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Patient information from Canterbury Health Limited Endocrine Services

INSULIN TOLERANCE TEST (ITT)

Introduction

Your doctor has requested a special test of your pituitary and adrenal gland function known as an insulin tolerance test (ITT for short).

The aim of the test is to measure the hormone response to the stress of a low blood sugar level. The medication insulin is used in people with diabetes to control high blood sugar levels. When given to non-diabetic people it may reduce blood sugar levels to lower than normal. The body recognizes that the sugar level is lower than it should be and responds by increasing the production of a number of hormones which act to increase the sugar level. Two of these are cortisol (produced by the adrenal gland, under control of a hormone called ACTH which is made by the pituitary gland) and growth hormone (made by the pituitary gland).

It has been well established how much cortisol and growth hormone should be made in response to a low blood sugar. Therefore this test will see if your pituitary and adrenal glands are working normally.

A fixed dose of insulin will be administered by injection into a vein after having nothing to eat or drink from midnight the previous night. Your blood sugar will soon begin to fall and should reach its lowest point 20 to 40 minutes after the injection. You may feel sweaty, drowsy, shaky, hungry and have trouble concentrating while your blood sugar is low.

These are expected effects. The blood sugar has to fall to less than 2.2 mmol/L for the hormone production to be stimulated. The symptoms are usually short lived and people start to feel better about an hour after the insulin, as the blood sugar starts to rise again.

Blood samples for glucose (sugar) and the hormones are taken every 15 minutes for the first hour and every 30 minutes for the second hour. You will be fed prior to leaving the Endocrine Test Center.

Risks

Provided the test is carefully planned and done by experienced staff, the ITT is a safe test. The unpleasant symptoms last about half an hour and recovery occurs even before eating. It is a good idea to have an extra something to eat later in the day after completion of the test. There are groups of patients who should not have an ITT - people with epilepsy, people who have had strokes and people with known heart disease (heart attacks, angina, irregular heart rhythms). If you have any of these, please inform your doctor and/or the endocrine test nurses. We routinely do a cardiograph (ECG) to screen for possible unknown heart conditions before performing an ITT. Having a low blood sugar may precipitate an epileptic seizure or a heart attack in susceptible people. The nursing staff are very experienced and know to administer glucose promptly if signs of a very low blood sugar develop. A doctor is always on hand nearby. Serious effects during an ITT are very rare. In a large review of over 6500 tests(1), only 2 cases of coma, 2 cases of seizures and 2 episodes of angina were reported. All reversed promptly on glucose administration.

Summary

The ITT is used to check your pituitary and adrenal gland function. Growth hormone and ACTH/cortisol are released in response to a low blood sugar You may feel some unpleasant effects of the low blood sugar such as sweating, drowsiness, "the shakes", hunger, poor concentration. These usually last about half an hour Serious side effects such as seizures, coma and angina are very rare (less than 1 in 1000) and are rapidly reversed with glucose

1. Fish HR, Chernow B, O'Brien JT. Endocrine and Neurophysiologic Responses of the Pituitary to Insulin-Induced Hypoglycaemia: A Review. Metabolism 1986,35:763-80.

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