MedTerms Medical Dictionary

Glucose tolerance test: A blood test done to make the diagnosis of diabetes mellitus. The test may also be done for other purposes such as to diagnose hypoglycemia (low blood sugar) or a malabsorption syndrome in which sugar is not absorbed properly through the intestines into the bloodstream.

The test was designed originally to determines the tolerance for the sugar glucose. "Tolerance" refers to the body's ability to handle (tolerate) glucose. However, the test is not that simple. The test depends on a number of factors including the ability of the intestine to absorb glucose, the power of the liver to take up and store glucose, the capacity of the pancreas to produce insulin, the amount of "active" insulin it produces, and the sensitivity of the cells in the body to the action of insulin.

For the test, you fast overnight and then are given a specific amount (100 grams) of glucose by mouth and then your blood glucose levels are monitored for 3 hours. Normally, the blood glucose should return to normal within 2 to 2½ hours. The outcome of the test may indicate:

• Normal glucose tolerance
• Abnormal glucose tolerance
  • Depressed glucose tolerance -- in which the blood glucose peaks sharply before declining slower then usual to normal levels -- as in:
    • Diabetes mellitus
    • Hemochromatosis (iron overload disease)
    • Cushing syndrome (too much cortisol ["cortisone"] hormone)
    • Pheochromocytoma (adrenaline-producing tumor)
    • Central nervous system lesions
  • Increased glucose tolerance -- in which the blood glucose levels peak at lower than normal levels -- as in the:
    • Malabsorption syndrome
    • Insulinoma (an insulin-producing tumor)
    • Addison disease (adrenocortical insufficiency)
    • Hypopituitarism (underactivity of the pituitary gland)
    • Hypothyroidism (underactivity of the thyroid gland)

Since the dose of glucose is taken by mouth, the test is sometimes called an oral glucose tolerance test.

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