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Also known as: Somatomedin C
Formal name: Insulin-like Growth Factor - 1
Related tests: Growth Hormone, Growth Hormone Releasing Hormone, Glucose Tests, Glucose Tolerance Test, Free T4, TSH, IGFBP-3

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The Test Sample

What is being tested?

Insulin-like growth factor-1 (IGF-1) is a hormone that, along with growth hormone (GH), helps promote normal bone and tissue growth and development. The test measures the amount of IGF-1 in the blood.

IGF-1 is produced by the liver and skeletal muscle as well as many other tissues in response to GH stimulation. IGF-1 mediates many of the actions of GH, stimulating the growth of bones and other tissues and promoting the production of lean muscle mass. Since GH is released into the blood in pulses throughout the day, it is difficult to interpret the results from a single GH test. IGF-1 mirrors GH excesses and deficiencies, but unlike GH, its level is stable throughout the day. This makes IGF-1 a useful indicator of average GH levels. The IGF-1 test is therefore often used to help evaluate for GH deficiency or GH excess.

IGF-1 levels, like GH, are normally low in early childhood, increase gradually during childhood, peak during puberty, and then decline in adult life. Deficiencies in GH and IGF-1 may be caused by a dysfunctional pituitary gland with decreased pituitary hormones (hypopituitarism) or by the presence of a non-GH-producing pituitary tumor that damages hormone-producing cells. Deficiencies in IGF-1 also occur where there is a lack of responsiveness to GH. This insensitivity may be primary (genetic) or secondary to conditions such as malnutrition, hypothyroidism, sex hormone deficiency, and chronic diseases. Genetic GH insensitivity (GH resistance) is very rare.

IGF-1 deficiencies early in life, usually the result of GH deficiency, can inhibit bone growth and overall development and can result in a child with a shorter than normal stature. In adults, decreased production can lead to low bone density, less muscle mass, and altered lipid levels. However, testing for IGF-1 deficiency, or GH deficiency, is not routine in adults who have decreased bone density and/or muscle strength or increased lipids. GH deficiency and consequent IGF-1 deficiency is a very rare cause of these disorders.

Excess GH and IGF-1 can cause abnormal growth of the skeleton and other signs and symptoms characteristic of two rare conditions, gigantism and acromegaly. In children, gigantism causes bones to grow longer, resulting in a very tall person with large feet and hands. In adults, acromegaly causes bones to thicken and soft tissues, such as the nose, to swell. Both conditions can lead to enlarged organs, such as the heart, and to other complications such as type 2 diabetes, increased risk of cardiovascular disease, high blood pressure, arthritis, and a decreased lifespan.

The most common reason for the pituitary to secrete excessive amounts of GH is a GH-producing pituitary tumor (usually benign). Frequently, the tumor can be surgically removed and/or treated with drugs or radiation. In most cases, this will cause GH and IGF-1 levels to return to normal or near normal levels.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

In general, no test preparation is needed; however, since this test may be performed at the same time as others, fasting for at least 12 hours may be required.