Familial Co-Aggregation of Neurodevelopmental Conditions and Childhood-Onset Type 1 Diabetes.

AIM: Childhood-onset Type 1 diabetes (T1D) is associated with neurodevelopmental conditions (NDCs). We examined whether this association extends to relatives and assessed phenotypic, genetic, and environmental correlations between T1D and NDCs. METHODS: Using Swedish registers, we identified 4 066 634 individuals born 1973-2015 and linked them to siblings and cousins; 23 212 (0.57%) had T1D before 18 years. Logistic regression estimated odds of receiving a diagnosis of any and each of the examined NDCs among individuals with T1D and their relatives. Bivariate quantitative genetic models estimated phenotypic, additive genetic, shared environmental, and non-shared environmental correlations. Analyses were conducted in April 2025. RESULTS: NDCs were more prevalent among individuals with T1D than in those without (10.3% vs. 6.4%). T1D was associated with higher odds of receiving a diagnosis of any NDCs (OR 1.43; 95% CI 1.37-1.49). Full siblings also had higher odds (OR 1.12; 95% CI 1.06-1.18), whereas no significant associations were observed among other relatives. Cross-trait phenotypic correlations were small (0.06-0.08). CONCLUSION: Individuals with childhood-onset T1D and their full siblings had higher odds of recorded NDC diagnoses. Shared familial factors appeared to contribute only modestly, while differential ascertainment and family-level consequences of T1D may also play a role. These findings support access to neurodevelopmental expertise in paediatric diabetes services.