Patient Test Information


Also known as:

Fasting Insulin

Formal name:

Insulin, serum

Related tests:

C-peptide, Glucose Tests

Why Get Tested?

To help evaluate insulin production by the beta cells in the pancreas; to help diagnose the presence of an insulin-producing tumor in the islet cells of the pancreas (insulinoma); to help determine the cause of low blood glucose (hypoglycemia); to help identify insulin resistance, or to help determine when a type 2 diabetic might need to start taking insulin to supplement oral medications

When to Get Tested?

When you have low blood glucose levels with symptoms such as sweating, palpitations, dizziness, fainting; when you have diabetes and your health practitioner wants to monitor your insulin production; sometimes when it is suspected that you have insulin resistance

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

Typically, you will be asked to fast for 8 hours before the blood sample is collected, but occasionally a healthcare practitioner may do the testing when fasting is not possible, such as when a glucose tolerance test is done. In some cases, a healthcare practitioner may request that you fast longer than 8 hours.

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 to 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.

When is it ordered?

Insulin levels are most frequently ordered following a low blood glucose test result and/or when someone has acute or chronic symptoms of low blood glucose (hypoglycemia) caused by, for example, an insulinoma. Symptoms of hypoglycemia may include:

  • Sweating
  • Palpitations
  • Hunger
  • Confusion
  • Blurred vision
  • Dizziness
  • Fainting
  • In serious cases, seizures and loss of consciousness

These symptoms may indicate low blood glucose but may also be seen with other conditions.

An insulin test may also be done when an individual has or is suspected of having insulin resistance. This may include people with type 2 diabetes, polycystic ovary syndrome (PCOS), prediabetes, acanthosis nigricans, heart disease, or metabolic syndrome.

A health practitioner also may order insulin and C-peptide tests after an insulinoma has been successfully removed to verify the effectiveness of treatment and then order the tests periodically to monitor for recurrence.

Periodic testing may also be used to monitor the success of an islet cell transplant by measuring the insulin-producing capacity of the transplant.

What does the test result mean?

Insulin levels must be evaluated in context.

Results seen:

Disorderfasting insulin levelfasting glucose level
None Normal Normal
Insulin resistance High Normal or somewhat elevated
Not enough insulin produced by the beta cells (as seen in diabetes, pancreatitis, for example) Low High
hypoglycemia due to excess insulin (may be seen in insulinomas, Cushing syndrome, excess administration of exogenous insulin, etc.) Normal or high Low

Elevated insulin levels are seen with:

  1. Acromegaly
  2. Cushing syndrome
  3. Use of drugs such as corticosteroids, levodopa, oral contraceptives
  4. Fructose or galactose intolerance
  5. Insulinomas
  6. Obesity
  7. insulin resistance, such as appears in type 2 diabetes, acanthosis nigricans, and metabolic syndrome

Decreased insulin levels are seen with:

  1. Diabetes
  2. Hypopituitarism
  3. Pancreatic diseases such as chronic pancreatitis (including cystic fibrosis) and pancreatic cancer

Is there anything else I should know?

Insulin for injection used to come strictly from animal sources (cow and pig pancreas cells). Most insulin used today is synthetic, made by biochemical synthesis to identically match the biological activity of the insulin produced by human cells.

There are different pharmaceutical formulations of insulin with different properties. Some are rapid-release and quick-acting and others are slow-release preparations that act over a prolonged period. Diabetics may take mixtures and/or different types of insulin throughout the day.

Insulin assays are designed to measure endogenous human insulin. However, different assays react variably with exogenous (animal or synthetic) insulin. If someone is receiving insulin, these effects should be clarified with the testing laboratory. If several or periodic insulin assays will be performed, they should be analyzed by the same laboratory to ensure consistency.

If someone has developed antibodies against insulin, especially as a result of taking non-human (animal or synthetic) insulin, these can interfere with insulin testing. In this case, a C-peptide may be performed as an alternative way to evaluate insulin production. Note also that most people with type 1 diabetes will also have autoantibodies against insulin.

What is being tested?

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 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 with disorders related to the pituitary or adrenal glands.

Other than in insulin resistance, hyperinsulinemia is most often seen in people with tumor 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 the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Typically, a person will be asked to fast for 8 hours before blood is collected, but occasionally a health practitioner may do testing when fasting is not possible, such as when a glucose tolerance test (see Glucose) is done. In some cases, the health practitioner may request that a person fast longer than 8 hours.

  1. Can I do an insulin test at home?

    No. Although glucose levels can be monitored at home, insulin tests require specialized instruments and training are are performed at laboratories.

  2. Why does insulin have to be injected?

    Insulin must be injected or given via an insulin pump. It cannot be given orally because it is a protein and is broken down in the stomach before it can be absorbed. In February 2015, a short-acting inhalable insulin became available.

  3. How is an insulinoma treated?

    Insulinomas are insulin-producing tumors that are usually benign. They are typically treated by being located and removed. Once removed, generally they do not return.

  4. What is an insulin tolerance test?

    The insulin tolerance test (ITT) is not widely used, but is one method for determining insulin sensitivity (or resistance), especially in obese individuals and those with PCOS. This test involves an IV-infusion of insulin, with subsequent measurements of glucose and insulin levels.