T3 (10.20 pmol/L), low free T4 (7.80 pmol/L), and borderline high TSH (5.20 μIU/mL), with a T3/T4 ratio of 1.31 pmol/pmol. Conclusions 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.