IGF-1: why the range is age-banded, and what secretagogues do to it

IGF-1 is the marker people use to tell whether a growth hormone secretagogue is doing anything. It is also the marker most often quoted without the one piece of context it cannot be interpreted without: how old the person is.

What IGF-1 measures, and why it is used instead of GH

Growth hormone is secreted in pulses, mostly at night, and a random GH measurement is close to meaningless as a result. IGF-1 is produced largely by the liver in response to that GH exposure, circulates bound to binding proteins with a much longer half-life, and is therefore stable enough across a day to be measured once and interpreted.

That is the whole reason IGF-1 is the standard proxy: it integrates GH exposure over time instead of sampling a pulse. IGFBP-3, the main binding protein, is sometimes measured alongside it and moves in the same direction.

The age problem

Established clinical use IGF-1 peaks in adolescence and declines continuously through adult life.¹ A value of 200 ng/mL is unremarkable at twenty-five and high at sixty-five. This is not a minor adjustment: published adult reference intervals run roughly 114–400 ng/mL in the 25–39 band and roughly 70–290 ng/mL over 54.¹

That is why the registry marks age as context this marker requires rather than as a nicety, and why a raw IGF-1 quoted with no age attached — as they almost always are in forum posts — cannot be interpreted. Many labs report a Z-score or standard deviation score against an age- and sex-adjusted median for exactly this reason, and where you have one it is more informative than the raw number.

Off-label or community practice The generic band in the fact box is a broad adult default. It is a fallback for when your report did not carry an age-adjusted interval, and it is weaker than the one your lab printed, which is built for your age band.

Assay standardisation, which is not settled

Established clinical use IGF-1 assays have historically disagreed with each other enough to matter. Calibration against the WHO reference standard has been recommended, and assays conforming to that recommendation have been validated against large multicentre cohorts.¹² Not every assay in use is calibrated the same way, and studies comparing two modern immunoassays in the same cohort still find differences worth knowing about.²

The practical consequence is the same as for every other marker on this site: a trend across two laboratories is partly a comparison of two assays. Stay with one lab if you are watching a number move.

What secretagogues do to it

Off-label or community practice Growth hormone secretagogues — the GHRH analogues like sermorelin, CJC-1295 and tesamorelin, and the ghrelin-receptor agonists like ipamorelin and MK-677 — raise IGF-1 by increasing endogenous GH release rather than by supplying GH. IGF-1 is therefore the reasonable marker for whether one is doing anything measurable, and a rise is the expected finding. Most of these are research compounds with no approval for this use.

The number worth watching alongside it is fasting glucose, and the reason is mechanistic rather than theoretical: growth hormone antagonises insulin. A rising IGF-1 with a drifting fasting glucose is the trade, and the app's own guidance is that a rising fasting glucose on a secretagogue is the signal to reconsider the dose. The HbA1c and fasting glucose page covers what to watch and how often.

How high is too high is not a question with a clean published answer for people using these compounds off-label, because the trials that would establish it have not been done. What is documented is what sustained GH excess does over years in acromegaly, which is the reason the ceiling is worth taking seriously rather than treating a high-normal IGF-1 as a target. That is a discussion for a doctor, with an age-adjusted result in front of you.

What the app records for this marker

Compounds whose monitoring panel includes this marker

Generated by matching this marker's own aliases against the monitoring note on every compound in the app's reference, so the list is the app's, not an author's.

• CJC-1295 IGF-1 every 3 months. Fasting glucose.
• Ipamorelin IGF-1 at baseline and every 3 months. Fasting glucose.
• HGH Fragment 176-191 Body composition tracking, waist circumference, fasting glucose (should be unaffected). IGF-1 should remain unchanged — if it rises the product may be mislabeled.
• Hexarelin IGF-1, Prolactin (elevated with Hexarelin), Cortisol if symptomatic. Cardiac assessment if using for cardiac indications.
• GHRP-2 IGF-1 every 3 months, Prolactin, Cortisol if symptomatic, Fasting glucose.
• GHRP-6 IGF-1 every 3 months, Prolactin, Cortisol, Fasting glucose, Body weight.
• Sermorelin IGF-1 at baseline and every 3 months. Fasting glucose.
• Tesamorelin IGF-1 every 3 months (critical), fasting glucose, HbA1c, visceral fat assessment, lipid panel.
• MK-677 / Ibutamoren IGF-1 every 6–8 weeks, fasting glucose, HbA1c, prolactin, cortisol, lipid panel. Baseline before starting.
• Recombinant HGH IGF-1 (critical — maintain upper third of normal range), Fasting glucose, HbA1c, Lipid panel every 3 months. Wrist and joint assessment for carpal tunnel.
• IGF-1 LR3 Fasting glucose (hypoglycemia risk critical), IGF-1 levels, tumor markers awareness, lipid panel.
• CJC-1295 with DAC IGF-1 every 6-8 weeks, fasting glucose, water retention assessment.

Questions people actually ask

What IGF-1 should I be aiming for?

This page will not give you a target, because one does not exist for off-label use. The honest position is that the reference interval for your age says what is ordinary, a Z-score says how far from the age-adjusted median you are, and no trial has established an optimal value for someone taking a secretagogue for body composition or recovery. Anyone quoting a specific target number is quoting a convention, not a finding.

Does IGF-1 need to be fasted?

Not in the way glucose does — its long half-life is what makes it stable. Consistency still helps: same lab, same rough time of day, same relationship to your dosing schedule if you are on something that moves it.

Why does my report show a Z-score instead of a range?

Because the age adjustment is the whole point, and a Z-score expresses it directly: it is roughly how many standard deviations you sit from the median for your age and sex. If your report gives one, prefer it to comparing a raw number against a generic adult band.

Sources

Every claim above that is not a definition is either labelled by evidence tier or carries a numbered reference to one of these.

1. Bidlingmaier M et al., Reference Intervals for Insulin-like Growth Factor-1 (IGF-I) From Birth to Senescence, J Clin Endocrinol Metab 99(5):1712 (2014) Multicentre age-banded reference intervals on an assay conforming to the international calibration recommendation.
2. Reference values for IGF-I serum concentration in an adult population: use of the VARIETE cohort for two new immunoassays Adult reference values across two assays, and how far modern assays still differ.
3. Reference ranges for serum insulin-like growth factor I (IGF-I) in healthy adults An independent population confirming the age-related decline.