Diagnostic Dilemma in a Patient With Insulinoma

Abstract

B.Y. is a 70-year-old woman who was referred to the diabetes clinic by her primary care provider for work-up of hypoglycemia. She had known coronary artery disease and was status post-coronary artery bypass grafting. Her symptoms included fatigue and some depression. Initial laboratory testing revealed two blood glucose levels in the 30- to 40-mg/dl range. She was referred to our clinic for further evaluation. She was asked to come in fasting. History revealed that she did, indeed, have severe fatigue. She did not complain of syncope, palpitations, or diaphoresis. She had experienced two episodes of mild dizziness with standing, but each of these had resolved spontaneously and had been short-lived. Physical exam revealed an elevated blood pressure of 194/97 mmHg. Her weight was 245 lb and had not changed recently. Fasting serum glucose, insulin, proinsulin, C-peptide, beta-hydroxybutyrate, sulfonylurea drug screen, thyroid function tests, somatomedin C, and a random morning cortisol level were measured. At 8:45 A.M., her blood glucose was 90 mg/dl, insulin was 22.8 μU/ml (range 2.6-24.4 μU/ml), and proinsulin was 302.6 pmol/l (range 2.1-26.8 pmol/l). However, by 4:00 P.M., her glucose was 37 mg/dl, insulin was persistently elevated at 22.2 μU/ml, and proinsulin was 280.5 pmol/l. Her C-peptide levels were slightly elevated at 3.7 ng/ml in the morning and 3.8 ng/ml in the afternoon (range 0.8-3.5 ng/ml). Random morning cortisol, thyroid-stimulating hormone, and somatomedin C were normal. Her beta-hydroxybutyrate was 2.2 mg/dl (range < 3.1 mg/dl), suggesting that overproduction of insulin was preventing formation of ketoacids despite fasting. Biochemical evidence of hypoglycemia with hyperin-sulinemia led to radiological investigation for an insulinoma. Computed tomography (CT) scanning of the abdomen showed mild steatosis of the liver, benign appearing …