Difficult diagnosis in a 17-year-old patient: Type 1 diabetes? Type 2 diabetes? Or "double diabetes"?

Medical history and clinical findings: We report on a 17-year-old boy with elevated blood glucose levels, elevated liver enzymes and obesity (BMI 32.3 kg/m(2)). Clinical examination showed acanthosis nigricans and a vitiligo. The rest of the physical examination was without pathological findings. Investigations: The HbA(1c) value was 8.6 % (71 mmol/mol), and postprandial C-peptide showed a maximum level of 1.3 nmol/l. The type 1 diabetes-associated autoantibodies against protein tyrosine phosphatase IA-2 and zinc-transporter-8 were positive, while autoantibodies to glutamic acid decarboxylase and insulin were negative. There was no ketonuria. Ultrasound showed steatohepatitis. Treatment and course: Under therapy with metformin up to 2x1 g, blood glucose levels and liver enzymes normalized after a few weeks. After two months, the HbA1c value was 6.0 % (42.1 mmol/mol), and a weight loss of 5 kg was recorded. Conclusion: In obese adolescent patients with diabetes, a clear classification right from the beginning is not always possible. Characteristic findings of type 1 and type 2 diabetes may be present simultaneously. In the presented patient, monotherapy with metformin was sufficient in the first year. Close monitoring