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Insulin-like Growth Factor 1, or IGF-1, is one of the most useful markers in endocrinology. It gives clinicians an indirect but stable indication of how much growth hormone your body is producing, and it plays a central role in investigating conditions where growth hormone is either too high or too low.
This guide explains what IGF-1 is, how it relates to growth hormone, what an IGF-1 blood test actually measures, whether you need to fast, and how to make sense of your result in the context of age, sex and laboratory reference ranges.
This article is for education. It does not diagnose any condition and does not replace advice from your GP, endocrinologist or another qualified clinician.
IGF-1 (Insulin-like Growth Factor 1), historically also called somatomedin C, is a hormone produced mainly by the liver in response to growth hormone (GH) released by the pituitary gland. It circulates in the bloodstream and acts on tissues throughout the body, where it promotes cell growth, tissue repair and metabolism.
Two features make IGF-1 particularly useful clinically:
That combination is why endocrinologists often measure IGF-1 as an initial screening marker when they suspect a problem with the growth hormone axis, rather than trying to catch GH itself with a single blood draw.
To understand IGF-1 results, it helps to picture the system it belongs to. In simplified terms:
Because GH is released in pulses — sometimes only a few times a day — a single random GH measurement can be misleadingly high or low. IGF-1, by contrast, tends to sit at a steadier level, integrating the effects of GH secretion over hours to days.
No. Growth hormone is the pituitary hormone that stimulates IGF-1 production. IGF-1 is the downstream messenger that carries out many of GH’s growth-related effects. They are related, but they are separate hormones measured by separate tests.
An IGF-1 blood test measures the concentration of IGF-1 in a sample of serum — the liquid part of your blood, once cells have been separated in the laboratory. It is often reported alongside a reference range appropriate for your age and sex.
Clinicians typically request an IGF-1 test to help investigate:
An IGF-1 result is almost always interpreted in context. On its own, a single figure rarely confirms or excludes a condition; it points a clinician towards the next appropriate investigation.
For a standalone IGF-1 blood test, fasting is not generally required, and IGF-1 levels do not fluctuate significantly with meals in the way that glucose or triglycerides do.
However:
Always follow the preparation advice given by your provider or on your appointment confirmation.
There is no single “normal” IGF-1 number. Levels change substantially across life:
Because of this pattern, IGF-1 results must be interpreted against an age- and sex-specific reference range. A value that is entirely healthy in a 16-year-old may be well above the expected range for a 65-year-old, and vice versa.
Reference ranges also vary between laboratories because different immunoassays produce slightly different numerical values. Your report will normally state the exact reference interval used for your sample. Some laboratories additionally report an IGF-1 z-score or standard deviation score (SDS), which expresses how far your result sits from the mean value expected for your age and sex — a format that is often easier for clinicians to compare across labs.
For all of these reasons, the most important reference range for your result is the one printed on your own laboratory report.
IGF-1 does not exist in isolation. Several biological and clinical factors can raise or lower it independently of the growth hormone system itself:
These influences are one of the reasons that a single IGF-1 measurement is best interpreted alongside a clinical history and other tests, rather than taken in isolation.
Because IGF-1 is a marker rather than a diagnosis, the safest way to think about your result is directional.
A result within your laboratory’s age- and sex-adjusted reference interval is reassuring in the context of screening. Importantly, however, a normal IGF-1 does not by itself rule out growth hormone deficiency in every case. Where clinical suspicion remains high, an endocrinologist may still arrange further investigations such as dynamic GH testing.
An elevated IGF-1 may prompt further assessment for growth hormone excess, including conditions such as acromegaly. Confirmation typically requires additional tests, such as an oral glucose tolerance test with GH measurement, and often pituitary imaging.
An IGF-1 that is only slightly above the reference range is not, on its own, a diagnosis of anything. It is a signal that further evaluation may be worthwhile.
A low IGF-1 may raise the possibility of growth hormone deficiency, or it may reflect one of the many non-GH factors listed above — nutritional status, chronic illness, liver or kidney disease, thyroid problems, or poorly controlled diabetes. Interpretation depends on the clinical picture.
Regardless of direction, an out-of-range IGF-1 is a reason to discuss the result with a GP or, where appropriate, an endocrinologist. It is not, by itself, a diagnosis.
Most IGF-1 in the bloodstream is not floating freely. It is bound to carrier proteins, principally IGFBP-3 (IGF-binding protein 3), which extends its half-life and modulates its activity.
Some clinicians request IGFBP-3 alongside IGF-1, particularly when investigating growth issues in children, because measuring both markers can give a fuller picture of the GH–IGF-1 axis.
Other investigations that may sit alongside IGF-1 in a fuller assessment include:
For adults exploring their wider endocrine picture privately, a combined metabolic hormones test that includes cortisol, insulin and IGF-1 is one common starting point, followed by clinician-guided investigation where indicated.
An IGF-1 generation test is a specialist endocrine investigation, not a routine screening test. In this test, growth hormone is administered over several days and IGF-1 is then measured to see how well the body responds. It is used in secondary and tertiary care to investigate rare conditions affecting the GH–IGF-1 axis, such as growth hormone insensitivity.
Because the test requires close clinical supervision and specialist interpretation, it is not offered as an over-the-counter service. If you have been advised that this test may be relevant to you, it will normally be arranged through an NHS or private endocrinology service.
In the UK, IGF-1 testing is usually initiated by a clinician — most often an endocrinologist — when there is a specific clinical reason such as suspected acromegaly, pituitary follow-up, or investigation of growth-related concerns.
Private IGF-1 testing is used by adults who:
In every case, the value of the test depends on interpretation. A private IGF-1 result that arrives without clinical context is significantly less useful than the same result reviewed by a clinician who knows your history.
If you are considering a private test, our metabolic hormones blood test measures IGF-1 alongside cortisol and insulin, and our broader hormone panels may be appropriate where a fuller endocrine picture is wanted.
“A normal IGF-1 means my growth hormone is definitely fine.”
Not always. A within-range IGF-1 is reassuring but cannot by itself rule out growth hormone deficiency in every clinical scenario. Dynamic testing may still be needed if the clinical picture warrants it.
“A single high IGF-1 means I have acromegaly.”
An elevated IGF-1 is a signal to investigate further, not a diagnosis. Confirmation of acromegaly involves additional tests, including GH measurement during an oral glucose tolerance test and pituitary imaging.
“There is a universal normal IGF-1 range.”
There isn’t. Reference ranges differ by age, sex and the specific assay a laboratory uses. Interpret your result against the range printed on your own report.
“IGF-1 is a fitness or anti-ageing marker.”
IGF-1 is a clinical marker used in endocrinology. Some fitness and wellness websites frame it as an optimisation target; that is not how it is used in medical practice. Attempting to manipulate IGF-1 outside a supervised clinical setting is not advisable.
IGF-1 is a hormone made mainly by the liver in response to growth hormone. It carries out many of the growth-related effects of growth hormone in the body and is used as a stable blood marker of growth hormone activity.
Standalone IGF-1 tests generally do not require fasting. If IGF-1 is being taken as part of a larger panel, follow the preparation instructions provided by your test provider.
There is no single universal figure. IGF-1 reference ranges are age- and sex-specific and vary between laboratories because different assays produce slightly different values. Always interpret your result against the reference range printed on your own report.
A raised IGF-1 may suggest growth hormone excess, but it is not on its own a diagnosis. Elevated results usually lead to further tests, such as an oral glucose tolerance test with GH measurement and, where appropriate, pituitary imaging.
A low IGF-1 can indicate reduced growth hormone activity, but it can also reflect other factors such as poor nutrition, chronic liver or kidney disease, poorly controlled diabetes, thyroid disease, or the effects of certain medications. Interpretation depends on the wider clinical picture.
Some private services offer home phlebotomy kits, though a properly collected venous serum sample analysed in an accredited laboratory is the reliable method. Home collection may be acceptable when it meets those requirements; interpretation, however, is a job for a clinician rather than a home report.
Testing without a clear reason risks producing results that are difficult to interpret and can cause unnecessary worry. If you are considering an IGF-1 test, it is worth discussing your reasons with a GP or specialist first, so that any result you receive can be interpreted in context.
If your clinician has requested an IGF-1 measurement, or if you want to include IGF-1 as part of a broader hormone panel, you can find full details on our metabolic hormones (cortisol, insulin, IGF-1) blood test. We recommend reviewing any private hormone result with a GP or endocrinologist, especially before making decisions about further investigation or treatment.
