How to read your IGF-1 lab — what the number actually tells you

The confusion is understandable. IGF-1 is simultaneously one of the most informative markers in this space and one of the most frequently misread.

Let's start with what the molecule is. IGF-1 stands for insulin-like growth factor 1. It's a peptide hormone produced primarily in the liver, and it's produced in response to growth hormone stimulation. When GH pulses — during slow-wave sleep, after exercise, during fasting — it binds to receptors on hepatocytes and triggers IGF-1 synthesis. The liver then releases IGF-1 into circulation, where it carries the anabolic and repair signals that GH initiated. Most of the tissue-level effects we associate with GH — protein synthesis, cellular repair, fat metabolism, collagen production — are mediated downstream through IGF-1. GH is the signal. IGF-1 is a large part of what the signal produces.

Once it's in circulation, most IGF-1 doesn't float freely. About 75 to 80 percent is bound to IGFBP-3, insulin-like growth factor binding protein 3, which acts as a reservoir and extends the half-life of IGF-1 dramatically. A smaller fraction is bound to other IGFBPs. A very small fraction — roughly 1 percent — circulates as free IGF-1, which is the biologically active form that can engage receptors directly. The bound forms are storage and transport. The free form is what's actually doing work in tissues at any given moment.

This is why IGF-1 is used as a proxy for GH status rather than GH itself. GH is pulsatile. It spikes during sleep, surges briefly after exercise, and falls back to nearly undetectable levels between pulses. A single random GH measurement tells you almost nothing about overall GH output — you might catch a peak, you might catch a trough, and without knowing which you have you can't interpret the number. IGF-1, because it integrates GH stimulation over hours to days and circulates in a relatively stable pool, is a much more informative snapshot. It's not perfect — several factors influence it — but it's the best single-number proxy for GH axis activity that routine labs can offer.

Now, the range. Your lab report has a reference range, and it is almost certainly age-adjusted, because IGF-1 declines substantially with age. A healthy 25-year-old might have an IGF-1 of 250 to 350 ng/mL. By 50, the same lab's reference range may bottom out near 100 ng/mL as the normal floor. Most labs define reference ranges from population distribution — roughly the middle 95 percent of tested individuals in each age bracket. Being within that range means you're not an outlier. It doesn't mean you're optimal.

This is where the reference range versus optimal range conversation becomes relevant, and where mainstream endocrinology and functional or peptide-focused medicine sometimes diverge. Standard endocrinology is primarily interested in identifying GH deficiency — a clinical condition with specific diagnostic criteria, usually involving stimulation testing and symptoms — and the reference range is calibrated around that question. If you're within range, you don't meet the diagnostic threshold for deficiency. That's the extent of what the reference range tells you. Functional medicine practitioners and providers who work with GH-axis peptide protocols often take a different approach: they're interested in where within the range you fall, and specifically whether your IGF-1 sits in the upper quartile for your age, which some clinical work suggests is associated with better body composition, recovery capacity, and tissue maintenance. A 55-year-old with an IGF-1 of 115 ng/mL and a reference range of 90–240 ng/mL is technically normal. They're also at the low end of a range that extends two-and-a-half times higher. Whether that distinction matters clinically depends on symptoms, context, and what you're trying to evaluate.

If you're on a peptide protocol — sermorelin, ipamorelin, CJC-1295, or a combination — the most important thing the IGF-1 number tells you is not whether you're within range but whether and how much you've moved from your own baseline. A before-and-after comparison anchored to your personal starting point is more informative than comparing your post-protocol number to a population distribution. You are not the average person in the reference range study. You have your own physiology, your own starting point, your own GH pulse pattern. If you began at 140 ng/mL and are now at 220 ng/mL, that shift tells you something about how your liver is responding to GH stimulation, which is a proxy for how the peptide is affecting GH axis activity. If you began at 140 ng/mL and are now at 142 ng/mL after eight weeks, that's also information — it suggests either the protocol isn't meaningfully affecting GH output, the dose timing is off, the peptide quality is variable, or your GH axis isn't responding as expected for reasons worth investigating.

There are things that move IGF-1 independent of GH axis interventions, and ignoring them makes interpretation difficult. Nutritional status matters substantially — protein intake in particular, since IGF-1 synthesis in the liver requires adequate amino acid availability. Someone on a low-protein or calorically restricted diet may have blunted IGF-1 regardless of GH stimulation. Liver function matters — since that's where IGF-1 is made, hepatic dysfunction of any kind can suppress IGF-1 independent of GH status. Insulin and glucose metabolism interact with IGF-1; insulin promotes IGF-1 synthesis and affects IGFBP-3 levels, so metabolic status is a confounding variable. Thyroid function matters, because hypothyroidism can blunt IGF-1 synthesis. Interpreting IGF-1 in isolation, without considering these factors, is like interpreting a speedometer reading without knowing what gear the car is in.

The high end of the range deserves honest attention, not because a slightly elevated IGF-1 in the context of a thoughtfully managed protocol is alarming, but because the biology is real. IGF-1 is a growth signal. It promotes cellular proliferation. Chronically elevated IGF-1 — not the upper quartile of normal range, but genuinely supraphysiological levels — has been associated in epidemiological research with modestly increased risk for certain hormone-sensitive cancers, including prostate, breast, and colorectal cancers. The association is from observational data and doesn't establish causation cleanly, but it's credible enough that it should inform clinical decision-making. The Laron syndrome data is instructive in the other direction: people with IGF-1 receptor mutations who have virtually no IGF-1 signaling have extremely low rates of cancer — and also extremely poor metabolic and physical function, and substantially reduced quality of life. This is the longevity-versus-recovery tension in clear view. Suppressing IGF-1 to zero is not a health strategy. Running it chronically supraphysiological probably isn't either. Where exactly the optimal window sits — and whether it differs meaningfully between someone optimizing for longevity versus recovery — is a question that's being actively researched and doesn't have a clean answer yet.

The low end carries its own concerns. People with consistently low IGF-1 for their age — not low-normal, but genuinely suppressed — show patterns across the literature that include poor recovery from exercise, difficulty maintaining lean mass, impaired wound healing, and in some research, associations with cognitive decline and increased all-cause mortality in older populations. Exactly how much of this is caused by low IGF-1 versus how much both are driven by the same underlying conditions is not perfectly clear. What is clear is that IGF-1 in the basement, when it's there because GH axis activity is blunted, is not a neutral finding.

What you want to bring to the conversation with your prescribing provider is not just the number but the context: your baseline IGF-1 before the protocol started, your current number, your fasting glucose and insulin (because GH and IGF-1 can affect insulin sensitivity, particularly at higher levels), your lipid panel if it hasn't been checked recently, your thyroid function, and your subjective experience — sleep quality, recovery rate, body composition trends, energy. The number in isolation is a starting point. The pattern across markers, tracked against your clinical picture, is what actually supports useful clinical decision-making.

One more thing. It is genuinely tempting to use a rising IGF-1 as the signal to keep pushing — higher dose, more frequent dosing — and a static IGF-1 as the signal to escalate. This framing puts a lot of faith in a proxy measure to do something it was never designed to do, which is serve as a real-time titration dial for a peptide protocol. IGF-1 is a useful tracking marker. It is not a precise dosing guide. The decision about whether, how, and when to adjust a GH-axis peptide protocol should involve your prescribing provider, your full clinical picture, and your baseline labs — not a single number compared against a population average. The number tells you something real. It doesn't tell you everything, and the difference between those two things is where clinical judgment lives.