IGF-1 reference ranges by age: what is normal for men over 40

You got your bloodwork back. Your IGF-1 is 118 ng/mL, and the lab flags it as normal. But you are 44, you train four days a week, you sleep well, and something is still slipping — recovery, body composition, the edge you used to have. Before you accept that number at face value, it helps to understand what that reference range actually represents, and whether it means anything useful for someone at your age and activity level.

What IGF-1 is and why it matters after 40

Insulin-like growth factor 1 (IGF-1) is a peptide hormone produced primarily in the liver in response to growth hormone (GH) signaling. It mediates most of the tissue-building effects of GH — muscle protein synthesis, fat metabolism, bone turnover, and cellular repair. Unlike GH itself, which is released in pulses throughout the day, IGF-1 stays relatively stable in the bloodstream. That stability makes it a far more practical marker for assessing your GH axis than a single GH draw.

GH secretion peaks in your 20s and drops roughly 14 percent per decade after that. IGF-1 follows the same trajectory. By the time you are in your mid-40s, your IGF-1 is likely 30 to 40 percent lower than it was at 25 — even if you are doing everything right. This is not pathology in the clinical sense. It is a normal biological process called somatopause. The problem is that normal is doing a lot of heavy lifting in that sentence.

How IGF-1 reference ranges are built — and where they fall short

Labs construct their reference ranges by measuring IGF-1 in a population of apparently healthy individuals and calculating a statistical distribution. The standard range covers the middle 95 percent of that population. Anyone below the 2.5th percentile is flagged as low. Anyone above the 97.5th percentile is flagged as high. Everyone else is reported as normal.

Here is the limitation you need to understand: the reference population includes everyone — sedentary people, people with poor metabolic health, people who sleep five hours a night. If most 44-year-old men in the reference cohort are deconditioned and hormonally blunted, the resulting normal range reflects that average. Your 118 ng/mL might technically fall within range while still sitting at the bottom third of what a healthy, active man your age could be producing.

Most commercial labs use age-stratified ranges. A common breakdown for adult men looks like this:

• Ages 20 to 29: approximately 160 to 400 ng/mL
• Ages 30 to 39: approximately 130 to 330 ng/mL
• Ages 40 to 49: approximately 100 to 290 ng/mL
• Ages 50 to 59: approximately 80 to 230 ng/mL
• Ages 60 and older: approximately 60 to 180 ng/mL

These numbers vary by lab and by assay method. Quest, LabCorp, and other major labs do not use identical reference intervals. Before you compare your result to any table you find online, confirm which assay your lab used and what their specific age-matched range is. A result that looks low on one lab's scale may sit differently on another's.

What low-normal IGF-1 actually looks like in practice

Clinical GH deficiency in adults — the kind diagnosed after pituitary damage or surgery — produces IGF-1 values that fall clearly below the reference range. That is a different category from what most men over 40 experience. What you are more likely dealing with is age-related GH axis blunting: your values are technically within range but functionally suboptimal for someone with your training load and health goals.

Research on this distinction is limited by the fact that most studies have used either GH-deficient patients or young athletes, not healthy middle-aged men with moderate decline. One frequently cited trial by Khorram et al. (1997, PMID 9141536) examined the effect of low-dose growth hormone-releasing hormone (GHRH) administration on IGF-1 and body composition in older adults. They found that even modest stimulation of endogenous GH secretion raised IGF-1 levels and produced measurable improvements in lean mass and fat mass — without the side effect burden seen with direct GH injections. This matters because it suggests the GH axis in older adults remains responsive; it simply needs a signal.

A 2006 review by Walker (PMC2699646) provides useful context on GHRH analogs and age-related GH decline, noting that the pituitary retains secretory capacity well into older age. The decline is largely driven by reduced hypothalamic GHRH output and increased somatostatin tone — both of which are upstream of the pituitary, meaning the pituitary itself is not broken. Sermorelin, a GHRH analog, works by stimulating that still-functional pituitary to release more GH in a pulsatile, physiological pattern. The result is a gradual increase in IGF-1 toward the upper portion of the age-appropriate range.

To be direct about the evidence: most trials in this space are small, some are decades old, and very few were designed specifically around healthy men in their 40s who simply want to recover the performance they had at 35. The mechanistic rationale is solid. The clinical trial base is thinner than anyone should pretend otherwise. What exists is encouraging, not conclusive.

How to use your IGF-1 result as a starting point, not a verdict

If your IGF-1 comes back in the lower half of your age-matched range, that is worth a conversation — not a crisis, but a signal. A few things to consider when reviewing your result:

• Where in the age-matched range does your result sit? Bottom quartile is different from bottom half.
• What assay did the lab use? Chemiluminescent immunoassays and mass spectrometry-based assays can produce different absolute values.
• Is your result trending? A single point is less informative than two values taken a year apart.
• Are your symptoms consistent with GH axis decline — reduced lean mass despite training, slower recovery, increased central fat, disrupted sleep architecture?
• Are other variables ruled out? Thyroid function, testosterone, and nutritional status all affect IGF-1 independently.

IGF-1 is a useful proxy for GH axis function, not a standalone diagnosis. A provider who understands the full clinical picture — your training history, sleep quality, body composition trends, and symptom pattern — can give that number much more meaning than the lab printout alone.

If you are in the lower third of your age range and you have ruled out other causes, it is reasonable to ask whether supporting your GH axis with a GHRH analog like sermorelin makes sense. The mechanism is physiological, the safety profile is favorable relative to exogenous GH, and the goal is not supraphysiologic IGF-1 — it is moving from the bottom of your normal range toward the middle or upper portion of it.