Celik, NurullahDemir, KorcanDibeklioglu, Saime ErgenDundar, Bumin NuriHatipoglu, NihalMutlu, Gul YesiltepeArslan, Emrullah2025-05-042025-05-0420240340-61991432-1076https://doi.org/10.1007/s00431-024-05931-7https://hdl.handle.net/20.500.12418/35623Allan- Herndon-Dudley syndrome is a neurodevelopmental disorder characterized by motor and intellectual disabilities. Despite its rarity, there has been a rise in interest due to ongoing research and emerging therapy suggestions. In this multicenter, retrospective, cross-sectional study, the genetic characteristics and clinical data of twenty-one cases of genetically confirmed MCT8 deficiency were evaluated. The median age at the diagnosis was 2.4 ( 1.29; 5.9) years, which ranged from 0.5 to 14.0 years. The median follow-up period was 2.34 years, ranging from four months to 7.9 years. In 21 patients, 17 different variants were detected in the SLC16A2 gene. Eleven of these variants (c. 1456delC, c.439G > T, c.949C > A, c.1392dupC, c.1612C > T, c. 407dup, c. 781del, c.589C > A, c.712G > A, c. 311 T > A, c.1461del) have not been previously reported. In this study, with the exception of three cases with fT3/fT4 ratios of 4.95, 3.58, and 4.52, all cases exhibited fT3/fT4 ratios higher than five (9.9 (7.9; 12.0)). Conclusion: MCT8 deficiency is a rare and devastating disorder characterized by central hypothyroidism and peripheral thyrotoxicosis. The fT3/fT4 ratio can be used as a useful diagnostic indicator of MCT8 deficiency in males with mental and motor retardation. There is a need to raise clinicians' awareness of this potentially treatable condition with the emergence of new and promising treatments.en10.1007/s00431-024-05931-7info:eu-repo/semantics/closedAccessMCT8 deficiencyAllan-Herndon-Dudley syndromeThyroid diseaseThyroid hormone transportArticle1841396995932-s2.0-85212784310Q1WOS:001445069500001Q1