The growth hormone suppression test determines whether growth hormone (GH) production is suppressed by high blood sugar.
How the test is performed
A blood sample is needed. For information on how this is done, see: Venipuncture
At least three blood samples are taken. The first is collected between 6 a.m. and 8 a.m before you eat or drink anything. Then you are asked to drink a water solution containing 75 grams of glucose. You may be asked to drink this slowly to avoid becoming nauseated. However, you must drink the solution within 5 minutes or the test results may be changed.
The next blood samples are usually collected for 1 - 2 hours after you finish drinking the glucose solution. Sometimes they are taken every 30 or 60 minutes. Each sample should be taken to the laboratory immediately. The lab measures glucose and GH levels in each sample.
How to prepare for the test
Do not eat anything and limit physical activity for 10 - 12 hours before the test, so that you don't affect the test results.
Some medications can affect test results. If you are taking medications, your health care provider may ask that you stop taking them before the test. Check with your health care provider before stopping any medications.
You will be asked to relax for at least 90 minutes before the test, as exercise or increased activity can change GH levels.
If your child is to have this test performed, it may be helpful to explain how the test will feel and even demonstrate on a doll. The more familiar your child is with what will happen and why, the less anxiety the child will feel.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
Normal test results show a GH level of less than 1 ng/mL. In children, the levels may be increased due to reactive hypoglycemia.
The examples above are common measurements for results of these tests. Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
If the GH levels are not changed and stay high during the suppression test, then the health care provider will suspect gigantism or acromegaly. You may need to be retested at the same time and under the same conditions on another day to confirm the test results.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Melmed S, Kleinberg D, Ho K. Pituitary physiology and diagnostic evaluation. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 8.
Molitch ME. Anterior pituitary. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 231.