IGF-1 Test: The Steady Stand-In for a Hormone Too Restless to Catch

Results are reported in ng/mL or µg/L, which are the same number. Because the expected value changes so steeply with age, MedlinePlus explains that results must be compared with a range for the person's age, and many labs also print a z-score (or SDS): how far the result sits from the average for that age, in standard deviations. A z-score near zero is average for your age, whatever the ng/mL figure looks like. A lab that prints a single adult range will flag many healthy older adults as low.

In an adult, a clearly high IGF-1 points first to acromegaly (gigantism when it starts in childhood), usually caused by a benign pituitary tumor making too much growth hormone. NIDDK describes it as rare, with an estimated 3 to 14 of every 100,000 people diagnosed, most often in middle age. The features are often recognized only in hindsight: larger hands and feet, larger lips, nose and tongue, a heavier brow and jaw, thick oily skin, more sweating and a deeper voice.

A worked example: if the upper limit for someone's age is 200 ng/mL, a result of 230 is borderline and usually repeated, while 280 is above the 1.3 × line of 260 and, with typical features, supports the diagnosis. The excess is then confirmed with an oral glucose tolerance test, in which growth hormone is measured every half hour for two hours after a sugary drink and fails to fall as it normally would. An MRI looks for the tumor.

Two other explanations come up often. Young people naturally have high levels around puberty and into their early twenties. And IGF-1 can be different between labs: reference ranges are specific to each method, and methods differ most at the highest values, so a borderline result at one lab can be within range at another.

In adults, a low IGF-1 is more often about age, nutrition or the liver than the pituitary, and MedlinePlus notes it may simply reflect the normal decline with age. In children, a low value raises the question of growth hormone deficiency, or of insensitivity, as in Laron syndrome, where growth hormone is made but the body cannot respond to it.

A normal IGF-1 does not rule out growth hormone deficiency, and a low one does not prove it. The diagnosis rests on a growth hormone stimulation test, in which a drug is given to prompt the pituitary and the peak response is measured, often along with other pituitary hormones such as ACTH and prolactin.

Not as far as the evidence shows. A 2022 meta-analysis in Aging Cell, pooling 19 studies and 30,876 people, found a U-shaped pattern: both high and low IGF-1 were linked to higher death rates, with the lowest rates between about 120 and 160 ng/mL. These are associations, not proof of cause, and the study's direct comparison of the highest group with the lowest was not statistically significant. Still, nothing in the data supports treating a bigger number as a goal, which is the premise of most supplements marketed to raise IGF-1. Anything taken for that purpose is worth discussing with a clinician first.

IGF-1 is used to monitor treatment as well as to diagnose, and repeat results only compare well when they come from the same assay. The Acromegaly Consensus Group recommends using one validated assay for all of a patient's tests, so a jump after switching labs is often the method rather than the person. It also says pregnancy should be taken into account before reading a new rise.

For people treated with injected somatostatin analogs, the group specifies that IGF-1 used for dose decisions be measured in the last week before the next injection. The guide to comparing lab results covers why switching labs causes this kind of jump.

On a hormone panel, IGF-1 is read together with growth hormone; the guide to reading a hormone panel explains how those pairs are interpreted.