Insulinoma with normal plasma insulin concentrations and insulin/glucose ratios during hypoglycemic episodes.

A patient with insulinoma had frequent hypoglycemic episodes with normal plasma insulin levels and insulin/glucose ratios. When immunoreactive insulin (IRI) concentrations in this patient were compared among plasma samples with the same C-peptide immunoreactivity (CPR) levels, the concentrations were significantly lower than in control patients with insulinoma and equal to or lower than those of normal subjects. In hepatic venous samples, CPR levels were significantly higher and the IRI/CPR molar ratios were lower than those in a control subject. These results may indicate that normoinsulinemia in this patient could be explained by increased hepatic extraction of insulin.