Insulinoma with low circulating insulin levels: the diagnostic value of proinsulin measurements.

Hypoglycemia coexisting with very low plasma immunoreactive insulin concentrations was found in a 78-year-old woman with an insulinoma. However, the absolute proinsulin levels during hypoglycemia were elevated (0.9 to 1.4 ng/ml), accounting for 66.5% of the total circulating immunoreactive insulinlike material. The raised serum proinsulin concentrations together with an abnormal proinsulin:insulin ratio proved to be of critical importance in establishing the diagnosis of an islet-cell tumor in this patient.