IGF-1 (Insulin-like Growth Factor 1)
A liver-derived hormone produced in response to growth hormone. Because GH is secreted in short pulses, IGF-1 is the stable proxy used to assess GH status.
The single marker for anyone running GH secretagogues. It's the only practical way to know whether ipamorelin, CJC-1295, sermorelin, tesamorelin or MK-677 are doing anything — and whether you've pushed into a range with real risk.
Check a IGF-1 (Insulin-like Growth Factor 1) result against this range →
What IGF-1 (Insulin-like Growth Factor 1) actually measures — the analyte, and the assay
IGF-1 never circulates free, and that is the assay's whole problem. Over 99% of it is carried in a ternary complex of IGF-1, IGF-binding protein 3 and the acid-labile subunit, a roughly 150 kDa assembly that keeps the peptide in the circulation for hours instead of minutes. Any assay must first strip the binding proteins off, classically with an acid-ethanol extraction and now more often by acidification with an excess of IGF-2 to occupy the released binding sites Huang 2024.
If the stripping is incomplete, residual binding protein interferes, and the direction of the error depends on the assay format — which is why IGF-1 methods have historically disagreed more than their manufacturers' precision claims suggest.
Six immunoassays measured against each other on the same sera produced six sets of reference values. The comparison exists because a result cannot be read against a generic interval Chanson 2016. LC-MS/MS methods traceable to the NIST and WHO standards now agree closely enough that reference intervals can be harmonized between laboratories — which is a statement about mass spectrometry, not about the immunoassays Ezra 2023.
Everything is expressed as a standard deviation score for age. IGF-1 falls steeply and continuously across adult life, so a raw number is meaningless without an age; the SDS is the number clinicians actually use, and it inherits every error in the age-specific interval it was computed from Yuen 2023.
IGF-1 (Insulin-like Growth Factor 1): what changes the blood, and what only changes the reading
What changes the IGF-1 in you:
- Growth hormone secretion, which IGF-1 exists to integrate. GH itself is pulsatile with a half-life of minutes; IGF-1 smooths it into a stable daily average, which is the whole reason the test is used Yuen 2023. Exercise is one lever on it: a single bout of high-intensity interval exercise increased measured GH secretion over the following 12.5 hours Deemer 2018.
- Protein and energy intake, which are separate levers. Nutritional regulation of the IGFs is the classic account of why a fasting person has a low IGF-1 despite a high GH Thissen 1994. In humans practicing long-term calorie restriction, serum IGF-1 was not lower than in controls unless protein intake was also reduced — protein, not calories, was the lever Fontana 2008. Controlled caloric and protein restriction move IGF-1 and its binding proteins in children and adults alike Smith 1995.
- Age, which halves it between the twenties and the seventies.
- Liver function, since the liver makes most circulating IGF-1 under GH drive; cirrhosis lowers it.
- Thyroid status, insulin and systemic illness, all of which modulate hepatic GH sensitivity.
- Exogenous GH, GH secretagogues and IGF-1 itself, which is why this is the monitoring number for those interventions.
What changes only the reading:
- Which immunoassay, and which of its reference intervals Chanson 2016 Ezra 2023.
- Incomplete separation from IGFBP-3, the format-specific failure Huang 2024.
- The age band used to compute the SDS. A person near a band boundary can change SDS by crossing a birthday Yuen 2023.
- Biotin, on streptavidin platforms Li 2020.
- Heterophile antibodies, which bridge two-site formats Ghazal 2022.
- A laboratory change mid-series, which is the commonest reason a monitored IGF-1 appears to jump.
Reference interval or decision threshold — which kind of number IGF-1 (Insulin-like Growth Factor 1) is
Reference interval, age-stratified, and method-specific to an unusual degree. The comparison of six immunoassays produced six different sets of reference values on the same samples, which means an IGF-1 read against the wrong manufacturer's interval can move by a standard deviation or more Chanson 2016.
The SDS is the unit that transfers, and only if both intervals came from the same method. Harmonization of reference intervals between laboratories became possible for LC-MS assays traceable to common standards Ezra 2023; it has not generally been achieved for immunoassays.
There is a decision threshold, and it belongs to acromegaly and GH deficiency, not to longevity. An IGF-1 SDS above the age-specific range triggers a growth hormone suppression test; a low SDS is one input into GH deficiency testing, whose accuracy, caveats and assay-dependence are the subject of its own guidance Yuen 2023.
No number here is a longevity target. The observational literature relating IGF-1 to lifespan is U-shaped and confounded by nutrition, body composition and illness, and no trial has assigned anyone an IGF-1 level Fontana 2008.
How you would know your IGF-1 (Insulin-like Growth Factor 1) was wrong — and when to redraw
Six weeks, because IGF-1 integrates growth hormone over days and responds to nutrition over weeks. Its own half-life in the ternary complex is roughly 12 to 15 hours, but the pool being measured is set by hepatic production, so a meaningful change needs a sustained change upstream.
The fasting state matters less than people expect and the nutritional history matters more. IGF-1 does not spike after a meal, but several days of underfeeding will lower it Thissen 1994 Smith 1995.
Conditions that must match: the same laboratory and assay Chanson 2016; the same age band for the SDS; stable protein intake for at least a week; no acute illness; and no biotin for 48 hours.
What would have to change for the second number to mean something:
- IGF-1 low with normal nutrition? The confirmatory test is a provocative growth hormone test, not a repeat IGF-1 — a low IGF-1 alone does not establish GH deficiency in an adult Yuen 2023.
- IGF-1 low and you are dieting? That is the expected effect of restricted protein and energy and needs no endocrine investigation Fontana 2008.
- IGF-1 high? Before anything else, confirm on the same method and check that the age band is right. A genuinely raised SDS is followed by an oral glucose tolerance test with growth hormone measured through it Yuen 2023.
- IGF-1 unchanged on a secretagogue? Check fasting insulin and ALT in the CMP: hepatic GH resistance is the commonest reason drive rises and IGF-1 does not.
- A step change in a series? Ask whether the laboratory changed platform Ezra 2023.
What IGF-1 (Insulin-like Growth Factor 1) cannot tell you
It cannot measure growth hormone secretion directly. It is an integrator, and a normal IGF-1 does not exclude an abnormal GH profile Yuen 2023.
It cannot separate GH deficiency from undernutrition or liver disease, all of which lower it by the same final pathway Thissen 1994.
It cannot be compared between assays without an SDS from the same method Chanson 2016 Ezra 2023.
It cannot tell you anything about local tissue IGF-1. Muscle and other tissues express IGF-1 that acts where it is made and never enters the circulation at a measurable concentration.
The wrong inference readers actually draw is that IGF-1 is a dial to be set — high for muscle, low for longevity. Neither direction has a trial behind it, the number is dominated by age and protein intake, and the same value read on two manufacturers' assays can differ by more than most interventions move it Chanson 2016 Fontana 2008.
Sources read for these sections
- Ezra S, et al. Agreement of LC-MS assays for IGF-1 traceable to NIST and WHO standards permits harmonization of reference intervals between laboratories. Clinical Biochemistry 2023 · PMID 37031902
- Chanson P, et al. Reference Values for IGF-I Serum Concentrations: Comparison of Six Immunoassays. Journal of Clinical Endocrinology and Metabolism 2016 · PMID 27167056
- Huang R, et al. Challenges of insulin-like growth factor-1 testing. Critical Reviews in Clinical Laboratory Sciences 2024 · PMID 38323343
- Thissen JP, et al. Nutritional regulation of the insulin-like growth factors. Endocrine Reviews 1994 · PMID 8156941
- Fontana L, et al. Long-term effects of calorie or protein restriction on serum IGF-1 and IGFBP-3 concentration in humans. Aging Cell 2008 · PMID 18843793
- Smith WJ, et al. Effects of caloric or protein restriction on insulin-like growth factor-I (IGF-I) and IGF-binding proteins in children and adults. Journal of Clinical Endocrinology and Metabolism 1995 · PMID 7531712
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
Age. Falls markedly with age, so an adult range spanning all ages is close to useless. Insist on an age-matched reference range. - Read it with its partner
If you run GH peptides without ever testing IGF-1, you're guessing. This is the objective readout. Draw it alongside: Fasting Insulin, HbA1c (Hemoglobin A1c), Free T4 (Thyroxine). - Work out which direction is yours
If it's high — Watch for fluid retention, carpal tunnel symptoms, joint pain, jaw/hand/foot growth, and worsening insulin sensitivity. Persistent elevation warrants stopping and reassessing.
If it's low — Poor recovery, reduced lean mass, poor sleep quality, low energy. Note IGF-1 falls in under-eating regardless of GH status. - Fix it in this order
Nutrition. IGF-1 is exquisitely protein- and energy-sensitive — it drops fast in caloric restriction and low protein intake. Adequate protein (0.7–1 g/lb) and energy are prerequisites before blaming the GH axis.
Lifestyle. Deep sleep is when the majority of GH is released — sleep quality is the biggest free lever. Resistance training and high-intensity work produce acute GH pulses.
Supplements. Nothing reliably raises IGF-1 beyond adequate nutrition. Arginine/ornithine claims are largely unsupported at oral doses. Address hypothyroidism and liver health.
Hormones. Exogenous GH raises IGF-1 potently but carries insulin resistance, edema, carpal tunnel and cost/legal considerations. Correct hypothyroidism first — it suppresses the GH axis.
Compounds. Baseline before starting; retest at 6–8 weeks. Ipamorelin/CJC-1295/sermorelin stimulate pulsatile GH; tesamorelin has the strongest human trial data (FDA-approved for HIV lipodystrophy, reduces visceral fat); MK-677 (ibutamoren) raises IGF-1 substantially but reliably worsens fasting glucose/insulin and causes water retention and appetite spikes — pair it with HbA1c and fasting insulin.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
Baseline, then 6–8 weeks after starting or changing dose; every 6 months on an ongoing protocol. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Bidlingmaier M et al., J Clin Endocrinol Metab 2014 — age/sex-specific IGF-1 reference intervals. Falutz J et al., NEJM 2007 — tesamorelin RCT.
This page can tell you what could have made your IGF-1 (Insulin-like Growth Factor 1) wrong. It cannot tell you whether it did.
Everything above is free and stays free — the assay, what changes the reading rather than the blood, the retest window and the sources. What no page can do is look at your draw: which laboratory ran it, at what hour, what you were taking that week, and what else was flagged beside it. Every one of those changes the answer, and none of them is on any page. Bringing a real result to people who know that list is what the members' area is for.
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Order this test — 10% off → Browse all 103 markers →What IGF-1 (Insulin-like Growth Factor 1) is usually tested alongside
One marker is a data point. These panels add the markers that make IGF-1 (Insulin-like Growth Factor 1) interpretable, name why each is on the list, and load the set into your cart at 10% off.
includes this + 7 more markers — Anyone handed a raised prolactin — on a routine hormone panel, during a fertility workup, or after being told it explains low libido, missing periods or gynecomastia.
includes this + 8 more markers — Using ipamorelin, CJC-1295, tesamorelin, sermorelin or MK-677 (ibutamoren).
What people use IGF-1 (Insulin-like Growth Factor 1) to decide
Nobody orders a test for its own sake. IGF-1 (Insulin-like Growth Factor 1) is on the test list for these pathways — each one links to what the pathway claims, and what its test list is read for before you spend anything on it.
Muscle loss on an aggressive deficit is invisible on the scale and obvious in IGF-1 and testosterone, both of which fall in a sustained deficit. Worth a baseline before you start and a recheck at eight weeks — this is the panel built specifically for people running a GLP-1.
IGF-1 is the only honest read on whether a GH protocol is doing anything — GH itself is pulsatile and a single draw is close to meaningless. Track fasting glucose and insulin alongside, because the most predictable cost of this pathway is insulin resistance and it arrives quietly.
BDNF isn't clinically measurable, but the things that suppress it are — inflammation, insulin resistance and poor sleep. ApoE status is a one-time test that changes how seriously you take this whole pathway.
Eight hours and still unrecovered is a depth problem. Low IGF-1 and low morning testosterone are downstream evidence of it, since both are produced largely during slow-wave sleep.
IGF-1 (Insulin-like Growth Factor 1) is also on the test list for these, where it narrows the picture rather than settling it:
Myostatin & activin inhibition · Recovery, sleep & the anabolic window between sessions
What moves your IGF-1 (Insulin-like Growth Factor 1)
14 compounds in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 278 compounds & 371 supplements →
Would you feel it? Symptoms IGF-1 (Insulin-like Growth Factor 1) helps explain
People search for how they feel, not for a marker. These are the complaints where this one is worth checking, and whether it is first-line or a follow-up.
Why your IGF-1 (Insulin-like Growth Factor 1) might be wrong
Most abnormal results are interference, not disease. Check these before you change anything. Each says whether the number is wrong (repeat it), badly timed (redraw it), or real with a cause.
Falls markedly with age, so an adult range spanning all ages is close to useless.
Insist on an age-matched reference range.
Under-eating produces hepatic GH resistance — GH goes up while IGF-1 falls. A low IGF-1 in a lean, hard-training person is usually under-fueling, not deficiency.
Interpret against intake and training load.
IGF-1 is made in the liver, so impaired function lowers it regardless of GH.
Read alongside liver enzymes.
What IGF-1 (Insulin-like Growth Factor 1) means in combination
One marker tells you a little; combinations tell you the story. These are the named patterns this one takes part in.
Almost certainly nothing. GH is secreted in pulses during deep sleep and is undetectable between them, so a random draw catches a trough far more often than a peak. A single low GH is not evidence of deficiency.
IGF-1 is the stable proxy and it is the number to read. Genuine GH deficiency is diagnosed by stimulation testing, not a random level. If you are considering secretagogues, IGF-1 is the before-and-after marker.
Hepatic growth hormone resistance. The pituitary is shouting and the liver will not answer, because IGF-1 production requires adequate energy. This is the endocrine fingerprint of eating too little for your training load — not a growth hormone problem.
Eat more. No secretagogue overcomes this, because the missing input is calories rather than signal. See the RED-S and performance panels.
What to test next
These put IGF-1 (Insulin-like Growth Factor 1) in context — each with its own full breakdown.
Frequently asked questions
Strongly age-dependent: ~115–355 ng/mL at age 20–30, declining each decade. Always use the age-adjusted range on your report. Ranges vary by laboratory and assay — always compare to the range printed on your own report.
Upper-normal for your age — not above it. Supraphysiologic IGF-1 raises theoretical concerns for insulin resistance, tissue overgrowth and cancer-promoting signaling. More is explicitly not better.
Watch for fluid retention, carpal tunnel symptoms, joint pain, jaw/hand/foot growth, and worsening insulin sensitivity. Persistent elevation warrants stopping and reassessing.
Poor recovery, reduced lean mass, poor sleep quality, low energy. Note IGF-1 falls in under-eating regardless of GH status.
You can order IGF-1 (Insulin-like Growth Factor 1) directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.
Where this goes next
This page is the free framework. The protocol itself — the dosing, the order to correct things in, the week-by-week schedule and what to retest — is a lesson inside Skool.