Hormones
IGF-1
Growth factor from the liver that shows how much growth hormone is active on average.
Everything about this testWhat it measures
IGF-1 (insulin-like growth factor 1, also called somatomedin C) is a protein hormone made mainly in the liver under the influence of growth hormone from the pituitary gland. Together with growth hormone, it regulates the growth of bones and muscles. Growth hormone is released in peaks and varies strongly from moment to moment, while IGF-1 is much more stable over the day; this makes IGF-1 a better picture of the average activity of growth hormone. The value depends strongly on age: high during puberty and then gradually lower.
- Acromegaly: too much growth hormone, usually due to a benign tumor of the pituitary gland
- A shortage of growth hormone, for example due to damage to the pituitary gland after surgery or radiotherapy, or due to a pituitary tumor
- Monitoring of treatment with growth hormone or of treatment for acromegaly
- Use of growth hormone or of substances that stimulate the release of growth hormone
High
A high IGF-1 is normal during puberty and pregnancy. In adults, a raised value is often due to the use of growth hormone or of substances that stimulate its release. A value that is and stays clearly too high for your age may point to acromegaly: too much growth hormone due to a usually benign tumor of the pituitary gland. The symptoms develop slowly over years: larger hands and feet (rings and shoes that get tight), coarsening facial features, heavy sweating, joint and muscle complaints, snoring, headache and tingling in the fingers.
Low
IGF-1 gradually falls with age; a low value in older people is usually normal. Malnutrition, rapid weight loss, a poorly working liver, an untreated underactive thyroid, poorly controlled diabetes and taking estrogens can also lower the value. A low value may fit a shortage of growth hormone due to a damaged pituitary gland, with symptoms such as less energy, less muscle strength, a changed body composition and bone loss. A low value does not prove that shortage and a normal value does not rule it out: that requires a stimulation test in hospital.
If your result is abnormal
With a value that is too high for your age and symptoms that fit, you are referred to an internist-endocrinologist for a glucose drink test and, if needed, an MRI of the pituitary gland. With a low value and a known pituitary condition, additional hormone testing follows and possibly a stimulation test in hospital. With a slightly abnormal value without symptoms, the GP first looks at age, diet, illnesses and medicines and repeats the measurement.
Reliability
In adults, IGF-1 is the usual first blood test when acromegaly is suspected, and it is used to monitor treatment with growth hormone or for acromegaly. A raised value is confirmed with a glucose drink test: in healthy people, growth hormone falls after sugar, in acromegaly it does not. For showing a shortage, IGF-1 is less reliable; that requires a stimulation test. As a measure of aging or fitness in people without symptoms, IGF-1 is not supported by evidence.
Limitations
- Age: the reference values differ strongly by age group; a result can only be interpreted against the limits for that age.
- Measuring methods give different results from each other and there is no general table of reference values; preferably compare results from the same laboratory.
- Use of growth hormone, IGF-1 or substances that stimulate the release of growth hormone raises the value; estrogens taken by mouth lower it.
- Malnutrition, liver disease, an untreated underactive thyroid and poorly controlled diabetes lower the value; puberty and pregnancy raise it.
How common is this
Acromegaly is rare: in the Netherlands, it is diagnosed in about 50 to 100 people per year. Because the symptoms develop slowly, it often takes years before the disease is recognized. A shortage of growth hormone in adults mainly occurs in people with a known pituitary condition or after pituitary surgery or radiotherapy.
Who is this useful for
Useful with symptoms that fit acromegaly, such as slowly enlarging hands and feet, coarsening facial features and heavy sweating, and in people with a known pituitary condition or after pituitary surgery or radiotherapy. Also useful for monitoring treatment with growth hormone. Not useful as a measure of aging, fitness or muscle building in healthy people: the value is not supported by evidence for that, and a low value is no reason for treatment with growth hormone.
Downsides and risks
- A slightly abnormal value without symptoms can lead to unnecessary worry about the pituitary gland and to unnecessary tests such as an MRI.
- A low value is sometimes taken as a reason to use growth hormone; use without a proven shortage has no medical basis and is on the doping list.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results