What Is an Insulin Tolerance Test?

An Insulin Tolerance Test (ITT) is a medical test an endocrinologist may recommend to evaluate a patient with a suspected adrenal or pituitary gland problem. In this test, the patient receives an injection of insulin to lower blood sugar, forcing the body into hypoglycemia. This should cause a stress reaction where levels of cortisol and growth hormone rise. If they do not, the patient’s endocrine system may not be functioning correctly. It could be necessary to supplement growth hormone or run some tests to find out more about what is happening.

Before the test, patients may not be allowed to eat for several hours. They also need to temporarily stop taking steroid medications, as these could throw the test results off. It can take several hours to complete an insulin tolerance test and it is a good idea to have a ride home afterward. Patients often feel tired and shaky after the test, and may not be able to drive or return to work for the rest of the day.

The test starts with the collection of a baseline blood sample. A care provider injects insulin and remains in the room at all times to monitor the patient, taking periodic samples to check blood sugar levels. Samples can also be pulled to check on the concentration of hormones in the blood, which should start to rise as the patient becomes hypoglycemic. When the test is over, the care provider can give the patient some juice or a snack to raise blood sugar levels.

Patients may not feel very well during an insulin tolerance test. Hypoglycemia can make people feel shaky, sweaty, tired, and irritable. Those who lose consciousness may need a bolus of medication to elevate blood sugar levels rapidly. Constant monitoring is necessary to make sure the early warning signs of complications are identified in time, and drugs may be kept on hand to reverse the effects of the insulin if necessary. Fatigue and dizziness aren’t uncommon even after blood sugar goes back up, because the patient’s body may still be adjusting.

If the adrenal gland is not functioning properly, cortisol levels won’t rise during an insulin tolerance test. Growth hormone levels may also be low, indicating that there is a problem with the pituitary gland. The insulin tolerance test can be definitive for an endocrinologist with concerns about a patient, which is why it may be recommended despite the risks. As long as a patient is adequately supervised, the chances of serious complications are low.