Details of Growth Hormone
What is Growth Hormone?
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 small gland located at the base of the brain behind the bridge of the nose. It is normally released into the blood 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.
Therefore, procedures called growth hormone stimulation and suppression tests are most often used to diagnose conditions caused by growth hormone deficiency or excess. These procedures are used in conjunction with signs and symptoms and growth factor levels (i.e., insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding protein 3). See “How is the test used?” under Common Questions for more detail.
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, a category referred to as “idiopathic.”
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 begin 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).
Excess GH is most often due to a pituitary tumor (usually benign) that produces GH.
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 feet or more. Those with excess GH may also have thickening of facial features, general weakness, delayed puberty, and headaches. Pituitary tumors releasing excess growth hormone can cause loss of vision if they grow very large.
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.
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.
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?
Growth hormone is released in pulses, so a single measurement of the blood level is not normally clinically useful. Therefore, growth hormone levels are usually measured using growth hormone suppression or stimulation procedures. 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.
Is any test preparation needed to ensure the quality of the sample?
Follow any instructions you are given. In most cases, you should be fasting. Resting or strenuous exercise for a time period before collection may be required.
Children and pre-teens 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 healthcare practitioner who is ordering the GH testing should provide the child’s parents with a prescription for such medications. Some healthcare practitioners will also prescribe propranolol prior to testing to reduce the number of falsely low GH responses to stimulation.