To help diagnose growth hormone (GH) deficiency or, less commonly, growth hormone excess; to evaluate pituitary function; to monitor the effectiveness of treatment for excess production of GH
When a child has slow growth, short stature, and delayed development or sometimes when an adult has decreased bone density and/or muscle strength and elevated blood lipid levels that could be related to insufficient GH production; when a child or adult has signs and symptoms suggestive of GH excess, which can cause two rare conditions, gigantism and acromegaly, respectively; when a pituitary disorder is suspected and other pituitary hormone levels are abnormal
Usually several blood samples, drawn at timed intervals from veins in your arms; sometimes a single sample of blood, drawn following a fast and rest, after a period of strenuous exercise, or after ingesting a glucose (sugar) solution
Follow any instructions you are given, such as to fast, in order to prepare for this test.
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.
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.
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.
- How is it used?
Growth hormone (GH) testing is primarily used to identify 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 essential for normal growth and development in children and helps regulate metabolism in both children and adults. The pituitary gland produces and releases GH into the blood in pulses throughout the day. (For more, see the "What is being tested?" section.)
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.
- 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, a person is 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. The most commonly used stimulant is arginine, but others include clonidine and glucagon. 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. For a suppression test, a sample of blood is drawn after 10-12 hours of fasting. A person is then given a standard glucose solution (usually 100 grams of glucose) 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.
- Growth hormone stimulates the production of insulin-like growth factor-1 (IGF-1). IGF-1 is a hormone that mediates 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.
- A GH suppression test and IGF-testing can also be used to monitor treatment of a GH-producing pituitary tumor. 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.
Other blood tests that may be used to evaluate pituitary gland function include prolactin, free T4, TSH, cortisol, FSH, LH, and 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, hypothryoidism must be treated prior to testing for GH deficiency in children; otherwise, a falsely low GH result may be seen.
- When is it ordered?
GH stimulation testing is ordered for a child when there are signs and symptoms of growth hormone deficiency (GHD), such as:
- A growth rate that slows down in early childhood
- Shorter stature than other children of the same chronological age
- 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 GHD 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. It is possible in adults if GH deficiency was diagnosed in childhood or there is a history of hypothalamic or pituitary disease.
GH suppression testing is uncommon but may be done when children or adults show signs and symptoms of GH excess (gigantism and acromegaly).
Suppression testing may be done when a pituitary tumor is suspected of causing excess GH and may sometimes be used along with IGF-1 levels and other hormone levels to monitor the effectiveness of treatment for these conditions. GH and IGF-1 testing may be ordered at regular intervals for many years to monitor for recurrence of GH abnormalities in cases of GH excess.
- 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 the person has signs and symptoms of GHD (and a low IGF-1 level, if it is measured), then it is likely that there is a GH deficiency that the health practitioner may treat.
If the person's TSH and/or T4 level is abnormal, then that will likely be addressed first as thyroid disorders can cause symptoms similar to GHD. A person may also have hypopituitarism and/or a more general decrease in pituitary function. GH testing for GH deficiency should not be performed until a person's 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 a person exercises vigorously and does not experience an increase in GH levels, then they may have a GH deficiency. This finding would need to be followed up with further testing.
GH suppression test
If a person's GH levels are not significantly suppressed during a GH suppression test (they stay higher than they should) and the person has signs and symptoms of excess GH (gigantism or acromegaly) and a high IGF-1 level (if measured), then it is likely that the person tested is producing too much GH. If a mass shows up on an X-ray, CT scan, or MRI, then a pituitary tumor (usually benign) is likely present. If someone is being monitored for a previous tumor, then increases in GH may indicate a recurrence.
Pituitary tumors are the most common cause of excess GH production, but they may also cause deficiencies. The presence of a pituitary tumor can affect not only GH production but may also affect production of other pituitary hormones, such as ACTH (Cushing syndrome) or prolactin. If the tumor is relatively large, it may inhibit all pituitary hormone production and cause damage to the surrounding tissues.
- Is there anything else I should know?
Since GH is released by the pituitary in bursts, measuring GH levels in samples collected at random times is not very useful. There is too much overlap between abnormal results and normal daily variation. GH levels will usually be higher first thing in the morning and will increase with exercise and stress.
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)
Abnormal GH levels can usually be modified once the causes are identified. Synthetic GH is available to alleviate deficiencies in children (treatment of adults with GHD is more controversial). Combinations of surgery, medication, and radiation can be used to treat pituitary tumors that are causing excess GH production. The important thing is to identify GH abnormalities 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 a person's risk of developing colon cancer), diabetes, high blood pressure, and visual abnormalities. If a pituitary tumor permanently damages pituitary cells, then multiple hormone replacement may be necessary. Increased bone growth may also lead to trapped nerves (carpal tunnel syndrome), arthritis, and weak bones.
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, 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
Treating children with GH replacement who are short but do not have GHD is controversial. This is also true of adults, whether or not they have documented GHD. The medicine has associated risks and side effects and is expensive. Furthermore, there are not enough data to support its benefits.
GH replacement is sometimes given to those with HIV/AIDS-related wasting (loss of lean muscle mass) to help maintain body weight.
- Why would athletes be tested for GH?
Because GH promotes muscle growth in adults, it may be taken by some adults as a performance-enhancing agent. Athletes may be tested for GH or IGF-1 when they are being tested for other performance-enhancing drugs (see the article on Drugs of Abuse for more on sports-related testing).
- What other tests might be ordered to evaluate my GH production?
Although they are not routinely ordered, other tests, such as insulin-like growth factor binding protein-3 (IGFBP-3), are sometimes ordered to help evaluate GH production.