Insulinoma masquerading as factitious hypoglycemia

: A 36-year-old woman without significant medical history complained of "spells" of diplopia, fatigue, and dizziness. On formal fasting, her glucose dropped to 40 mg/dL, with simultaneous insulin levels of 15 microU/mL (normal <6 microU/mL) and C-peptide of 2.5 ng/ml (normal <2 ng/mL). An isolated plasma sulfonylurea screen done during the fast was positive for tolbutamide, suggesting the diagnosis of factitious hypoglycemia, but further workup revealed multiple pancreatic masses resulting in an eventual diagnosis of multiple insulinomas that was confirmed surgically. We discuss the approach to hypoglycemia caused by insulin excess and distinguishing clinical and biochemical features.