The characteristic features of insulinomas include: (a) hypoglycemic symptoms; (b) blood glucose <50 mg/dL during the symptomatic episodes; and (c) relief of symptoms by intravenous injection of glucose (Whipple's triad). Diagnosis is confirmed by demonstration of fasting hypoglycemia in the presence of inappropriately elevated levels of insulin in the blood. A ratio of plasma insulin/glucose >0.3 is diagnostic. Circulating levels of C-peptide are usually elevated in patients with insulinoma but not in patients with such other causes of hypoglycemia as tumors of mesenchymal origin and liver tumors. Patients who surreptitiously administer insulin develop insulin antibodies.