Acute-onset autoimmune type 1 diabetes presenting with diabetic ketoacidosis in an obese adult.

Autoimmune type 1 diabetes mellitus (T1DM) has traditionally been associated with early onset disease and a lean phenotype; however, adult-onset disease increasingly presents with heterogeneous features that overlap with type 2 diabetes. We describe a 51-year-old obese man presenting with severe diabetic ketoacidosis (DKA). Laboratory evaluation demonstrated marked hyperglycemia (1297 mg/dL), metabolic acidosis (bicarbonate 7 mmol/L, anion gap 42), β-hydroxybutyrate >9 mmol/L, and hemoglobin A1c 13.3%. Autoimmune testing revealed strong glutamic acid decarboxylase antibody positivity with very low C-peptide, confirming profound insulin deficiency despite obesity. The patient improved with standard DKA management and was discharged on basal bolus insulin therapy. This case highlights the evolving phenotype of adult-onset autoimmune diabetes and emphasizes that obesity does not exclude T1DM. Early recognition using antibody testing and assessment of endogenous insulin secretion is essential to guide therapy and reduce recurrent metabolic decompensation.