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 childre; 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 stimulation tests help diagnose growth hormone deficiency (GHD) in children and adults.
- GH suppression tests help diagnose in children and in adults. Along with other blood tests and imaging scans, they help identify and locate pituitary
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:
- A child's growth rate slows down in early childhood and he or she is significantly shorter than others of the same age
- Delayed puberty
- X-rays show delayed bone development
- A doctor suspects that a child’s pituitary gland is under-active.
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 test result mean?
GH Stimulaton 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 treatd 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 sumptoms 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:
- Stress, exercise, and low blood glucose levels
- Drugs that can increase GH include: amphetamines, arginine, dopamine, oestrogens, glucagon, histamine, insulin, levodopa, methyldopa and nicotinic acid
- Drugs that can decrease GH levels: 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 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 shoud 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.