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?
Insulin and C-peptide are produced by the body at the same rate as part of the activation and division of proinsulin in the pancreas. Both may be ordered to evaluate how much insulin in the blood is due to endogenous production (what your body is making) and how much is from exogenous (produced outside the body) sources. Insulin tests will reflect the total, while C-peptide will reflect only the endogenous insulin.
Your doctor also may order both tests to verify that an insulinoma has been successfully removed. If you are one of the few people who have received an islet cell transplant to restore your insulin-producing capability, your insulin level may be monitored to determine whether or not this procedure is successful over time.
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, etc.) |
normal or ↑↑ | ↓↓ |
| ↑ = somewhat increased; ↑↑ = greatly increased; ↓↓ = greatly decreased |
||
Elevated insulin levels are seen with:
- Acromegaly
- Cushing's syndrome
- 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 you are receiving insulin, these effects should be clarified with the testing laboratory. If you are going to have several insulin assays done, they should be performed 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 you have developed antibodies against insulin, especially as a result of taking non-human (animal or synthetic) insulin, they can interfere with your test. Your doctor and the testing laboratory must determine if they are affecting your test result.
