Insulin Autoimmune Syndrome Associated With Hashimoto Thyroiditis and Nodular Thyroid Disease: Clinical Management With Flash Glucose Monitoring and Prednisolone Therapy

BACKGROUND Insulin autoimmune syndrome (IAS) is a rare cause of spontaneous hyperinsulinemic hypoglycemia, characterized by high titers of insulin autoantibodies in the absence of prior insulin exposure. It is most frequently reported in Asian populations and is often associated with other autoimmune conditions. Reports in White patients remain uncommon. CASE REPORT We describe a 63-year-old White woman presenting with recurrent symptomatic hypoglycemia, with plasma glucose as low as 27 mg/dL. Episodes were accompanied by sweating and transient loss of consciousness. Several weeks before the first episode of hypoglycemia, she experienced a single episode of a pruritic rash, which resolved spontaneously. The patient had no prior exposure to insulin therapy or sulfhydryl-containing drugs. Laboratory evaluation revealed markedly elevated insulin concentrations (reaching 19 300 μU/mL), increased C-peptide levels, and insulin autoantibody titer of 99.9%. Abdominal MRI excluded insulinoma. Continuous flash glucose monitoring demonstrated frequent nocturnal and postprandial hypoglycemic episodes. Additional testing confirmed Hashimoto's thyroiditis and a benign thyroid nodule. Management included dietary modification with frequent small meals and carbohydrate restriction. Because these measures were insufficient, prednisolone was initiated, resulting in rapid improvement in glycemic stability. Corticosteroid dose was tapered over 4 months, and the patient remained euglycemic. CONCLUSIONS IAS should be considered in patients with unexplained hyperinsulinemic hypoglycemia to avoid unnecessary invasive evaluation for insulinoma. When dietary measures alone are inadequate, prednisolone therapy is effective. Given the frequent coexistence of IAS with other autoimmune disorders, systematic screening is advisable, and incidental thyroid findings in the setting of autoimmune thyroiditis warrant careful assessment to exclude malignancy.