IGF-1
Also reported as: Insulin-like growth factor 1 · Somatomedin C
The hormone through which most of growth hormone's effects are actually delivered. Because it is stable across the day where growth hormone is pulsatile, IGF-1 is the practical way to read GH status from a single blood draw.
Strongly age-dependent — IGF-1 peaks in adolescence and falls steadily thereafter, so a good lab reports an age-matched range alongside the raw figure.
The band associated with lowest all-cause mortality in pooled cohort data. Some clinicians work to 120-200; the case for the tighter range is that it stays clear of both the frailty signal below and the cancer signal above.
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Where the thresholds sit
| Band | Range (ng/mL) | Note |
|---|---|---|
| Low | < 100 | Predicts frailty, sarcopenia and functional decline, particularly over 65. |
| Below optimal | 100–120 | — |
| Optimal | 120–160 | — |
| Acceptable | 160–200 | — |
| Elevated | 200–250 | Cancer associations begin to appear in cohort data. |
| High | > 250 | Investigate acromegaly if unexplained; review any GH secretagogue use. |
Why it matters
One of the clearest cases on this site where more is not better. A meta-analysis of all-cause mortality found a U-shaped curve: low IGF-1 raised risk by about a third versus the middle range, and high IGF-1 raised it by about a quarter. Low levels predict frailty and functional decline in older adults; persistently high levels track with breast, prostate and colorectal cancer incidence. Anyone taking a growth hormone secretagogue is moving this number, and should know which direction is actually good.
What raises it
- Growth hormone and GH secretagogues — tesamorelin, CJC-1295, ipamorelin
- High protein intake, particularly dairy
- Insulin and a positive energy balance
- Acromegaly, in which it is the screening test
What lowers it
- Caloric restriction and protein restriction
- Fasting, sharply and quickly
- Liver disease — IGF-1 is made in the liver
- Hypothyroidism
- Ageing, unavoidably
How to move it
- 1Decide which direction you are aiming before you act. If your IGF-1 is 140, it is where you want it, and raising it is a downgrade.
- 2If it is low and you are older, adequate protein and resistance training are the first levers, not a peptide.
- 3If it is high and unexplained, get acromegaly ruled out before assuming it is diet.
- 4If you take a GH secretagogue, test IGF-1 before starting and every 8-12 weeks after. That is the point of the marker.
- 5Interpret against an age-matched range, not the raw laboratory bounds.
What moves this marker
Interventions on this site with evidence behind them for this specific marker. None of them replaces working out why yours is where it is.
What makes a single reading misleading
- Falls steeply with age, so the same number means different things at 30 and 70.
- Suppressed by fasting, malnutrition and liver disease independent of growth hormone status.
- Assay variation between laboratories is substantial — track with the same lab.
- The mortality relationship is observational; it does not prove that moving your number moves your risk.
Sources
Read next
Educational only. Reference ranges are not diagnostic thresholds and this page cannot account for your medical history, medication or symptoms. Discuss any result that concerns you with a qualified clinician. See our medical disclaimer.