The growth hormone (GH) is a protein hormone released from the anterior pituitary gland under the control of the hypothalamus. In children, GH has growth-promoting effects on the body. It stimulates the secretion of somatomedins from the liver, which are a family of insulin-like growth factor (IGF) hormones. These, along with GH and thyroid hormone, stimulate linear skeletal growth in children. In adults, GH stimulates protein synthesis in muscle and the release of fatty acids from adipose tissue (anabolic effects). It inhibits uptake of glucose by muscle while stimulating uptake of amino acids. The amino acids are used in the synthesis of proteins, and the muscle shifts to using fatty acids as a source of energy. GH secretion occurs in a pulsatile (short, concentrated secretion) and sporadic manner. Thus, a single test of the GH level is usually not performed.
Because of the sporadic release of GH, the patient will have his blood drawn a total of five times over a few hours. Instead of the traditional method of blood drawing (veinipuncture), the blood is taken through an IV (angiocatheter).
How to prepare for the test:
You should fast and limit physical activity for 10 to 12 hours before the test. If you are taking certain medications, your health-care provider may ask that you withhold these before the test, as some can affect results.
You will be asked to relax for at least 90 minutes before the test, as exercise or increased activity can alter hGH levels. Inform your health-care provider if you or your child has had a radioactive scan performed within a week of this test, as radioactive scans can affect test results.
If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. This test requires temporary placement of an angiocatheter, an IV, and this should be explained to your child. The more familiar your child is with what will happen to him, and the purpose for the procedure, the less anxiety he will feel. How the test will feel: When the needle is inserted, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
What the risks are:
Risks associated with venipuncture are slight:
- excessive bleeding fainting or
- feeling lightheaded hematoma (blood accumulating under the skin)
- infection (a slight risk any time the skin is broken)
- multiple punctures to locate veins
- clinical signs and symptoms of hypoglycemia if IV insulin is administered.
What abnormal results mean: A diminished amount of hGH indicates a problem either in the hypothalamus or the pituitary. Additional testing can illustrate the exact site of the deficiency. In children, a deficiency of GH results in dwarfism. In adults, it may be associated with panhypopituitarism. About 10% to 15% of people with normal functioning pituitaries may fail to respond on the first test. The test is often repeated.