What Is a Glucose Tolerance Test (GTT)? - Definition & Side Effects

Instructor: Jayne Yenko

Jayne has taught health/nutrition and education at the college level and has a master's degree in education.

Some people avoid the glucose tolerance test when their doctors order it. It sounds scary, but is it really? (Answer: no, not really.) Discover what the glucose tolerance test is, and what it is used for.


The glucose tolerance test (GTT) is performed to determine how your body responds to glucose. You drink a glucose solution, and your blood is taken at intervals to see how quickly the glucose is removed from the blood.

The GTT is used to diagnose type 2 diabetes and insulin resistance. There are a number of variations of the glucose tolerance test used for different purposes, but the most common one is the oral glucose tolerance test.

What is Glucose?

All carbohydrate foods break down into their smallest particle, which is glucose. Glucose is a single molecule of what we know as sugar. It is what fuels our bodies, particularly the brain. It doesn't need to be digested. Consider the 'sugar rush' you experience when drinking soft drinks or eating a candy bar. Those single molecules get absorbed directly into the blood stream, which results in the 'rush' we feel.

Sometimes, the body doesn't quite respond to glucose as it should. What should happen is that after eating, our blood sugar goes up and the pancreas releases insulin to encourage the cells to absorb the excess blood sugar. If this doesn't happen, our blood sugar levels remain high and over time, we can develop type 2 diabetes.

The pancreas is an organ in our abdomen that produces the hormone insulin. In the case of type 1 diabetes, the pancreas does not produce any, or very little, insulin, requiring injections of insulin. In the case of type 2 diabetes, the pancreas produces insulin, but the body doesn't respond. Type 2 diabetes can be treated with oral medications, but not insulin. Insulin is responsible for getting the body's cells to take up the glucose in the blood to perform various cellular functions.

Side Effects of GTT

Yes, like most everything, there can be side effects to the glucose tolerance test. These tend to be a bit uncomfortable. Serious side effects are very uncommon.

The side effects include:

• Nausea

• Sweating

• Light-headedness or faintness

• Shortness of breath

• Pain at the needle site

The risks from having blood drawn include:

• Excessive bleeding

• Hematoma (otherwise known as a bruise)

• Infection

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