IGF-1

What IGF-1 Measures

IGF-1 (Insulin-like Growth Factor 1) is a protein hormone produced primarily in the liver in response to growth hormone (GH) signaling. Because pulsatile GH is difficult to measure directly - its levels fluctuate wildly over minutes and hours - IGF-1 serves as the stable, clinically useful readout of overall growth hormone activity. It is the single most important biomarker for anyone running GHRH analogs, GH secretagogues, or exogenous HGH.

IGF-1 levels decline steadily with age, with most people losing 20–40% of their peak IGF-1 between their 20s and 50s. Optimization protocols often target the upper quartile of the age-adjusted reference range rather than the "normal" range, which can span from genuinely deficient to peak optimization. This is one of the biomarkers where the reference range and the optimal range differ meaningfully.

What Affects This Biomarker

IGF-1 is influenced by: baseline growth hormone production (declines with age), nutritional status (low protein depresses IGF-1), sleep quality (deep sleep drives GH pulses), liver function (IGF-1 is produced there), insulin resistance (alters GH-IGF signaling), and directly by pharmacologic GH secretagogues (tesamorelin, ipamorelin/CJC-1295, MK-677, HGH). Acute illness and inflammation suppress IGF-1. Laboratory assays can vary, so using the same lab for serial measurements is important.

In the Context of Peptide Protocols

For peptide and HGH users, IGF-1 is typically drawn at baseline, then at 6 weeks into a protocol (the standard response-assessment point), then every 3 months. A typical responder on tesamorelin 1–2 mg nightly will see IGF-1 rise 40–80% from baseline. Non-response at 6 weeks usually means compliance or technique issue (often an incomplete fasting window). Very high IGF-1 elevations warrant caution - above ~300 ng/mL in most adults suggests the dose is higher than needed. IGF-1 LR3 users will see suppressed native IGF-1 on standard assays (different epitope) - this is normal and expected.

Deep Dive

What IGF-1 actually measures: a mostly bound hormone

Almost all circulating IGF-1, roughly 99%, is not floating free. It travels locked in a ternary complex with IGF binding protein 3 (IGFBP-3) and the acid-labile subunit (ALS), and only a small fraction is biologically free and available to bind the IGF-1 receptor at any moment. This matters twice over. First, a standard test reports total IGF-1, not the active free fraction, so two people with the same total value can differ in free IGF-1 depending on their binding-protein status. Second, that binding is exactly what makes IGF-1 a stable readout: free IGF-1 has a half-life of only minutes, while the bound complex persists for roughly 12 to 16 hours. That buffering smooths the minute-to-minute swings of pulsatile GH into a steady number you can actually track over time.

Some endocrinologists add IGFBP-3 and look at the IGF-1 to IGFBP-3 molar ratio as a rough proxy for free, bioavailable IGF-1. It is not a routine order, but it explains why a catabolic or low-protein state can move IGF-1 and IGFBP-3 together while true bioavailability shifts less than the raw number implies.

The U-shaped mortality curve: why higher is not automatically better

This is the single most important nuance the standard reference range hides. Across large populations, IGF-1 relates to mortality in a U-shape: both unusually low and unusually high levels associate with higher all-cause mortality. A 2011 meta-analysis in the Journal of Clinical Endocrinology and Metabolism (Burgers et al.) described exactly this non-linear pattern, with elevated risk at both the low and high ends of the distribution.

The low end reflects frailty, undernutrition, and chronic illness. The high end is where optimizers tend to slow down. Higher circulating IGF-1 has been linked in large prospective cohorts, including UK Biobank, to modestly increased risk of certain cancers, notably prostate, breast, and colorectal. On the other side, populations with genetically low IGF-1 signaling appear strikingly protected: people with Laron syndrome (GH receptor deficiency, studied in the Ecuadorian cohort by Guevara-Aguirre and colleagues) show very low rates of diabetes and cancer, and among Ashkenazi Jewish adults already at exceptional ages studied by Milman, Barzilai, and colleagues, those with lower IGF-1 tended to live longer still, an association seen in women. None of this makes "low" a target to chase. It means the pursuit of an ever-higher IGF-1 weighs a possible short-term performance signal against a plausible long-term risk, and that trade belongs in a conversation with your clinician, not a dosing spreadsheet. It is also why acromegaly, a state of chronic IGF-1 excess, is monitored so closely: sustained elevation is associated with cardiomyopathy, arthropathy, and metabolic disease.

Reading the number: age, sex, and the assay trap

A raw IGF-1 value is close to meaningless without its age and sex context. IGF-1 peaks in adolescence, falls sharply through the 20s, and keeps drifting down for life, so 180 ng/mL is high-normal for a 60-year-old and unremarkable for a 25-year-old. Endocrinologists rarely read the raw number. They convert it to an IGF-1 standard deviation score (SDS, or z-score) against an age- and sex-matched population. Women, particularly those on oral estrogen, tend to run lower than men at the same age.

There is also an assay problem. IGF-1 immunoassays were historically poorly harmonized across manufacturers, and a value from one platform did not cleanly translate to another. Standardizing against the WHO international reference (IS 02/254) improved this, but real between-lab differences persist. This is the concrete reason serial tracking should use the same lab and ideally the same assay. A 30 ng/mL move between two labs may be pure method difference, not a real physiological change.

What reliably raises and lowers IGF-1

Beyond GH itself, the levers that move IGF-1 are largely nutritional and hepatic:

- Raises: adequate calories and protein, dairy intake (milk is one of the most consistent dietary correlates of higher IGF-1), deep slow-wave sleep, and, more modestly and less consistently, regular resistance training. - Lowers: caloric restriction and multi-day fasting (a core mechanism behind fasting's longevity research), chronic protein restriction, systemic inflammation and acute illness, liver disease, and poorly controlled diabetes.

One underappreciated lever deserves its own line: route of estrogen. Oral estrogen passes through the liver first and suppresses hepatic IGF-1 production, so women on oral hormone therapy often show notably lower IGF-1, while transdermal estrogen, which bypasses first-pass metabolism, does not. If IGF-1 looks unexpectedly low in a woman, it is worth noting what form of estrogen she is taking before drawing any conclusion about GH status, a point to raise with her clinician.

Testing timing and interpretation pitfalls

Because the bound complex is stable through the day, IGF-1 does not require same-day fasting the way glucose or insulin do. You can draw it at most times and get a comparable number. What matters is the recent nutritional trajectory: a crash diet, a several-day fast, or an acute illness in the days before the draw will pull IGF-1 down independent of the GH axis. IGF-1 is also a lagging indicator on a protocol. It rises over days to weeks after a secretagogue takes hold, which is why the roughly 6-week check exists rather than a same-week draw. Keeping the draw timing consistent relative to dosing across measurements makes the trend easier to read.

Peptide and protocol tracking context

Different GH-directed agents move IGF-1 through different physiology, and the shape of the change matters as much as the size. GHRH analogs such as tesamorelin, sermorelin, and CJC-1295 amplify the body's own pulsatile GH release, so IGF-1 rises while the natural rhythm and negative feedback stay intact. MK-677 and exogenous HGH instead tend to produce a more sustained elevation, which is why they are the agents most likely to push IGF-1 high and to nudge fasting glucose and insulin upward. That is the practical reason to never track IGF-1 in isolation on these protocols. Pairing it with fasting insulin, fasting glucose, and HbA1c helps you see whether a rising IGF-1 that reads like progress is quietly accompanied by a slide in insulin sensitivity. Logging these together on one timeline is what turns scattered lab values into a picture you can review and reason about with your clinician.

Peptides That Commonly Move IGF-1

Related Reading

• Igf 1 Blood Test Guide →
• Igf1 Optimal Ranges →
• Tesamorelin Igf1 Results →
• How To Interpret Igf1 Results →

Frequently Asked Questions

Where This Fits in Your Panel

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