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 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 health 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.
How is the test used?
Growth hormone (GH) testing is primarily used to help diagnose growth hormone deficiency and to help evaluate pituitary gland function, usually as a follow-up to other abnormal pituitary hormone test results.
GH testing is also used to detect excess GH and to help diagnose and monitor the treatment of acromegaly and gigantism.
Growth hormone is released in pulses, so 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.
- GH stimulation tests help to diagnose GH deficiency and hypopituitarism. For a stimulation test, a sample of blood is drawn after 10-12 hours of fasting. Then, under close medical supervision, you are given an intravenous solution of a substance that normally stimulates the release of GH from the pituitary. Blood samples are then drawn at timed intervals and GH levels are tested in each to see if the pituitary gland was stimulated to produce the expected levels of GH. Since exercise normally causes an increase in GH, vigorous exercise may also be used as the stimulant for GH release.
- GH suppression tests help to diagnose GH excess by determining if GH production is being suppressed by high blood sugar. For a suppression test, a sample of blood is drawn after 10-12 hours of fasting. You are then given a standard glucose solution (either weight-based in children or a standard 100 grams of glucose in adults) to drink. Blood samples are drawn at timed intervals and tested for GH to see if the pituitary gland is sufficiently suppressed by the dose of glucose.
Other tests that may be done include:
- IGF-1—growth hormone stimulates the production of insulin-like growth factor-1 (IGF-1). IGF-1 is a hormone that carries out many of the effects of GH and helps promote normal bone and tissue growth and development. However, unlike GH, its level is stable in the blood throughout the day. This makes IGF-1 a useful indicator of average GH levels and the IGF-1 test is often used to help evaluate GH deficiency or GH excess.
- IGFBP3—growth hormone also stimulates the production of IGFBP3 (insulin-like growth factor binding protein 3). The circulating level of this protein is also used as a measure of growth hormone action/activity and is especially useful in children younger than 3 years of age.
GH suppression testing and IGF-1 testing can also be used to monitor treatment of a pituitary tumor that produces excess GH. If a tumor is located, GH and IGF-1 levels can be measured after the tumor’s removal to determine whether the entire tumor was successfully removed. Tests may be ordered at regular intervals for years afterward to monitor GH production and to detect tumor recurrence.
When is it ordered?
GH stimulation testing is ordered for a child with signs and symptoms of growth hormone deficiency, such as:
- A growth rate that slows down in early childhood
- Shorter stature than other children of the same chronological age despite normal or enhanced weight gain
- Delayed puberty
- Delayed bone development (as seen in X-rays)
Stimulation testing may be ordered for an adult when there are signs and symptoms of growth hormone deficiency and/or hypopituitarism, such as:
- Decreased bone density
- Adverse lipid changes, such as high cholesterol
- Reduced exercise tolerance
Other hormone testing, such as thyroid testing, is typically done first to rule out other conditions that may cause similar symptoms. GH deficiency is rare in children and adults.
GH suppression testing is not commonly ordered but may be done when children or adults show signs and symptoms of GH excess (gigantism and acromegaly).
GH and IGF-1 testing may be ordered at regular intervals for many years to monitor for excess GH.
What does the test result mean?
GH stimulation test
If GH levels are not significantly stimulated during a GH stimulation test (they stay lower than they should) and you have signs and symptoms of growth hormone deficiency (including a low IGF-1 and/or IGFBP3), then it is likely that you have a GH deficiency that your healthcare practitioner may treat.
If your TSH and/or T4 level is abnormal, then that will likely be addressed first as thyroid disorders can cause symptoms similar to growth hormone deficiency. You may also have hypopituitarism and/or a more general decrease in pituitary function. GH testing for GH deficiency should not be performed until your thyroid function has been evaluated. If hypothyroidism is present in a child, it should be treated and the child’s growth rate evaluated before GH testing is considered.
If you exercise vigorously and does not experience an increase in GH levels, then you may have a GH deficiency. This finding would need to be followed up with further testing.
Children diagnosed with GH deficiency who have been receiving GH medications during childhood will need to be retested when they have completed growing.
GH suppression test
If your GH levels are not significantly suppressed during a GH suppression test (they stay higher than they should) and you have signs and symptoms of excess GH (gigantism or acromegaly) and a high IGF-1 level (if measured), then it is likely that you are producing too much GH. If a mass is found on an imaging scan (usually MRI), then a pituitary tumor (usually benign) is likely present. If you are being monitored after successful treatment of a tumor, then increases in GH may indicate a recurrence.
In addition to growth hormone testing, what other laboratory tests might be done?
Other blood tests that may be used to evaluate pituitary gland function include prolactin, free T4 or total T4, TSH, cortisol, FSH, LH, and/or testosterone. These tests are usually performed prior to GH testing to make sure that they are normal and/or controlled with medication before GH testing is done. For example, hypothyroidism must be treated prior to testing for GH deficiency in children; otherwise, a falsely low GH result may be seen.
Other tests, such as insulin-like growth factor binding protein-3 (IGFBP-3), are sometimes ordered to help evaluate GH production.
How are abnormal GH levels treated?
Abnormal GH levels can usually be treated once the causes are identified.
- Synthetic GH is available to treat deficiencies.
- Combinations of surgery, medication, and radiation can be used to treat pituitary tumors that cause excess GH.
It is important to identify GH excess or deficiency as soon as possible for good outcomes. If left untreated, a child with GH deficiency will continue to have a short stature compared to peers. Likewise, the bone growth changes associated with gigantism and acromegaly are permanent.
There can be long-term complications from GH abnormalities. Acromegaly, for instance, can cause colon polyps (increasing your risk of developing colon cancer), diabetes, high blood pressure, and problems with vision. If a pituitary tumor permanently damages pituitary cells, then multiple hormone replacement therapies may be necessary. Increased bone growth may also lead to trapped nerves (carpal tunnel syndrome), arthritis, and weak bones.
Is there anything else I should know?
Factors that can interfere with GH testing include:
- Drugs that can increase GH (e.g., amphetamines, arginine, dopamine, estrogens, glucagon, histamine, insulin, levodopa, methyldopa, and nicotinic acid)
- Drugs that can decrease GH levels (e.g., corticosteroids and phenothiazines)
It should be noted that most cases of short stature are not due to GH deficiencies. They can also be related to familial traits, a variety of diseases and conditions, and to other genetic disorders. Rarely, a person may have symptoms associated with GH deficiency but normal or elevated GH levels due to an inherited resistance to GH. In cases of GH resistance, whereas GH levels are high and IGF-1 levels are low.
What conditions are treated with growth hormone therapy?
Besides growth hormone deficiency (GHD), children may be treated with growth hormone replacement if they have:
- Chronic kidney disease
- Prader Willi syndrome (for information, see Genetics Home Reference: Prader Willi syndrome)
- Turner syndrome
- Noonan’s syndrome
- Small for gestational age with failure to display catch-up growth
- Idiopathic short stature