Growth hormone suppression test

Definition
The growth hormone suppression test determines whether human growth hormone (hGH) is suppressed by hyperglycemia.

How the test is performed

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to cause the veins to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. Alternatively, an intravenous catheter or a small needle called a butterfly may be placed in the arm or hand to minimize the number of needle sticks.

Three blood specimens are taken. The first is collected between 6 and 8 a.m. Then you are asked to drink a water solution with 75 grams of glucose. You may be advised to drink this slowly to avoid becoming nauseated. However, the solution must be consumed within 5 minutes or the test results may be altered.

The second and third blood samples are collected 1 to 2 hours after you finish drinking the glucose solution. Each sample should be taken to the laboratory immediately. Glucose and hGH are measured in each sample.

How to prepare for the test

Fast and limit physical activity for 10 to 12 hours before the test, or the test results may be altered.

If you are taking medications, your health care provider may ask that you withhold these before the test, as some can affect results. Check with your health care provider before discontinuing any medications.

You will be asked to relax for at least 90 minutes before the test, as exercise or increased activity can alter hGH 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 he or she 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

This test is to evaluate for elevated levels of hGH, a condition that leads to gigantism in children and acromegaly in adults. This is not a normal screening tool, rather a test that is performed if you show signs of increased hGH.

Normal Values

Normal test results would reveal an hGH level of less than 2 ng/mL. In children, if the second blood sample is taken after the second hour, there may be increased levels from a rebound phenomenon due to the occurrence of reactive hypoglycemia.

What abnormal results mean

If the hGH levels are unaltered and remain high during the suppression test, then gigantism or acromegaly is suspected. The test may need to be confirmed by retesting at the same time and under the same conditions on another day.

What the risks are

Risks associated with drawing blood are slight:

     
  • Excessive bleeding  
  • Fainting or feeling light-headed  
  • Hematoma (blood accumulating under the skin)  
  • Infection (a slight risk any time the skin is broken)  
  • Multiple punctures to locate veins

Special considerations

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.

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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