Insulin
The Test
How is it used?
Insulin levels are 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, particularly in obese individuals.
When is it ordered?
- Sweating
- Palpitations
- Hunger
- Confusion
- Blurred vision
- Dizziness
- Fainting
- In serious cases, seizures
These symptoms may indicate low blood glucose but may also be seen with other conditions.
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 may be ordered to evaluate how much insulin in the blood is made by the body (endogenous) and how much is from exogenous sources. The test for insulin measures insulin from both sources while the C-peptide test reflects insulin produced by the pancreas (endogenous insulin).
A doctor also may order both tests to verify that an insulinoma has been successfully removed and 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?
Results seen:
| Disorder | fasting insulin level | fasting glucose level |
|---|---|---|
| None | normal | normal |
| Insulin resistance |
↑↑ | normal or ↑ |
| Not enough insulin produced by the beta cells (as seen in diabetes, pancreatitis, for example) |
↓↓ | ↑↑ |
| Hypoglycemia due to excess insulin (may be seen in insulinomas, Cushings, excess administration of exogenous insulin, etc.) |
normal or ↑↑ | ↓↓ |
| ↑ = somewhat increased; ↑↑ = greatly increased; ↓↓ = greatly decreased |
||
Elevated insulin levels are seen with:
- Acromegaly
- Cushing's syndrome
- Use of drugs such as corticosteroids, levodopa, oral contraceptives
- Fructose or galactose intolerance
- Insulinomas
- Obesity
- Insulin resistance, such as appears in type 2 diabetes and metabolic syndrome
Decreased insulin levels are seen with:
- Diabetes
- Hypopituitarism
- Pancreatic diseases such as chronic pancreatitis (including cystic fibrosis) and pancreatic cancer
Is there anything else I should know?
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.
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.
If someone has developed antibodies against insulin, especially as a result of taking non-human (animal or synthetic) insulin, they 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 persons with type I diabetes will also have autoantibodies against insulin.
