Insulin Autoimmune Syndrome Presenting With Recurrent Hypoglycemia and Extreme Hyperinsulinemia

Insulin autoimmune syndrome (IAS) is a rare but important cause of hypoglycemia. Herein, we describe a case of a 67-year-old White man with recurrent episodes of confusion and fatigue relieved by food intake. Biochemical evaluation revealed profound hyperinsulinemia and an elevated C-peptide level, suggesting endogenous insulin production, with no pancreatic lesion on imaging. The presence of insulin autoantibodies confirmed the diagnosis. Fluctuating beta-hydroxybutyrate levels provided an additional physiologic clue, reflecting variable insulin activity resulting from antibody binding and release. This case highlights how characteristic biochemical patterns, including variable ketone levels, can facilitate timely recognition of IAS and help avoid unnecessary diagnostic procedures.