Insulinoma With Concomitant Insulin Antibodies

OBJECTIVE: The coexistence of insulinoma and insulin antibodies is extremely rare. The aim of this novel case report is to inform physicians of the possibility of an insulinoma with concomitant insulin antibodies. METHODS: In this report, we describe a patient with symptomatic hypoglycemia confirmed with a 72-hour fast, who was subsequently found to have an insulinoma with concomitant elevation in his immunoglobulin G insulin antibody titer. RESULTS: The patient presented with initial symptoms of diaphoresis, confusion, and disorientation and was found unresponsive by a bystander. He had a fingerstick blood glucose of 36 mg/dL (reference 74-99 mg/dL), without exogenous insulin or sulfonylurea use. His symptoms resolved with administration of glucose. He was subsequently admitted for a 72-hour fast in which he developed neuroglycopenic symptoms 4 hours into the fast with fingerstick glucose of 47 mg/dL and serum glucose of 44 mg/dL (reference 74-99 mg/dL), C-peptide of 10.8 ng/mL (reference 0.5-2.7 ng/mL), insulin level of 106 μIU/mL (reference <25 μIU/mL), and a proinsulin level of 675 pmol/mL (reference <22 pmol/mL). His insulin-to-C-peptide ratio was 0.20, in which a ratio <1 is indicative of an insulinoma. Endoscopic ultrasound demonstrated a 16 x 11 mm biopsy-proven neuroendocrine tumor. He was found to have a high titer insulin antibody titer at 2.4 U/mL (reference <0.4 U/mL), was started on prednisone, and underwent successful radiofrequency ablation. He was able to be successfully tapered off steroids without recurrence. CONCLUSION: The coexistence of insulinoma with insulin antibodies is novel, and to our knowledge, has never been published.