Padeira, GonçaloJacinto, SandraVenâncio, MargaridaMarcão, AnaConceição, CarlaFerreira, Ana Cristina2026-02-102026-02-102025-03-27http://hdl.handle.net/10400.18/10879X-linked intellectual disability (XLID) is characterized by extensive genetic heterogeneity. Next-generation sequencing (NGS) have been used in these cases as a cost-effective diagnosis approach. Genetic findings often reveal variants unforeseen during clinical investigation, prompting the need for reevaluation of specific features designated as reverse phenotyping (RP). X-linked creatine transporter deficiency (CTD) is a potentially treatable intellectual disability caused by pathogenic variants in the SLC6A8 gene leading to impaired creatine transport into the brain. A 7-year-old boy with intellectual disability, speech delay, hyperactivity and epilepsy was referred to Metabolic and Neuropediatric Clinic. Family history identified a mother with learning difficulties and a maternal uncle with intellectual disability, indicating a possible X-linked inheritance. NGS intellectual disability panel identified a variant classified as probably pathogenic (c.880_881del (p(Lys294Alafs*2)) in the SLC6A8 gene, in hemizygosity which prompted referral to Metabolic and Neuropediatric Clinic. Reverse phenotyping was carried out with biochemical and imaging assessment that showed: high urinary Creatine-Creatinine ratio (2.17; RV 0.04-1.07) with normal guanidinoacetate acid and absence of creatine peak in brain MRI spectroscopy, confirming the diagnosis. Genetic studies on female family members are ongoing. He started treatment with creatine, arginine and glycine in the last appointment. CTD is a rare disease that has been reported in more than 150 individuals worldwide. We present a case in which the diagnostic approach was reverse phenotyping, through biochemical and imaging studies, after the identification of pathogenic variants in SLC6A8 by NGS panel. The efficacy of its treatment remains controversial with variable results, and a close evaluation will be needed.engX-Linked Intellectual Disability (XLID)X-linked Creatine Transporter Deficiency (CTD)Creatine TransporterDoenças GenéticasTransportador da Creatinaconference object