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The Test Sample
What is being tested?
Growth hormone (GH) is a hormone that is essential for normal growth and development in children. It promotes proper linear bone growth from birth through puberty. In both children and adults, growth hormone helps regulate the rate at which the body both produces energy from food (metabolism) and makes lipids, proteins, and glucose (sugar). It also helps regulate the production of red blood cells and muscle mass.
Growth hormone is produced by the pituitary gland, a grape-sized gland located at the base of the brain behind the bridge of the nose. It is normally released into the bloodstream in pulses throughout the day and night with peaks that occur mostly during the night. Because of this, a single measurement of the level of GH in blood is difficult to interpret and not usually clinically useful. The value will be higher if the sample is taken during a pulse and lower if it is taken during a period between pulses. GH stimulation and suppression tests are therefore often used to diagnose GH abnormalities. (See the "How is it used?" section.)
Children with insufficient GH production grow more slowly and are smaller in size for their age. Some children have GH deficiency at birth (congenital), but some may develop a deficiency later due, for example, to a brain injury or tumor. These conditions can affect the pituitary gland, causing a decrease in pituitary function, resulting in a lowered production of pituitary hormones (hypopituitarism). Sometimes, the cause of the deficiency is not known.
In adults, growth hormone plays a role in regulating bone density, muscle mass, and glucose and lipid metabolism. It can also affect heart and kidney function. Deficiencies may have begun in childhood or develop in adulthood. A deficiency can develop, for example, because of damage to the pituitary gland caused by a head injury, brain tumor, or surgery or radiation treatment. This can result in a decrease in pituitary hormones (hypopituitarism). The deficiency in GH can lead to decreased bone density, less muscle mass, and altered lipid levels. However, testing for GH deficiency is not routine in adults who have decreased bone density and/or muscle strength or increased lipids. GH deficiency is a very rare cause of these disorders.
Excess GH is most often due to a GH-secreting pituitary tumor (usually benign). Frequently, the pituitary tumor causing the excess can be surgically removed and/or treated with drugs or radiation. In most cases, this will cause GH and IGF-1 levels to return to normal or near normal levels.
Too much GH in children can cause their long bones to continue to grow beyond puberty, resulting in the extremely rare condition, gigantism, with heights of 7 or more feet. Those with excess GH may also have thickening of facial features, general weakness, delayed puberty, and headaches.
Excess GH in adults can lead to the rare condition, acromegaly, marked not by bone lengthening but by bone thickening. Although symptoms such as skin thickening, sweating, fatigue, headaches, and joint pain can be subtle at first, increased GH levels can lead to enlarged hands and feet, enlarged facial bones, carpal tunnel syndrome, and abnormally enlarged internal organs. Excess GH can also cause skin tags and intestinal polyps.
If left untreated, both acromegaly and gigantism can lead to complications such as type 2 diabetes, increased risk of cardiovascular disease, high blood pressure, arthritis, and in general, a decreased lifespan.
How is the sample collected for testing?
Because growth hormone is released in pulses, a single measurement of the blood level is not normally clinically useful. Therefore, testing for the suppression or stimulation of growth hormone release from the pituitary is usually done. After fasting for 10 to 12 hours, a blood sample is drawn from a vein in the arm. Then, under medical supervision, the suppression or stimulation procedure is performed. This may involve the use of an intravenous (IV) infusion. Blood samples are then drawn from a vein (or from the IV) at timed intervals.
Sometimes, a single sample of blood is drawn following a fast and rest or after a period of strenuous exercise.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Follow any instructions you are given. In most cases, a person should be fasting. Resting or strenuous exercise for a time period before collection may be required.
Children and early adolescents who are being tested for GH deficiency should be prepared for treatment with sex steroids for a few days prior to testing. Such treatment reduces the number of falsely low GH responses to stimulation. The health practitioner who is ordering the GH testing should provide the child's parents with a prescription for such medications. Some healthcare providers will also prescribe propranolol prior to testing to reduce the number of falsely low GH responses to stimulation.