Pheochromocytoma unmasked by rapid deterioration of previously stable type 2 diabetes.

A 46-year-old man with a 15-year history of type 2 diabetes had maintained hemoglobin A1c (HbA1c) around 6.5% (SI: 48 mmol/mol) (reference range, 4.7%-6.2% [28-44 mmol/mol]) for approximately 12 years on saxagliptin monotherapy, but glycemic control worsened over 1 year despite additional ipragliflozin and metformin. Abdominal computed tomography (CT), performed after health-screening urine dipstick protein positivity, detected a left adrenal mass. He had tachycardia and weight loss but no palpitations, headache, pallor, or diaphoresis. Biochemical tests showed marked catecholamine excess and elevated 24-hour urinary metanephrines, and iodine-123 metaiodobenzylguanidine (123I-MIBG) scintigraphy showed concordant uptake. Before surgery, fasting C-peptide was 0.64 ng/mL (SI: 0.21 nmol/L) (reference range, 0.80-2.50 ng/mL [SI: 0.26-0.83 nmol/L]), C-peptide index was 0.51, and homeostasis model assessment of beta-cell function (HOMA-beta) was 11.0. After alpha blockade and volume expansion, laparoscopic left adrenalectomy confirmed pheochromocytoma. Antihypertensive and glucose-lowering medications were discontinued postoperatively, and glycemic control remained stable without antidiabetic medication at 6 months. This case highlights pheochromocytoma as a reversible endocrine cause of rapid worsening of previously stable type 2 diabetes.