A disappearing diabetes: extreme and reversible hyperglycemia during an ulcerative colitis flare.

Stress hyperglycemia commonly occurs during acute illness; however, extreme hyperglycemia accompanied by markedly elevated glycated hemoglobin (HbA1c) in individuals without pre-existing diabetes is rare and may mimic new-onset diabetes. We describe a 56-year-old man with no prior history of diabetes who presented with severe diarrhea, abdominal pain, and weight loss and was found to have profound hyperglycemia, 603 mg/dL (SI: 33.5 mmol/L) (reference range, 70-99 mg/dL [SI: 3.9-5.5 mmol/L]), without metabolic acidosis. HbA1c was markedly elevated at 9.4% (reference range, 4.0%-5.7%), despite a normal value documented 2 months earlier. Evaluation demonstrated preserved endogenous insulin secretion, elevated homeostatic model assessment of insulin resistance, and negative autoimmune markers. Imaging excluded pancreatic pathology, while colonoscopy and histopathology confirmed active moderate ulcerative colitis (UC). The patient was treated with intravenous corticosteroids and insulin therapy, with rapid normalization of glycemia allowing discontinuation of insulin. Sustained euglycemia was observed after remission, with HbA1c normalizing to 4.7% at 3-month follow-up. This case highlights that severe inflammation during an UC flare can induce profound yet reversible hyperglycemia, emphasizing the need for careful interpretation and longitudinal follow-up.