What is being tested?
Growth hormone (GH) is produced by the anterior pituitary gland, a grape sized organ found at the base of the brain. Essential for a child’s normal growth and development, GH promotes proper linear bone growth from birth through to puberty. Children with insufficient GH levels grow more slowly and are smaller in size for their age, one of the first symptoms of growth hormone deficiency (GHD). It should be noted that short stature can also be related to familial traits or other genetic disorders. Excess GH is most often due to a GH-secreting pituitary tumour (usually benign). Too much GH can cause a child’s long bones to continue to grow beyond puberty, resulting in a condition called gigantism. Those with excess GH may also have thickening of facial features, general weakness, delayed puberty and headaches.
Although GH is not as active in adults, it does play a role in regulating bone density, muscle mass and lipid metabolism. Deficiencies can lead to decreased bone densities, less muscle mass and altered lipid levels. Excess GH in adults can lead to condition called acromegaly, marked 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. If untreated, acromegaly (and gigantism in children) can lead to complications such as type 2 diabetes, increased cardiovascular disease risk, high blood pressure, arthritis, and in general, a decreased life span.
How is it used?
GH testing is usually provocative using either a GH stimulation test or a GH suppression test to track GH levels over time. GH testing may be used to test for abnormal pituitary function and to help diagnose the condition causing the abnormality, its severity, and the complications that have arisen because of it.
Often other blood tests that reflect pituitary function, such as T4, TSH, cortisol, FSH, LH, and testosterone (in men), are also ordered. 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.
Glucose levels are run on the samples collected during the GH suppression test, both to track glucose levels and to make sure that the patient’s system is sufficiently challenged by the glucose solution.
IGF-1 (Insulin-like growth factor–1) is often measured before or during GH provocation testing and can be used by itself or with GH as a monitoring tool. Produced in the liver, IGF-1 mirrors GH excesses and deficiencies, but its level is stable throughout the day, making it a useful indicator of average GH levels.
GH testing is usually ordered on those with symptoms of growth hormone abnormalities or as a follow-up to other abnormal hormone test results. It is not recommended for general screening. GH tests may be ordered to help evaluate pituitary function:
Periodic GH measurements may also be performed on children who have had radiation treatment of the central nervous system or whole body irradiation prior to stem cell transplants. This is common in acute lymphoblastic leukaemia (ALL), where irradiation of the brain can affect the hypothalamus and pituitary gland, thus affecting growth.
Growth hormone tests help identify excess and diminished GH production and give your doctor information about the severity of your condition and are part of the diagnostic work-up to find the reason for abnormal hormone production.
GH and IGF-1 levels are often monitored for extended periods of time following treatment for GH deficiency, gigantism and acromegaly, and are monitored following surgery, drug treatment, and/or radiation therapy for a pituitary tumour.
When is it requested?
GH stimulation testing is ordered when a child has symptoms of growth hormone deficiency (GHD), such as when:
Once GHD is diagnosed, your doctor may use stimulation testing to confirm the diagnosis, along with IGF-1 to monitor the effectiveness of GH replacement (if indicated), and as a child reaches adulthood to see if continued supplementation is necessary. GH levels are also monitored in children who have received radiation therapy.
Stimulation testing is ordered in adults when patients have symptoms of GHD and/or hypopituitarism, such as decreased bone density, fatigue, adverse lipid changes, and reduced exercise tolerance. Other hormone testing is done first to rule out other conditions that may cause similar symptoms.
GH suppression testing is done when children show signs of gigantism, when adults show signs of acromegaly, and/or when their doctor suspects hyperpituitarism. Suppression testing may be done when a pituitary tumour is suspected and may be used along with IGF-1 levels and other hormone levels to monitor the effectiveness of treatment for these conditions. Monitoring may continue at regular intervals for many years to watch for recurrence.
Since GH is released by the pituitary in bursts, random GH levels are not very useful. There is too much overlap between abnormal results and normal daily variation. GH levels will be higher first thing in the morning and will increase with exercise and stress.
What does the 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 shows signs and symptoms of GHD ( and a low IGF-1 level, if it is measured), then it is likely that there is GH deficiency that the doctor may treat.
If the person's TSH and/or T4 level is abnormal, then that will likely be addressed first as thyroid disorders can cuase 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 funcion 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 pitutary tumour (usually benign) may be present. If someone is being monitored for a previous tumour, then increases in GH may indicated a recurrence.
Is there anything else I should know?
Pituitary tumours are the most common cause of excess GH production but may also cause deficiencies. If the tumour produces another of the pituitary hormones, such as ACTH or prolactin, it may inhibit GH secretion. If the tumour is relatively large, it may inhibit all pituitary hormone production and cause damage to the surrounding tissues. Some other conditions also cause general hyper- or hypopituitarism. These may be genetic, due to disease, or resulting from trauma.
Factors that can interfere with GH testing include:
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 tumours that are causing excess GH production. The important thing is to identify GH abnormalities as soon as possible to improve patient outcomes. The bone growth changes associated with gigantism and acromegaly are permanent and, if left untreated, the GH deficient child’s short stature will remain.
There can be long term complications from GH abnormalities. Acromegaly, for instance, can cause colon polyps (increasing a patient’s risk of developing colon cancer), diabetes, high blood pressure, and visual abnormalities. If a pituitary tumour 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.
Common questions
Besides growth hormone deficiency (GHD), children may be treated with growth hormone replacement if they have:
Treating children with GH replacement who are short but do not have GHD is controversial. Treating adults with GH replacement is also controversial, whether or not they have documented GHD. The medicine has associated risks and side effects, is expensive, and there are not enough data to support its benefits in these groups of people.
GH replacement is sometimes given to those with HIV/AIDS-related wasting (loss of lean muscle mass) to help maintain body weight.
Other tests, such as insulin-like growth factor-1, binding protein-3 (IGFBP-3), and growth hormone releasing hormone (GHRH) are sometimes ordered to help evaluate GH production.
GH may be taken by some adults as a performance enhancing steroid because GH promotes muscle growth in adults. Athletes may be tested for GH when they are being tested for other performance enhancing drugs.
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