IGF-1 Levels by Age: What's Normal After 35?

If you have ever had bloodwork come back with an IGF-1 number on it, you have probably wondered what that number is supposed to look like. And if you are over 35, you have likely noticed that the reference range printed next to your result looks a lot narrower than the ones you see quoted online. That is not an error. IGF-1 levels by age shift considerably across adulthood, and what counts as normal after 35 is genuinely different from what counted as normal at 22.

Below you will find actual published reference ranges by age and sex from clinical laboratories, an explanation of why the numbers drift downward, what the research says about low versus high IGF-1, and how to read your own result without over-interpreting it.

Quick Summary

• IGF-1 (insulin-like growth factor 1) is made mostly by your liver in response to growth hormone. Because it stays stable in the blood while growth hormone comes in pulses, IGF-1 is the standard blood marker used to gauge growth hormone activity.

• For men, published reference intervals run roughly 83 to 233 ng/mL at ages 36 to 40, dropping to about 67 to 205 ng/mL by ages 46 to 50, based on Mayo Clinic Laboratories values.

• Growth hormone secretion declines about 15% per decade after the third decade of life, and falling IGF-1 largely reflects that reduced signal rather than the liver losing its ability to respond.

• Normal is defined against your own age and sex, not against a young adult. A result of 150 ng/mL is unremarkable at 40 and would be notably high at 80.

• Both low and high IGF-1 track with worse outcomes in large cohort studies. The relationship is U-shaped, not "more is better."

• Reference ranges are assay-specific and are not interchangeable between labs. Week-to-week variation within the same person runs roughly 10% to 30%.

• Sleep quality, visceral fat, protein and energy intake, thyroid status, and liver health all influence IGF-1. Exercise, notably, does not reliably raise resting levels in older adults.

• A single below-range IGF-1 does not diagnose growth hormone deficiency. Confirming that requires provocative stimulation testing ordered by a physician.

What Is IGF-1, and Why Does It Matter After 35?

IGF-1 is a 70-amino-acid polypeptide, and it is the main courier for growth hormone's anabolic effects. Your pituitary releases growth hormone, your liver reads that signal, and the liver produces IGF-1, which then acts on muscle, bone, connective tissue, and other targets. In practice, IGF-1 is the readout of your body's growth and repair signaling, which is exactly why it becomes interesting in your late thirties and forties.

Growth hormone plays real roles in adult life. It helps maintain lean and bone mass, drives lipolysis that limits visceral fat accumulation, and contributes to carbohydrate metabolism, cardiovascular function, aerobic capacity, and cognitive function. When that axis quiets down, the associated changes are familiar to anyone past 40: more abdominal fat, less lean mass, slower recovery, thinner skin, reduced exercise capacity.

Why doctors measure IGF-1 instead of growth hormone

Growth hormone is released in bursts, mostly overnight, with dramatic swings across the day. A random growth hormone draw tells you almost nothing. IGF-1 behaves differently. Uncomplexed IGF-1 has a half-life of only about 10 minutes, but the complex it forms with IGF binding protein 3 clears far more slowly, with a half-life around 12 hours. That buffering means IGF-1 shows only minor day-to-day fluctuation and functions as a stable, integrated measure of growth hormone output and tissue effect.

That stability is the whole reason IGF-1 became the first-tier test for evaluating growth disorders in children and adults.

IGF-1 Levels by Age Chart: Reference Ranges for Adults

The table below uses reference intervals published by Mayo Clinic Laboratories, derived using a chemiluminescence immunoassay. These are the kinds of numbers that would actually be printed on a lab report.

Look at what happens to the ceiling. For men, the upper limit of normal falls from roughly 355 ng/mL in the early twenties to about 233 by the late thirties, and to about 205 by age 50. That is a decline of more than 40% in the top of the range across about 25 years. The floor moves too, but far less dramatically.

The other pattern worth noticing: men run slightly higher than women through midlife, and the two converge and then largely overlap in later decades.

Why your lab's numbers may look different from this chart

They probably will, and that is expected rather than alarming.

IGF-1 assays vary meaningfully between manufacturers and platforms, and Mayo Clinic Laboratories states plainly that results obtained with different assay methods or kits should not be used interchangeably. Between-laboratory variability has been reported in the range of about 7% to 19% depending on calibration practices.

A concrete example: University of Iowa Diagnostic Laboratories publishes single-year intervals and lists a 40-year-old male range of 98.5 to 229 ng/mL. Mayo's 36-to-40 band is 83.4 to 232.7. Same underlying biology, same general shape, different boundaries.

The practical implication is straightforward. Compare your result to the range your lab printed, and if you are tracking IGF-1 over time, stay with the same laboratory.

What Counts as a Normal IGF-1 Level After 35?

A normal IGF-1 level after 35 is any value inside your laboratory's age-specific and sex-specific reference interval, which for most men in their late thirties falls between roughly 83 and 233 ng/mL and for most women between roughly 69 and 227 ng/mL. There is no single universal target number. The same result can be entirely normal at 40 and clearly abnormal at 75, which is why both age and sex are required to interpret an IGF-1 test at all.

Values below the 2.5th percentile for age are considered consistent with growth hormone deficiency or severe growth hormone resistance. Values above the range raise the question of growth hormone excess. Everything in between is, statistically speaking, unremarkable.

Understanding the Z-score on your report

Many labs now report IGF-1 alongside a Z-score, also called a standard deviation score. This is more useful than the raw number.

The Z-score tells you approximately how many standard deviations your result sits above or below the median for your age and sex. Most results inside the reference interval land between -2.0 and +2.0. A Z-score of 0 means you are at the age-adjusted median. A Z-score below -2 flags a result below the 2.5th percentile.

The advantage is that the Z-score already accounts for your age. It lets you see whether you are near the bottom, middle, or top of your peer group rather than just inside or outside a wide band.

Why IGF-1 Declines With Age

This is not mysterious, and it is not primarily a failure of the liver.

Growth hormone secretion declines roughly 15% per decade after the third decade of life. At puberty, secretion peaks around 150 micrograms per kilogram per day. By age 55, it has dropped to about 25 micrograms per kilogram per day. That reduction comes mainly from a marked drop in the amplitude of overnight growth hormone pulses, with little change in how often the pulses occur.

IGF-1 declines with age mainly because of that reduced growth hormone secretion, not because of increased growth hormone resistance at the liver. The signal gets quieter. The receiver is still working.

Sleep is deeply tangled up in this. Growth hormone release is tied to slow-wave sleep, and slow-wave sleep collapses earlier in adulthood than most people assume. In a study of 149 healthy men aged 16 to 83, deep slow-wave sleep fell from 18.9% of the night in early adulthood (ages 16 to 25) to just 3.4% by midlife (ages 36 to 50).

That decline was paralleled by a major drop in growth hormone secretion, and the amount of growth hormone secreted tracked with slow-wave sleep independently of age.

That timing matters. The steepest part of the sleep-related decline happens between the mid-twenties and the late forties, which is precisely the window when men start noticing changes and start searching for answers.

What lowers IGF-1 beyond age alone

Several things push IGF-1 down independently of how old you are:

• Visceral fat. Abdominal visceral fat mass is one of the strongest predictors of reduced growth hormone release, and elevated adiposity lowers serum IGF-1 independently of chronological age. Substantial weight loss through dietary restriction has been shown to restore growth hormone secretion.

• Poor or fragmented sleep. Serum IGF-1 falls after roughly 25 hours of sleep deprivation and recovers after a recovery night. Patients with chronic insomnia disorder show lower serum IGF-1 than controls.

• Low protein or low energy intake. In UK Biobank data covering 11,815 participants, higher total protein intake was associated with higher circulating IGF-1. Prolonged calorie restriction and malnutrition reliably lower it.

• Hypothyroidism, which reduces the liver's responsiveness to growth hormone.

• Chronic liver disease and kidney disease, since the liver is the main source of circulating IGF-1.

• Poorly controlled diabetes, which can create hepatic growth hormone resistance until glycemic control improves.

• High glucocorticoid exposure and oral estrogen, both of which blunt hepatic IGF-1 output.

• Heavy alcohol intake. In the same UK Biobank analysis, consuming more than 40 g of alcohol per day was inversely associated with IGF-1 versus under 1 g per day.

Is Low IGF-1 Bad? What the Research Actually Shows

Here is where a lot of online content goes wrong. The wellness framing treats IGF-1 as a "higher is better" number. The epidemiology does not support that.

The relationship between IGF-1 and mortality is U-shaped. Both the lowest and the highest levels associate with increased risk.

In UK Biobank, 380,997 participants with no history of cancer, cardiovascular disease, or diabetes at baseline were followed for a median of 8.8 years. Compared to the fifth decile of IGF-1, the lowest decile carried 39% higher all-cause mortality risk, 20% higher cancer mortality, and 39% higher cardiovascular mortality. The highest decile also showed elevated risk.

The EPIC-Heidelberg study, which followed participants for a median of 17.5 years, found the same shape: individuals at both the lowest and highest circulating IGF-1 levels had higher hazards of death from cancer, cardiovascular disease, and all causes.

A meta-analysis pooling 19 prospective cohort studies covering 30,876 participants confirmed it quantitatively. Compared to the middle IGF-1 category, low IGF-1 carried a hazard ratio of 1.33 for all-cause mortality, and high IGF-1 carried a hazard ratio of 1.23. The authors identified a 120 to 160 ng/mL band as associated with the lowest mortality, though that specific window should be read cautiously given how much assay methods differed across the pooled studies.

There is a further wrinkle. Animal research points in the opposite direction from the "boost it" narrative. Mice with growth hormone resistance and profoundly reduced IGF-1 are healthy and live longer.

Mice with mutations preventing pituitary growth hormone production show lifespan extended by 42% with delayed tumor development. Growth hormone administration shortens lifespan in those models. How much of that translates to humans is genuinely unresolved.

The honest read: extremely low IGF-1 is worth investigating, extremely high IGF-1 is worth investigating, and the middle of your age-appropriate range is a reasonable place to be.

Symptoms of Low IGF-1 (and Why Symptoms Alone Are Not Enough)

The changes associated with a quieter growth hormone axis overlap heavily with what people describe as normal aging:

• Increased fat mass, particularly abdominal

• Decreased lean body mass and muscle strength

• Reduced exercise performance and cardiac capacity

• Decreased bone mass

• Thin, dry skin

• Fatigue, low mood, reduced self-esteem, sleep disturbance

• Atherogenic shifts in lipid profile

That overlap is exactly the problem. These symptoms are nonspecific. They are equally consistent with untreated sleep apnea, hypothyroidism, depression, low testosterone, iron deficiency, poor sleep hygiene, or simple deconditioning. A symptom list cannot distinguish among them, and neither can a single lab value.

It is also worth knowing that aging by itself is not an indication for growth hormone deficiency testing. Most adult growth hormone deficiency traces to pituitary tumors or their treatment, or increasingly to traumatic brain injury. Clinical context drives the decision to test, not age.

How to Get Your IGF-1 Tested

1. Talk to a physician first about why you want the number. IGF-1 is most informative when there is a specific question behind it. If your real concern is fatigue or body composition, a broader panel including thyroid function, metabolic markers, and testosterone will tell you more than IGF-1 alone.

2. Stop high-dose biotin at least 12 hours before the draw. Biotin above about 5 mg, common in hair, skin, and nail supplements, interferes with these immunoassays and can produce falsely low IGF-1 results. Mayo Clinic Laboratories specifically instructs patients to avoid biotin-containing supplements for 12 hours beforehand.

3. Do not stress about fasting or timing. Because IGF-1 lacks significant circadian variation, most labs do not require fasting. Your provider may still ask you to fast if IGF-1 is bundled with glucose or lipids.

4. Make sure your age and sex are on the requisition. Both are required to generate a valid reference interval and Z-score. Without them, the raw number is uninterpretable.

5. Read your result against your own lab's range, not a chart from the internet. Then look at the Z-score if one is reported.

6. Repeat before drawing conclusions from a borderline value. Week-to-week intraindividual variation in IGF-1 runs roughly 10% to 30%, driven by protein intake, intercurrent illness, exercise, and stress rather than by assay error. One borderline result is a signal to retest, not a diagnosis.

If IGF-1 does come back low and there is genuine clinical suspicion, understand what comes next: serum IGF-1 alone is usually insufficient to diagnose adult growth hormone deficiency. Confirmation requires provocative testing such as an insulin tolerance test, glucagon stimulation test, or a combined GHRH-arginine test, performed and interpreted by an endocrinologist.

What Actually Moves IGF-1 in Adults

This section deserves more honesty than it usually gets.

Sleep is the highest-leverage lever. Growth hormone release is slow-wave-sleep dependent, sleep deprivation lowers IGF-1, and recovery sleep brings it back. Protecting deep sleep is not a wellness platitude here, it is the mechanism.

Reducing visceral fat helps. Abdominal visceral fat is among the best predictors of blunted growth hormone secretion, and meaningful weight loss has been shown to restore secretion.

Adequate protein and adequate calories matter. Under-eating protein or energy reduces hepatic IGF-1 output. This is one of the more common and most reversible causes of a low result in otherwise healthy adults.

Treating the underlying condition matters more than chasing the number. If hypothyroidism, poorly controlled diabetes, or fatty liver disease is dragging IGF-1 down, addressing that is the intervention.

Exercise, surprisingly, does not reliably raise resting IGF-1 in older adults. Vigorous exercise triggers an acute growth hormone spike, but overnight secretion afterward is actually impaired, and a six-month endurance training program in healthy older men and women did not alter IGF-1. The benefits of exercise on muscle mass and function appear to work largely independently of growth hormone. Train anyway, for every other reason. Just do not expect it to move this specific number.

Growth hormone injections are not the answer for age-related decline, and are not legal for that purpose. Both the American Association of Clinical Endocrinology and the Endocrine Society state that growth hormone therapy should be used only in adults with confirmed deficiency and should not be given for age-related declines in growth hormone or IGF-1 in otherwise healthy individuals.

In pooled trials of growth hormone in healthy older adults, soft tissue edema occurred in 50% of treated participants versus 8% of controls, carpal tunnel syndrome in 19% versus 1%, and arthralgias in 21% versus 5%. In the United States, distributing growth hormone for anti-aging purposes is prohibited by federal law.

What About Supplements That Claim to Raise IGF-1?

Be skeptical, and be specific about what is being claimed.

Deer and elk antler velvet is the most common ingredient marketed on an IGF-1 story, because velvet antler tissue genuinely does contain IGF-1 among its growth factors. The problem is what happens after you swallow it. In a randomized, double-blind trial of 38 active males taking deer antler velvet extract, powder, or placebo across a 10-week strength program, researchers measured testosterone, IGF-1, erythropoietin, red cell mass, plasma volume, and total blood volume. All groups improved strength roughly equally, and the study found no advantage for antler velvet on IGF-1 or the other circulating markers. A subsequent trial in male and female rowers found that antler velvet did not alter measured hormonal responses either. A separate controlled study of short-term sublingual antler velvet found no increase in IGF-1 versus placebo.

So the accurate statement is this: oral antler velvet has not been shown to raise circulating IGF-1 in humans. Any product claiming otherwise is ahead of its evidence.

Where BioPro+ fits

BioProtein Technology built BioPro+ on a deliberately different premise, and that difference is worth stating plainly.

BioPro+ is not formulated or marketed to raise your blood IGF-1, and it is not a hormone. It is a sublingual liquid supplement built from naturally derived ingredients: a proprietary elk antler velvet extract supplying amino acids, peptides, and structural compounds, shilajit as a source of fulvic acid and trace minerals studied in relation to cellular energy production, and functional botanicals including aloe vera, goji, and tribulus. The formula contains no synthetic hormones and no prescription compounds, and it is third-party tested.

The reason we like this approach is that it lines up with what the science above actually supports. The variables that genuinely influence the GH/IGF-1 axis in adults are sleep quality, body composition, and nutrient adequacy. Those are inputs, not levers you pull on a lab number.

BioPro+ is designed to support those everyday inputs by supplying naturally derived building blocks the body uses for tissue maintenance and recovery, alongside ingredients studied for their roles in cellular energy metabolism. It is taken once each morning, held under the tongue, and swallowed.

What BioPro+ is not: it is not a substitute for getting your IGF-1 tested, it is not a substitute for a physician's evaluation if your result is abnormal, and it is not a treatment for growth hormone deficiency or any other condition. If your IGF-1 comes back below range, the right next step is a conversation with your doctor, not a supplement.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

When to Talk to a Doctor About Your IGF-1

Bring it to a physician if any of the following apply:

• Your IGF-1 is below the 2.5th percentile for your age and sex, or your Z-score is below -2

• Your IGF-1 is above your lab's upper limit, particularly alongside enlarging hands or feet, changing jaw structure, new headaches, or new snoring, which together raise the question of acromegaly

• You have a history of pituitary disease, pituitary surgery, cranial radiation, or significant head injury

• Your result is low and you also have symptoms that have not been explained by a full workup

And a case where you probably do not need to act: your IGF-1 sits in the lower half of your age-appropriate range, you feel fine, and nothing else on your panel is unusual. That is a normal result. Age-related decline in this axis is a physiological process, not a disease, and it is clearly distinct from pathological growth hormone deficiency.

Key Takeaways

IGF-1 levels by age follow a clear and well-documented trajectory. They peak around age 15, drift downward through adulthood, and by your late thirties the reference interval has already narrowed substantially compared to your early twenties. That decline mostly reflects reduced overnight growth hormone pulse amplitude, which is itself tightly linked to the loss of slow-wave sleep that begins in the mid-twenties.

Normal after 35 means inside your own lab's age-adjusted and sex-adjusted range. Not inside a young adult's range, and not at some optimized target you read about. The mortality data are clear that the middle of the range is the better place to be than either extreme.

Practical next steps: if you want the number, get it through a physician who can put it in context. Avoid biotin for 12 hours beforehand. Read it against your lab's range and Z-score. Retest before drawing conclusions from a borderline value. And put your energy into the variables that demonstrably influence this axis, meaning sleep quality, visceral fat, and adequate protein and calories, rather than trying to force a single lab value upward.

Frequently Asked Questions

What is a good IGF-1 level for a 40-year-old man?

For a 40-year-old man, published reference intervals run approximately 83 to 233 ng/mL using Mayo Clinic Laboratories values, or 98.5 to 229 ng/mL using University of Iowa single-year values. Both are normal ranges, and the difference between them reflects different assays rather than disagreement about biology. Given that the pooled mortality research points to the middle of the range rather than the top, a mid-range result is a reasonable position. Compare your value to the range your own laboratory printed.

Does IGF-1 really drop 50% by age 35?

No, that figure overstates the case. The best-documented estimate is that growth hormone secretion declines about 15% per decade after the third decade of life, which works out to roughly 15% between ages 30 and 40, not 50%. Published reference intervals do show a substantial shift, with the male upper limit falling more than 40% between the early twenties and age 50, but that unfolds over about 25 years rather than by age 35.

Can you increase IGF-1 levels naturally?

You can influence the conditions that affect IGF-1, though results vary by individual. Improving sleep quality has direct mechanistic support, since growth hormone release depends on slow-wave sleep and IGF-1 falls with sleep deprivation. Reducing visceral fat and consuming adequate protein and calories both help, since visceral adiposity and undernutrition each suppress hepatic IGF-1 output. Treating hypothyroidism, poor glycemic control, or liver disease also matters. What does not reliably work is exercise, which does not raise resting IGF-1 in older adults despite its many other benefits.

What are the symptoms of low IGF-1 in adults?

Reduced growth hormone and IGF-1 activity in adults is associated with increased abdominal fat, decreased lean mass and muscle strength, lower exercise and cardiac capacity, reduced bone mass, thin dry skin, fatigue, low mood, and sleep disturbance. Every one of these symptoms is nonspecific and overlaps with sleep apnea, thyroid dysfunction, depression, low testosterone, and deconditioning. Symptoms cannot confirm low IGF-1, and low IGF-1 cannot by itself confirm growth hormone deficiency.

Should I get my IGF-1 tested if I feel fine?

Probably not on its own. IGF-1 is a first-tier test for evaluating growth disorders and monitoring growth hormone therapy, and aging by itself is not considered an indication for growth hormone deficiency testing. If you are asymptomatic and simply curious, understand that a low-but-normal result is common with age and that week-to-week variation of 10% to 30% makes single readings noisy. If you have unexplained symptoms, a history of pituitary disease or head injury, or signs suggesting growth hormone excess, then testing through a physician makes sense.

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These statements have not been evaluated by the Food and Drug Administration. BioPro+ is not intended to diagnose, treat, cure, or prevent any disease. This content is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making decisions about testing or supplementation.