C-peptide is made up of chemical compounds called amino acids. When the pancreas produces insulin, it releases C-peptide into the bloodstream in the same way that the production of heat from burning coal or wood releases smoke into the atmosphere.
The amount of C-peptide in the blood can indicate the presence or absence of disease. For example, abnormally low amounts of C-peptide in the blood suggest the insulin production is too low (or absent) because of type I diabetes, also known as juvenile or insulin-dependent diabetes. Abnormally high amounts of C-peptide warn of the possible presence of a tumor called an insulinoma that secretes insulin.
Normal levels of C-peptide may signal that all is well. However, in a person with diabetes, a normal level of C-peptide indicates the body is making plenty of insulin but the body is just not responding properly to it. This is the hallmark of type 2 diabetes (adult insulin-resistant diabetes). C-peptide, therefore, plays a crucial diagnostic role as regards insulin.
Insulin is a hormone that regulates the body's use of glucose (blood sugar). Muscle cells and other types of cells need glucose to generate energy. The body manufactures glucose from food, mainly carbohydrates. It is the job of insulin to deliver glucose to an energy-consuming body site. There it knocks on the front door and places the glucose into the hands of the occupant. The occupant then uses the glucose to help its master -- the body -- walk, run, throw, lift, and carry out other activities. Football players, mountain climbers and lumberjacks all thrive on the energy glucose provides. Insulin also prevents glucose overload in the bloodstream by lowering the level of blood glucose as necessary. Insulin is released by cells in the pancreas called the islets of Langerhans.
If the pancreas malfunctions, it may produce an inadequate supply of insulin, or no insulin at all. Blood sugar then increases because there is little or no insulin to regulate it. Type 1 diabetes then develops. Meanwhile, the reduced production of insulin also reduces the amount of C-peptide released into the bloodstream. If a patient reports symptoms of diabetes to a physician--symptoms that may include great thirst, frequent urination and fatigue--the physician can order a test that checks for the amount of C-peptide in the blood. If a patient reports symptoms consist with pancreatic cancer -- abdominal pain, cramps, weight loss, and jaundice-- the physician can again order the C-peptide test -- to determine the likelihood of a tumor.