Insulin may be used, often along with glucose and C-peptide levels, to help diagnose insulinomas and to help diagnose documented acute or chronic (fasting) hypoglycemia. Insulin and C-peptide levels may also be used to monitor endogenous (produced by the body) insulin, check for insulin resistance, and to help determine when a type 2 diabetic might need to start taking insulin injections to supplement oral medications.

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.

Insulin levels are most frequently ordered following an abnormal glucose test and/or when a patient has acute or chronic symptoms of hypoglycemia, such as sweating, palpitations, hunger, confusion, blurred vision, dizziness, fainting, and seizures (although these can be caused by other conditions along with low blood glucose). 

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.

Insulin levels must be evaluated in context.

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 ↑↑	↓↓

Elevated insulin levels are seen with:

1. Acromegaly
2. Cushing's syndrome
3. 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 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

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 insulin produced from 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 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.