Insulin is a protein hormone produced by the beta cells of the pancreas. It consists of two chains (A and B) connected by disulfide bridges. Insulin and C-peptide are produced by the pancreas as the result of proteolytic cleavage of proinsulin. Insulin is an anabolic hormone that stimulates the uptake of glucose into fat and muscle and promotes the formation of glycogen. Insulin stimulates protein synthesis and inhibits protein degradation. Glucose, amino acids, and certain pancreatic and gastrointestinal hormones (eg, glucagon, gastrin, secretin) stimulate the pancreas to secrete insulin. Insulin secretion is inhibited by hypoglycemia and somatostatin. In healthy individuals insulin is secreted in a pulsatile fashion that is closely controlled by glucose levels.
The primary clinical utility of insulin measurement is in the evaluation of patients with fasting hypoglycemia. Insulin levels tend to be inappropriately elevated in patients with insulin-secreting tumors. Fasting hypoglycemia in association with markedly elevated serum insulin levels is considered diagnostic for insulinoma. Some patients with insulin secreting tumors exhibit intermittent insulin elevations. Diagnostic Fast, C-Peptide Suppression, and Tolbutamide Tolerance tests described in the Endocrine Appendix can also be helpful in the evaluation of patients with possible insulinoma.
Insulin levels can be useful predicting susceptibility to the development of type II diabetes, although C-peptide has largely supplanted insulin measurement for this role. The American Diabetes Association recommendations for the diagnosis of diabetes do not include the measurement of insulin levels.
