What is Insulin?
Insulin is a hormone that is produced and stored in the beta cells of the pancreas. It is secreted in response to elevated blood glucose following a meal and is vital for the transportation and storage of glucose, the body’s main source of energy. Insulin helps transport glucose from the blood to within cells, thus helping regulate blood glucose levels, and has a role in lipid metabolism. This test measures the amount of insulin in the blood.
Insulin and glucose blood levels must be in balance. After a meal, carbohydrates usually are broken down into glucose and other simple sugars. These are absorbed into the blood, causing the blood glucose level to rise, which in turn stimulates the pancreas to release insulin into the blood. As glucose moves into cells, the level in the blood decreases, and the release of insulin by the pancreas decreases.
If an individual is not able to produce enough insulin, or if the body’s cells are resistant to its effects (insulin resistance), glucose cannot reach most of the body’s cells and the cells starve. Meanwhile, blood glucose rises to an unhealthy level. This can cause disturbances in normal metabolic processes that result in various disorders and complications, including kidney disease, cardiovascular disease, and vision and neurological problems.
Diabetes, a disorder associated with high glucose levels and decreased insulin effects, can be a life-threatening condition. People with type 1 diabetes produce very little insulin and so eventually require insulin supplementation therapy. Type 2 diabetes is generally related to insulin resistance, which increases with time.
With insulin resistance, many of the body’s cells are unable to respond to the effects of insulin, leaving glucose in the blood. The body compensates by producing additional amounts of the hormone. This results in a high level of insulin in the blood (hyperinsulinemia) and over-stimulation of some tissues that have remained insulin-sensitive. Over time, this process causes an imbalance in the relationship between glucose and insulin and, without treatment, may eventually cause health complications affecting various parts of the body.
In addition to type 2 diabetes, insulin resistance may be seen in those with polycystic ovary syndrome (PCOS), prediabetes or heart disease, metabolic syndrome, acanthosis nigricans, and disorders related to the pituitary or adrenal glands.
Other than insulin resistance, hyperinsulinemia is most often seen in people with tumors of the islet cells in the pancreas (insulinomas) or with an excess amount of administered (exogenous) insulin. Hyperinsulinemia causes low blood sugar (hypoglycemia), which can lead to sweating, palpitations, hunger, confusion, blurred vision, dizziness, fainting, and seizures. Since the brain is dependent on blood glucose as an energy source, severe glucose deprivation due to hyperinsulinemia can lead fairly quickly to insulin shock and death.
How is it used?
Insulin testing may be used to help:
- Diagnose an insulinoma, verify that removal of the tumor has been successful, and/or monitor for recurrence
- Diagnose the cause of hypoglycemia in an individual with signs and symptoms
- Identify insulin resistance
- Monitor the amount of insulin produced by the beta cells in the pancreas (endogenous); in this case, a C-peptide test may also be done. Insulin and C-peptide are produced by the body at the same rate as part of the conversion of proinsulin to insulin in the pancreas. Both tests may be ordered when a health practitioner wants to evaluate how much insulin in the blood is made by the body and how much is from outside (exogenous) sources such as insulin injections. The test for insulin measures insulin from both sources while the C-peptide test reflects insulin produced by the pancreas.
- Determine when a type 2 diabetic might need to start taking insulin to supplement oral medications
- Determine and monitor the success of an islet cell transplant intended to restore the ability to make insulin, by measuring the insulin-producing capacity of the transplant
Insulin testing may be ordered with glucose and C-peptide tests. Insulin levels are also sometimes used in conjunction with the glucose tolerance test (GTT). In this situation, blood glucose and insulin levels are measured at pre-established time intervals to evaluate insulin resistance.