Type 1 diabetes mellitus (T1DM) typically appears during childhood or adolescence, yet autoimmune diabetes in adults is increasingly recognized. Its presentation may be subtle and overlap clinically with type 2 diabetes, creating diagnostic uncertainty in primary care. Early identification of autoimmune etiology is crucial to avoid delays in insulin initiation and prevent metabolic decompensation. This case illustrates the role of primary care in recognizing atypical presentations and guiding timely referral.A 30-year-old man with no significant past medical history attended a routine occupational medical examination where an incidental hyperglycemia (334 mg/dL) was detected. He reported several weeks of dry mouth and polydipsia, without polyphagia, weight loss or polyuria. He denied alcohol or tobacco use and had a family history of diabetes in a paternal uncle. He presented to the emergency department, where capillary glucose was elevated and simple ketosis was confirmed. Insulin therapy with Lantus 12 units nightly was initiated, achieving metabolic stabilization. Physical examination revealed a BMI of 26.5 kg/m², normal vital signs, appropriate hydration, and no signs of infection or acute illness. Laboratory tests showed normal renal function and electrolytes. Autoimmune markers performed subsequently demonstrated positivity for anti-GAD antibodies, confirming the autoimmune origin. During follow-up, the patient required basal-bolus therapy with rapid-acting insulin titrated to 3–4–4 units per meals. HbA1c at three months was 6.0%, with fewer than one weekly hypoglycemia episode. Education on carbohydrate counting and self-adjustment of insulin doses was provided.Adult-onset autoimmune diabetes remains underdiagnosed and is often mistaken for type 2 diabetes, delaying appropriate treatment. This case highlights an atypical presentation without ketoacidosis or significant symptoms despite severe hyperglycemia.Incidental hyperglycemia, even in asymptomatic adults, requires structured evaluation including autoimmune markers. Early insulin therapy prevents progression to ketoacidosis and reduces long-term complicationsFamily physicians often encounter ambiguous hyperglycemia. Understanding latent autoimmune diabetes in adults facilitates earlier referral, personalized treatment and prevention of misclassification.Adult-onset T1DM should be considered in lean, asymptomatic adults presenting with unexplained hyperglycemia. Comprehensive assessment and coordinated care between primary and specialized services optimize outcomes.