Growth hormone (GH) is produced by the pituitary, situated at the base of the brain, behind the bridge of your nose and has growth promoting properties. It is measured to check if there is under or overproduction. In addition it is used to examine the function of the pituitary and to monitor the effectiveness of treatment.
The evaluation of GH status is based on clinical findings, medical history, imaging and biochemical tests. Slow growth in height and delayed development (in children),whilst decreased bone density and/or muscle strength, and increased lipids (in adults) could all be related to insufficient GH production. Symptoms suggestive of gigantism in children or acromegaly in adults may be a result of excess GH production. It is also measured as part of an evaluation of pituitary function.
After an overnight fast, several blood samples are taken at timed intervals from veins in your arm, as part of a stimulation or suppression test. Pre-adolescents require priming prior to performing a stimulation test. A sample is usually taken for measurement of insulin-like growth factor-1 (IGF-1) on the baseline sample. When monitoring treatment for GH excess a single sample of blood may be drawn following a fast.
In healthy adults GH is released in bursts throughout the day, it rises sharply 3-4 hours after a meal and within 60 minutes after the onset of sleep making random GH results in general uninterruptable. GH may be measured after stimulation or suppression testing. Fasting levels are used to monitor treatment for GH excess.
GH is needed for a child’s normal growth and development. It promotes growth of the long bones from birth through puberty. Children with insufficient GH production grow more slowly and are small 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. Constitutional delay (i.e. temporary delay in growth of no obvious cause) is the most common cause of short stature in childhood.
An excess of GH is most often due to a benign GH-secreting pituitary tumour i.e. a tumour that has not spread to other tissues, although larger tumours can have other effects e.g. headaches and impaired vision Gigantism is a disorder resulting from long-term secretion of too much GH, which increases the growth of muscle, bones and connective tissue in childhood or adolescence before the end of puberty. This results in a child becoming excessively tall (e.g. over 2.1 meters). Children with excessive GH production may also have thickening of their facial features, general weakness, delayed puberty, and headaches.
In adults GH plays a role in regulating bone density, muscle mass, and lipid metabolism. Deficiencies can lead to decreased bone densities, lower muscle mass, and altered lipid concentrations. Excess GH in adults can lead to acromegaly, with bone and skin thickening. Symptoms such as sweating, fatigue, headaches and joint pain can be subtle at first. Increased GH concentrations can lead to enlargement of the hands, feet, facial bones and internal organs and carpal tunnel syndrome (trapped nerves). If untreated, acromegaly in adults 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.
GH stimulates the secretion of the true growth factors, most notably IGF-1. The concentration of which represents the secretion of GH in the previous few days. This is measured prior to stimulation or suppression tests used to diagnose GH abnormalities and to monitor treatment.
How is the sample collected for testing?
A GH suppression or stimulation tests is performed after fasting for 10 to 12 hours when a blood sample is taken from a vein in the arm. Under medical supervision, a standard glucose solution is given to the patient to drink (for a suppression test), or an intravenous (IV) injection of a solution of insulin, glucagon, arginine, clonidine or GH releasing hormone (GHRH, for a stimulation test) is given through a vein in your arm. Blood samples are then taken from your veins at timed intervals. GH is measured on each sample collected to look at the change in levels over time. Sometimes it is necessary to perform a second test.
During treatment for growth hormone excess a sample of blood may be taken, following a fast, to monitor growth hormone production.
Is any test preparation needed to ensure the quality of the sample?
Samples collected for stimulation or suppression tests are collected after fasting. Pre-adolescents are usually primed with sex steroids prior to a stimulation test.
How is it used?
GH testing is not used as a screening test. It is largely requested in those with symptoms of growth hormone abnormalities, as a follow-up to other abnormal hormone test results, or to help understand how the pituitary gland is working. Where biochemical tests give abnormal results they are used in conjunction with imaging scans to help diagnose and locate a pituitary tumour.
Several GH measurements, as part of a stimulation test, 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 treatment with radiation can affect the hypothalamus and pituitary gland, thus affecting growth.
GH stimulation tests help identify lower GH production and gives your doctor information about the severity of your condition. They also help to diagnose hypopituitarism (where there is low or absent production of hormones(s). A sample of blood is taken after you have fasted for 10-12 hours. Then, under close medical supervision, you are given an intravenous solution of insulin, glucagon, arginine or GHRH. Blood samples are taken at timed intervals and GH levels are measured in each to see if your pituitary gland was stimulated to produce the expected levels of GH.
For a suppression test, a sample of blood is taken after a10-12 hours fast. After this a standard glucose solution is given to the patient to drink. Blood samples are taken at timed intervals and GH levels are tested in each to see if GH production from the pituitary gland is suppressed.
Often other blood tests that reflect pituitary function, such as T4, TSH, cortisol, FSH, LH, and testosterone (in men), are also requested. 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 but may also be measured as part of a stimulation test.
When is it requested?
GH stimulation testing is requested when your child has symptoms of GH deficiency (GHD), such as when:
- Your child's growth rate slows down in early childhood and (s)he is significantly shorter than others of similar age
- TSH tests show that your child is not hypothyroid (low thyroid concentrations in the bloodstream can also cause slowed growth)
- X-rays show delayed bone development
- Your doctor suspects that your child’s pituitary gland is underactive
- To monitor GH levels in children who have received radiation therapy.
Once GHD is suspected your doctor will use an IGF-1 and GH stimulation tests to confirm the diagnosis. To monitor treatment both measurement of IGF-1 and the pattern of your child’s growth will be looked at. Repeat testing for growth hormone deficiency may occur as a child reaches adulthood to see if continued replacement is necessary.
Stimulation testing is requested in adults when patients have symptoms of GHD. 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 as there is too much overlap between abnormal results and normal daily variation.
What does the test result mean?
GH suppression test:
If your GH levels are not significantly suppressed during a GH suppression test (i.e. they stay higher than they should) and:
- you have symptoms of gigantism or acromegaly
- other pituitary hormone levels are normal and/or controlled
- your IGF-1 levels are high
it is likely that you are producing too much GH and it is causing complications. If you have other pituitary hormones that are abnormal, then you may have a condition causing hyperpituitarism. If a mass shows up on an X-ray, CT scan, or MRI; then you may have a pituitary or very rarely another sort of tumour. If you are being monitored for a previous tumour, then you may be having a recurrence.
GH stimulation test:
If your TSH is low then that should be addressed first as thyroid deficiencies can cause symptoms similar to GHD.
If your GH levels are not significantly stimulated during a GH stimulation test (i.e. they stay lower than they should be) and:
- you have symptoms of GHD • other pituitary hormone levels are normal and/or controlled
- your IGF-1 level is low
then it is likely that you have a deficiency of GH and that your doctor may treat the symptoms. You may also have a more general decrease in pituitary function.
Is there anything else I should know?
Pituitary tumours are the most common cause of excess GH production but these tumours may also cause deficiencies of other pituitary hormones, such as ACTH (Cushing’s syndrome) or prolactin (galactorrhoea). If the tumour is relatively large it may inhibit all pituitary hormone production and cause damage to the surrounding tissues including the optic nerve.
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.
- A radioactive scan within a week of the test (with some laboratory methods)
Abnormal GH levels can usually be modified once the causes are identified. Synthetic GH is available to treat 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. It is important to identify GH abnormalities as soon as possible to get a good outcome. If left untreated, the GH deficient child will remain short.. 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 colonic 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.
What conditions are treated with GH therapy?
Besides GHD, children may be treated with growth hormone replacement if they have:
- Chronic renal insufficiency
- Prader-Willi syndrome
- Turner's syndrome
- Growth failure at 4 years or older and were born small for gestational age
- Short stature homeobox-containing gene (SHOX) deficiency
Treating children with GH replacement who are short but do not have GHD is controversial. This is also true of adults unless GHD has been diagnosed and other criteria met. This is because treatment has risks and side effects and existing data does not support its benefits.
Why would athletes be tested for GH?
What other tests might be requested to evaluate my GH production?