RESUMO
Allan-Herndon-Dudley syndrome is a rare X-linked genetic disorder, caused by a deficiency of the monocarboxylate transporter 8 (MCT8), a specific transporter of thyroid hormones, with functions mainly at the brain level. The syndrome produces an early onset of severe neurological disorder, in which hypotonia predominates. OBJECTIVE: To present a rare case with an unexpected diagnosis, highlighting the usefulness of requesting a complete thyroid profile in every hypotonic male infant without a specific cause. CLINICAL CASE: A 10-month-old male infant with severe axial and peripheral hypotonia, global weakness with little spontaneous mobility, without head support or stable sitting. Complete metabolic and peripheral neurophysiological studies were performed. Genetic studies for spinal muscular atrophy, Prader Willi syndrome, and myotonic dystrophy were also performed. The trio exome analysis detected a probably pathogenic variant c.359C>T;p.(Ser120Phe), hemizygous in exon 1 of the SLC16A2 gene, inherited from the mother. Thyroid abnormalities as increased free triiodothyronine (T3) and thyroid-stimulating hormone (TSH), and delayed myelination were ob served. CONCLUSIONS: MCT8 deficiency should be considered in the case of the male infant with unex plained hypotonia and weakness without a determined cause. The diagnosis is guided by a thyroid profile including free T3 hormone, because it presents a characteristic thyroid profile with decreased free thyroxine (T4), increased free T3, and normal or slightly elevated TSH levels. In this case, the implementation of the trio exome analysis allows establishing an early certain diagnosis.