Summary

• Growth hormone (GH) controls your body's growth. It also has an important role in many body systems including metabolism, brain function and cell repair.
• GH is made by the pituitary gland at the base of your brain and is regulated by a complex feedback system of hormones and glands. Many other hormones interact with GH.
• Health conditions caused by either low or high levels of GH occur in both children and adults, but they are rare.
• Assessing GH is usually done using tests that either stimulate the pituitary gland to make GH or suppress the amount of GH in circulation.
• GH works together with insulin-like growth factor 1 (IGF-1) and usually this is tested at the same time.

What is growth hormone (GH)?

Growth hormone (GH) is essential for normal growth and development. It induces growth in nearly every tissue and organ in the body. Although better known for its role in building muscle and bone, it affects many body systems.

GH is integral to metabolism - the chemical processes that change the food we eat into energy - and as part of this affects glucose and lipid (cholesterol and triglycerides) levels, as well as the distribution of body fat. It is also important for brain development and function, and is needed for cell repair and recovery, helping to heal injuries and maintain healthy tissues.

GH acts at all stages of life. Levels are low in infancy, increasing gradually during childhood, peaking at puberty, and then declining as we get older. Adults still need GH after they have stopped growing and it continues to have an important influence on body systems.

How GH levels are controlled

GH is made by the pituitary gland which sits at the base of your brain just behind your eyes. The amount of GH that is made and released into your body is tightly controlled by a complex feedback system of hormones and glands in response to exercise, sleep, stress, nutrition and the levels of GH already circulating in your body.

Hormones are chemical messengers that are made by your glands and travel in your bloodstream to control the actions of specific cells in your tissues or organs. When they reach their target, they attach to a cell’s receptors, stimulating a response.

• Your pituitary gland is controlled by a part of the brain called the hypothalamus. The hypothalamus and the pituitary are connected by a stalk of blood vessels and nerves.
• Your hypothalamus makes growth hormone-releasing hormone (GHRH) that signals to your pituitary to make GH.
• In response, your pituitary gland makes and releases GH into your bloodstream throughout the day. It does this in pulses.
• As GH levels in your blood rise, your liver responds by making insulin-like growth factor 1 (IGF-1). This growth factor acts on your tissues to make them grow.
• Levels of GH and IGF-1 are kept in balance by way of a negative feedback system - As IGF-1 levels rise, GH levels fall.
• Your stomach and GI tract make a hormone called somatostatin which acts like a brake, stopping the body from making more GH.
• Other hormones such as thyroxine (T4) made by your thyroid gland, or glucocorticoids and ghrelin which are made in your stomach, stimulate the release of GH.
• In this way, GH, IGF-1 and somatostatin regulate each other and keep levels in balance. This tightly controlled system helps ensure normal growth and development, as well as healthy metabolism, and cellular repair.

Measuring IGF-1

GH works together with IGF-1 which directly acts on the tissues to stimulate cells to grow.

Since GH is released into the blood in pulses throughout the day, it is difficult to accurately interpret the results from a single GH test. IGF-1 follows GH levels, and unlike GH, its level is stable throughout the day. This means the IGF-1 test is often used to help assess GH levels.

IGF-1 has a similar structure to insulin and plays a role in glucose regulation. Insulin controls the uptake of glucose into cells while IGF-1 causes cells to grow and increase their metabolism. IGF-1 increases cells sensitivity to insulin which has a glucose lowering effect.

GH and growth in children

Many hormones work together with GH and one of these is the thyroid hormone thyroxine (T4). T4 is made and released by the thyroid gland, located just at the base of the neck. It too is controlled by the hypothalamus. T4 is important for height gain in children. The sex hormones oestradiol and testosterone—from the ovaries and testicles—are responsible for the growth spurt during puberty.

Why get tested?

There are several conditions associated with having either too little or too much GH.

Low levels of GH

Growth hormone deficiency (GHD): This is a rare condition in which the body does not make enough GH. It can be due to problems with the pituitary gland, which can be present from the time a baby is born (congenital) or occur during childhood (acquired). Acquired GHD can also be due to pituitary gland damage later in life which can be caused by any process that injures the pituitary gland or the surrounding brain area, such as tumours, surgery or radiation.

Most childhood-onset GHD is termed idiopathic, meaning there is no known cause. There are many reasons for slow growth and below-average height in children. Often these resolve themselves.

Growth hormone resistance: Sometimes, GH production and circulation are normal, but the body does not respond. Being born small-for-gestational age is one condition which may lead to GH resistance, but some rare genetic conditions can also lead to this.

Hypothyroidism: Underactive thyroid function can cause a slow-down in height growth rate.

Cortisol: Having too much cortisol, a hormone made in the adrenal gland, can lead to stunted height growth and rapid weight gain in children. This can happen following treatment with high doses of cortisol-like steroids or through a rare condition called Cushing syndrome.

High levels of GH

Acromegaly is a rare but serious medical condition that happens when you have too much GH in your body. Acromegaly affects your body’s bones and tissues and causes them to grow in irregular ways. Initially the symptoms may be skin thickening, sweating, fatigue and headaches but these can go onto enlargement of hands and the feet as well as internal organs. In children excessive GH secretion leads to gigantism. The most common cause is a pituitary tumour.

Testing

If your doctor suspects that you or your child’s symptoms are due to a problem with GH levels, they will first order tests for the various hormones that can be involved or interact with GH. These include:

• thyroid stimulating hormone (TSH)
• T4
• Adrenocorticotropic hormone (ACTH)
• prolactin
• cortisol
• follicle stimulating hormone (FSH)
• luteinising hormone (LH)
• oestradiol (in females) and testosterone (in males).

The results of these tests will ensure that other hormones are working correctly before GH testing is done. For example, if your thyroid gland is not working normally and you have hypothyroidism – not enough T4 or thyroxine - this may lead to a falsely low GH level.

Assessing whether your pituitary gland is working properly or not is usually done by what is called provocative testing. This involves either a test to stimulate the pituitary or a test to supress the pituitary. Since GH is released in pulses throughout the day, a single random measurement isn't useful. Instead, your doctor will use a stimulation or suppression test to assess GH levels over a period of time.

Testing for too little GH

If your doctor suspects a deficiency of GH, you may be asked to have a GH stimulating test. This checks to see if your pituitary gland can produce GH when stimulated.

Various substances can be used for this. After an overnight fast, a sample of your blood is taken and then an intravenous solution of insulin, glucagon, arginine or GHRH is given which should stimulate your pituitary to produce GH. Blood samples are taken at timed intervals, with GH and IGF-1 levels measured to see if your pituitary gland has been stimulated to produce the expected levels of GH.

Testing for too much GH

If your symptoms suggest that you are making too much GH (such as in acromegaly) your will be asked to have a GH suppression test. After an overnight fast, you will be given a glucose drink and at various timed intervals, your glucose, GH and IGF-1 will be measured.

In a healthy person, the GH and IGF-1 levels will be supressed by glucose – this is a normal metabolic process. But if you have a pituitary tumour or another cause of excessive GH secretion, your GH and IGF-1 levels will remain higher for longer. Following these results, you are likely to have radiology investigations to see if a tumour is present.

Having the test

Sample

Blood

Any preparation?

Fasting is necessary before suppression and stimulation tests.

Your results

Reading your test report

Your results will generally be presented along with those of your other tests on the same form. You will see separate columns or lines for each of these tests.

Random GH measurements are not used to investigate possible abnormalities in GH secretion. Depending upon your symptoms and clinical features your doctor will decide whether a stimulation or suppression test is required.

Reference Intervals

Your results will be compared to reference intervals (sometimes called a normal range).

• Reference intervals are the range of results expected in healthy people
• When compared against them your results may be flagged high or low if they sit outside the range.
• Reference ranges may vary between different age groups.
• Reference intervals may vary between labs so only those results that are standardised or harmonised across most laboratories are given on this website.

If your results are flagged as high or low this does not necessarily mean that anything is wrong. Your doctor needs to take your medical history to help interpret your result.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods, medications or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record. You and your healthcare provider can now access your results whenever and wherever needed.

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