Growth Hormone Disorders: IGF-1 (Insulin-Like Growth Factor -1) Test

IGF-1 is measured to help diagnose the cause of growth abnormalities, evaluate pituitary function, and monitor treatment of GH deficiencies and excesses. It is not diagnostic of a GH deficiency but may be ordered along with GH stimulation tests to offer additional information. IGF-1 levels and the measurement of GH can also provide information related to GH insensitivity. Prior to performing definitive GH testing, if the IGF-1 level is found to be normal for age and sex, GH deficiency is excluded and definitive testing is not necessary.

IGF-1 may be ordered with other pituitary hormone tests, such as adrenocorticotropic hormone (ACTH), to help diagnose hypopituitarism. It may be used to monitor the effectiveness of treatment for growth hormone deficiencies and growth hormone insensitivity. However, IGF-1 is not routinely measured to monitor GH treatment of GH-deficient children. The best index of the effectiveness of GH treatment of children who are GH-deficient is observation of an increase in their height velocity and their absolute height.

IGF-1 testing and a GH suppression test can be used to detect a GH-producing pituitary tumor. An anterior pituitary tumor is then confirmed with imaging scans that help identify and locate the tumor. If surgery is necessary, GH and IGF-1 levels are measured after the tumor’s removal to determine whether all of the tumor was successfully removed. Drug and/or radiation therapy may be used in addition to, or sometimes instead of, surgery to try to decrease GH production and return IGF-1 to a normal or near normal concentration. IGF-1 may be ordered to monitor the effectiveness of this therapy at regular intervals for years afterward to monitor GH production and to detect tumor recurrence.

IGF-1 testing may be ordered, along with a GH stimulation test, when a child has symptoms of GH deficiency, such as a slowed growth rate and short stature. They also may be ordered when adults have symptoms that the doctor suspects may be due to a GH deficiency, such as decreased bone density, fatigue, adverse lipid changes, and reduced exercise tolerance. However, testing for IGF-1 deficiency is not routine in adults who have these symptoms; GH and IGF-1 deficiency are only very rare causes of these disorders.

An IGF-1 also may be ordered when a doctor suspects that someone has an underactive pituitary gland and at intervals to monitor those on GH therapy. However, IGF-1 is not routinely measured to monitor GH treatment of GH-deficient children. Again, the best index of the effectiveness of GH treatment of children who are GH-deficient is observation of an increase in their height velocity and their absolute height.

IGF-1 testing may be ordered, along with a GH suppression test, when a child has symptoms of gigantism or when an adult shows signs of acromegaly.

When a GH-producing pituitary tumor is found, GH and IGF-1 are ordered after the tumor is surgically removed to determine whether all of the tumor has been extracted. IGF-1 also is ordered at regular intervals when someone is undergoing the drug and/or radiation therapy that frequently follow tumor surgery.

IGF-1 levels may be ordered at regular intervals for many years to monitor a person’s GH production and to watch for pituitary tumor recurrence.

Interpretation

Normal
A normal concentration of IGF-1 must be considered in context. Some people can have a GH deficiency and still have a normal IGF-1 concentration.

Decreased IGF-1
If the IGF-1 concentration is decreased, then it is likely that there is a GH deficiency or an insensitivity to GH. If this is in a child, the GH deficiency may have already caused short stature and delayed development and may be treated with GH supplementation. Adults will have an age-related decrease in production, but lower than expected concentrations may reflect a GH deficiency or insensitivity.

If a decrease in IGF-1 is due to a more general decrease in pituitary function (hypopituitarism), then several of the person’s pituitary hormones will need to be evaluated and may be supplemented to bring them up to normal levels. Reduced pituitary function may be due to inherited defects or can develop as a result of pituitary damage following conditions such as trauma, infections, and inflammation.

Decreased levels of IGF-1 also may be seen with nutritional deficiencies (including anorexia nervosa), chronic kidney or liver disease, inactive/ineffective forms of GH, and with high doses of estrogen.

Increased IGF-1
Elevated levels of IGF-1 usually indicate an increased production of GH. Since GH levels vary throughout the day, IGF-1 concentrations are a reflection of average GH production, not of the actual amount of GH in the blood at the time that the sample for the IGF-1 measurement was taken. This is accurate up to the point at which the liver’s capacity to produce IGF-1 is reached. With severely increased GH production, the IGF-1 level will stabilize at an elevated maximum concentration.

Increased concentrations of GH and IGF-1 are normal during puberty and pregnancy but otherwise are most frequently due to pituitary tumors (usually benign). 

If IGF-1 is still elevated after the surgical removal of a pituitary tumor, then the surgery may not have been fully effective. Decreasing IGF-1 concentrations during subsequent drug and/or radiation therapies indicate that the treatment is lowering GH production. If levels of IGF-1 become “normalized,” then the person is no longer producing excess amounts of GH. When someone is undergoing long-term monitoring, an increase in IGF-1 levels may indicate a recurrence of the pituitary tumor.

If an IGF-1 level is normal and the doctor still strongly suspects a GH deficiency, then the doctor may order another test, an IGFBP-3 (insulin-like growth factor binding protein 3), to help confirm the GH deficiency. Almost all IGF-1 in the blood is bound to binding proteins, with IGFBP-3 being the most prevalent form.

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