Also Known As
Fasting Insulin
Formal Name
Insulin, serum
This article was last reviewed on
This article waslast modified on November 15, 2019.
At a Glance
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 healthcare 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. Occasionally, a healthcare practitioner may do 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.

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

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.

Accordion Title
Common Questions
  • 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 healthcare 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:

    Disorder fasting insulin level fasting 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.

  • Can I do an insulin test at home?

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

  • 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.

  • 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.

  • 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.

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2017 review performed by Alan F. Weir, PhD, DABCC, Instructor, Fox Valley Technical College.

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