IGF-1 blood test: what insulin-like growth factor 1 measures, normal levels by age, and what high or low IGF-1 means

Growth hormone (GH) is released by the pituitary gland in short pulses, mostly at night, so a single GH measurement is nearly meaningless — it can be undetectable in a healthy person one minute and high the next. Most of GH's effects, though, are carried out by IGF-1, which the liver produces in proportion to average GH exposure and which circulates bound to carrier proteins with a half-life of many hours. IGF-1 is therefore stable across the day and reflects GH output over the preceding day or two.

The test measures IGF-1 in nanograms per milliliter (ng/mL). It is the screening and monitoring test for acromegaly (GH excess in adults), gigantism (GH excess in children), adult and childhood GH deficiency, and the response to GH replacement. Outside endocrinology it is drawn to investigate short stature or unusual growth in children, and increasingly by adults using growth-hormone secretagogue peptides who want to know whether the compound is doing anything.

IGF-1 is not a general fitness or longevity score. It rises and falls with nutrition, illness and hormones in ways that have nothing to do with growth hormone, and observational links between IGF-1 levels and long-term health run in both directions.

IGF-1 peaks during puberty and declines steadily through adult life, so a value that is normal at 25 can be high at 65. Laboratories report age-specific reference ranges and often a z-score or standard-deviation score (SDS) that expresses how far the result sits from the age-matched mean; a result more than two standard deviations from the mean is outside the range. Assays differ between manufacturers, so ranges are not interchangeable between labs.

A sustained IGF-1 above the age-matched range is the screening signal for acromegaly, almost always caused by a benign GH-secreting pituitary adenoma. The Endocrine Society recommends IGF-1 as the first test in anyone with the typical features — enlarging hands and feet, coarsening facial features, a widening jaw, sweating, headaches, joint pain, carpal tunnel syndrome, sleep apnea, new diabetes or hypertension — and in people with a pituitary mass found for another reason. A clearly elevated IGF-1 is followed by a glucose-suppression test: in health, a glucose load suppresses GH; in acromegaly it does not.

Not every high IGF-1 is acromegaly. Puberty and late pregnancy raise it physiologically. Exogenous growth hormone and the GH-secretagogue peptides (sermorelin, tesamorelin, CJC-1295, ipamorelin, MK-677) raise it by design — which is also how their use is detected. Hyperthyroidism can raise it modestly. Some assays read high in the presence of certain binding-protein abnormalities. In an adult on a GH-stimulating peptide, an IGF-1 that has climbed into or above the upper range is the expected effect, not a sign of a pituitary tumor, though sustained supraphysiological levels are exactly what clinicians monitoring those protocols try to avoid.

A low IGF-1 raises the question of GH deficiency, but it is far less specific than a high value is for excess. Adult GH deficiency usually occurs in people with a reason for it — a pituitary tumor or its surgery or radiation, a head injury, a brain hemorrhage, or a childhood diagnosis carried into adulthood — and in that setting a low IGF-1 supports the diagnosis. A low IGF-1 in someone with no pituitary history rarely turns out to be GH deficiency, and the Endocrine Society requires a GH stimulation test to confirm it because a normal IGF-1 does not exclude deficiency and a low one does not prove it.

The common non-pituitary causes are the ones to think about first. Undernutrition and low-calorie or very low-protein diets lower IGF-1 quickly; so do anorexia nervosa and prolonged fasting. Liver disease reduces the liver's capacity to make it. Poorly controlled type 1 or type 2 diabetes, untreated hypothyroidism, chronic kidney disease, critical illness and chronic inflammatory disease all lower it. Oral estrogen — including combined oral contraceptives and oral menopausal hormone therapy — reduces IGF-1 by acting on the liver, whereas transdermal estrogen does not. Normal aging lowers it, which is exactly why age-matched ranges exist.

IGF-1 is the single most useful lab for anyone on growth hormone replacement or a GH-secretagogue protocol, because it integrates the effect of the drug over days. Endocrinologists titrate GH replacement to keep IGF-1 within the age-matched range — typically aiming for the middle, and avoiding the upper limit because long-term supraphysiological IGF-1 is the state acromegaly patients live in, with its associated joint, metabolic and cardiovascular consequences.

For peptide users the same logic applies: a baseline IGF-1 before starting, a repeat after eight to twelve weeks at a stable dose, and periodic checks thereafter. A level that has not moved suggests the compound, dose or timing is not producing a GH response; a level above the range is a reason to reduce rather than a sign of success. Fasting glucose or A1c is usually checked alongside, because GH excess raises blood sugar. TestWell's peptide therapy bloodwork guide covers the full monitoring set.

IGF-1 is a standard blood draw available at Quest and Labcorp, ordered by a clinician or directly through TestWell with physician review included; results post in a few business days with the lab's age-specific range. Fasting is not strictly required, but a morning fasting draw is the convention because it removes meal effects and makes repeat tests comparable. Avoid testing during an acute illness or a period of very low food intake, both of which lower the result for reasons unrelated to growth hormone.

Tell the ordering clinician about oral estrogen, glucocorticoids, growth hormone or peptides, since each changes the interpretation. And because assays differ, use the same laboratory for any result you intend to compare with a previous one.