IGF-1 Blood Test Explained: What It Measures and What Affects It
Insulin-like growth factor 1 (IGF-1) is a hormone made mainly by the liver in response to growth hormone (GH). Because it reflects GH activity in a single, stable blood value, it is the standard lab marker of the GH axis.
Why IGF-1 instead of growth hormone?
GH is released in short pulses, mostly during sleep, so a random GH level can be near zero or very high depending on timing. IGF-1 has a much longer half-life (it circulates bound to IGFBP-3 and another protein), so it stays fairly steady through the day and can be drawn at any time. That is why clinicians use IGF-1 to screen for GH deficiency and excess, and to adjust GH therapy, typically rechecking a few weeks after a change.
Growth-hormone secretagogues such as Sermorelin, Tesamorelin and Ipamorelin act through the body's own GH release, so studies of them report IGF-1 as a key outcome. See their half-life pages for reference data.
Reading the result
- Ranges are age- and sex-specific. IGF-1 peaks in adolescence and declines steadily with age, so the same number can be high for a 60-year-old and low for a 20-year-old.
- Units: usually ng/mL. To convert to nmol/L, multiply by 0.131.
- Z-score or SDS: some labs report how many standard deviations the value sits from the mean for your age. Roughly -2 to +2 covers most healthy people.
What raises or lowers IGF-1
| Lower IGF-1 | Higher IGF-1 |
|---|---|
| Older age | Puberty |
| Fasting, calorie or protein restriction | High protein and calorie intake |
| Liver disease, poorly controlled diabetes | GH therapy or GH-releasing compounds |
| Hypothyroidism, oral estrogen | Excess GH (acromegaly) |
LC/MS versus immunoassay
IGF-1 is measured either by immunoassay or by liquid chromatography-mass spectrometry (LC/MS). The methods can give noticeably different values, so compare results only within the same method and lab. Assays are designed for native IGF-1; modified analogs such as IGF-1 LR3 may not be measured accurately by them. IGFBP-3, the main binding protein, is sometimes ordered alongside.
This guide explains lab tests. It is not medical advice, does not interpret individual results, and does not endorse using any compound. Review your results with a licensed clinician.
FAQ
What does an IGF-1 test show?
It reflects growth-hormone activity. Because IGF-1 stays fairly steady through the day, unlike pulsatile GH, it is the standard blood marker for screening GH deficiency or excess and for adjusting GH therapy.
Do I need to fast for an IGF-1 test?
Many labs do not require fasting for IGF-1 alone, but follow the instructions for your order; panels that include glucose or lipids usually require 10-12 hours of fasting. Long-term fasting or low protein intake lowers IGF-1.
What is a normal IGF-1 level?
It depends on age and sex, so labs give age-specific reference ranges or a Z-score. Values decline steadily with age; compare your result with the range printed on your report.