To delineate the clinical and genetic characteristics of patients with Allan-Herndon-Dudley syndrome (AHDS).

Genetic testing was carried out by next generation sequencing on 117 patients featuring intellectual disability and developmental delay. Clinical information including clinical manifestation, brain magnetic resonance imaging(MRI), thyroid hormone levels, and electrocardiogram was collected for those with SLC16A2 mutations.

Five male patients with SLC16A2 gene mutations were identified, including 2 affected brothers and 3 sporadic cases. The ages of the patients ranged from 8 months to 8 years. All patients presented with severe intellectual disability and developmental delay including poor head control, inability to sit independently, no speech, and poor response to external stimuli. All patients presented with hypotonia, dystonia, and positive pyramidal signs. Three patients had sinus tachycardia. All patients had abnormal thyroid hormone levels with elevated free triiodothyronine (FT₃), decreased free tetraiodothyronine(FT₄), and normal thyroid stimulating hormone (TSH). Brain MRI on 3 patients showed delayed myelination. Among the 3 sporadic patients, 2 carried de novo mutations including c. 61G>T(p.E21X) and c. 695_699delATGGT(p.N232SfsX7), respectively, 1 carried a c. 42delC(p.W15GfsX69)mutation, which was inherited from his heterozygous mother. A nonsense mutation (c.916C>T, p. Q306X) was discovered in the two brothers, for which their mother was heterozygous.

AHDS is characterized by severe psychomotor developmental delay as well as congenital hypotonia, dystonia and positive pyramidal signs. Affected males may present with distinctive thyroid hormone abnormalities including increased FT₃ and low FT₄ accompanied by normal TSH. Delayed meylination of white matter is common. It is an X-linked mental retardation caused by SLC16A2 gene mutations.

Allan-Herndon-Dudley syndrome; Thyroid hormone; X-linked mental retardation; SLC16A2gene