A 55-yr-old woman with a history of Graves'; disease experienced attacks of postprandial hypoglycemia for 6 yr. An insulinoma could not be confirmed by repeated fasting tests and by surgical pancreas revision. Extracted pancreatic insulin was chemically normal. Fasting plasma total insulin (1.22 nM = 183 μU/ml) and proinsulin (0.48 nM) were elevated and greatly increased after oral glucose. Glucoseclamp studies revealed delayed insulin clearance. Plasma free-insulin levels were normal. Insulin-binding antibodies were detected by enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay with human insulin as ligand but not with pork or beef insulins. Analysis with a modified ELISA suggested a monotypic and monoclonal human insulin autoantibody, which showed a restriction to the λ-light chain. T-lymphocytes (predominantly helper) demonstrated increased responsiveness to beef, pork, and human insulins by proliferation assay. A T-lymphocyte line showed exclusively human insulin specificity. All this indicated cellular and humoral anti-human insulin autoimmunities. Clinically, the cause of hypoglycemia associated with elevated total insulin and proinsulin was misdiagnosed as atypical insulinoma. The study of total and free plasma insulin levels and sensitive antibody assays specific to human insulin were necessary to correctly diagnose autoimmune hypoglycemia.

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