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Data on the familial occurrence of type 1 diabetes mellitus (T1DM) in Korean pediatric populations are limited. This study evaluated the clinical characteristics of children with T1DM according to family history and estimated the T1DM prevalence among relatives.

We conducted a multicenter retrospective cohort study including patients aged ≤18 years newly diagnosed with T1DM at 18 university-affiliated hospitals in Korea between 2010 and 2024. The index child was defined as the first sibling diagnosed with T1DM and categorized according to the presence of affected parents or siblings. Familial T1DM prevalence was calculated for siblings, first-degree relatives, and twin pairs.

Among 936 index children, 32 (3.4%) exhibited a T1DM family history. Compared with index children, subsequent-affected children presented with lower plasma glucose (300.0 mg/dL vs. 412.0 mg/dL, P=0.009) and glycosylated hemoglobin levels (10.4% vs. 12.6%, P<0.001), and a lower frequency of diabetic ketoacidosis (13.8% vs. 49.7%, P<0.001). Venous pH and serum bicarbonate levels were higher (7.4 vs. 7.3, P=0.005; 22.0 mmol/L vs. 17.0 mmol/L, P=0.004, respectively), whereas urine ketone levels were significantly lower (P<0.001). Sibling, first-degree relative, and twin-pair prevalence rates were 3.0% (23/779), 1.3% (34/2,651), and 42.9% (3/7), respectively.

In this multicenter Korean cohort, familial T1DM accounted for 3.4% of pediatric cases, which was lower than in Western populations. Subsequent-affected children exhibited milder metabolic decompensation at diagnosis than did index children, likely reflecting earlier recognition through family awareness and screening. These findings underscore the importance of early education and monitoring of at-risk relatives within affected families.