This study aimed to investigate the clinical characteristics, glycemic control, and microvascular complications compared between young-onset type 1 (T1DM) and type 2 diabetes (T2DM) patients at Siriraj Hospital. Patients and Methods: We collected demographic, clinical, glycemic control, and microvascular complication data of young-onset (onset <30 years of age) T1DM and T2DM patients at our center using February 2019-December 2020 data from the Thai Type 1 Diabetes and Diabetes diagnosed Age before 30 years Registry, Care and Network (T1DDAR CN). Results: Of 396 patients, 76% had T1DM and 24% had T2DM. At diagnosis, T1DM were significantly younger (9.7±5.4 vs 16.9±6.4 years, p<0.001), had a lower body mass index (17.2±4.1 vs 30.8±7.9 kg/m 2 , p<0.001), higher prevalence of diabetic ketoacidosis (DKA) (66.1% vs 13.7%, p<0.001), and higher HbA 1c level (12.8±2.6% vs 10.9±3.1%, p=0.002) compared to T2DM. Regarding glycemic control, the mean HbA 1c at registry enrollment did not differ between groups (T1DM 8.3±1.8% vs T2DM 8.1±2.2%, p=0.303), but T1DM achieved HbA 1c <7% significantly less than T2DM (19.3% vs 47.8%, p<0.001). T1DM showed deterioration of glycemic control during 10-20 years of age, and gradually improved during 20-30 years of age, whereas patients with T2DM showed progressive worsening of glycemic control over time. Concerning microvascular complications, the prevalence of diabetic retinopathy (10.6% vs 9%, p=0.92) and diabetic neuropathy (3.4% vs 5.5%, p=0.514) between T1DM and T2DM was not significantly different. However, T2DM had a significantly higher prevalence of diabetic nephropathy (T1DM 10.1% vs T2DM 40.2%, p<0.001) that developed within a significantly shorter duration of diabetes (T1DM 11.0±6.8 vs T2DM 4.3±5.1 years, p<0.001) compared to T1DM. Conclusion: T1DM had a significantly high prevalence of DKA at presentation, and most T1DM did not achieve the glycemic target, especially during adolescence. T2DM had a significantly higher prevalence of diabetic nephropathy that developed within a shorter duration of diabetes compared to T1DM.
