Glucose Changes in a Patient With Insulin Autoimmune Syndrome Demonstrated by Continuous Glucose Monitoring

ABSTRACT Objective: Insulin autoimmune syndrome (IAS) is an unusual cause of hypoglycemia in individuals without underlying diseases. Continuous glucose monitoring (CGM) has rarely been applied for IAS. We present a case of IAS with available 6-day CGM data. Methods: A 61-year-old Taiwanese man was admitted because of impaired consciousness while driving, caused by a low blood glucose level of 30 mg/dL. He regained consciousness fully after parenteral glucose administration. Results: During the prolonged fasting test, his C-peptide and insulin levels were respectively 11 ng/mL and 169.34 μIU/mL when plasma glucose was 41 mg/dL. Abdominal magnetic resonance imaging did not show any pancreatic abnormality. His 6-day CGM data revealed fasting hypoglycemia, several instances of postprandial hyperglycemia, and low blood glucose levels before lunch and dinner. Additional diagnostic findings included elevated anti-insulin antibody of 78.2%, thyrotoxicosis due to Graves disease, and gastric ulcer. He was discharged home on prednisolone at 5 mg daily, methimazole at 10 mg daily, and esomeprazole at 40 mg daily. Hypoglycemia and impairment of consciousness did not recur throughout the subsequent year-long follow up. Conclusion: We proposed a novel approach using CGM coupled with measurements of plasma insulin, C-peptide, and anti-insulin antibodies as the initial investigation for hypoglycemia in non-diabetic subjects. These relatively inexpensive tests may lead to earlier detection of IAS and thus render hospital admission and more costly explorations unnecessary. Abbreviations: CGM = continuous glucose monitoring; IAS = insulin autoimmune syndrome