IGF-1 (insulin-like growth factor 1) levels follow a distinctive arc over a lifetime: low in early childhood, surging during puberty, then gradually declining from early adulthood onward. Because of this wide age-related swing, a single “normal” number does not exist. Instead, labs report reference ranges broken out by age group, and sometimes by sex or assay method. Those ranges matter clinically because IGF-1 is the primary blood marker used to evaluate growth hormone disorders in both children and adults. But the numbers on your lab report are less straightforward than they appear, shaped not only by your age but by the specific test your lab used, your nutritional status, and even your organ function.
The Lifespan Curve of IGF-1
IGF-1 is produced mainly by the liver in response to growth hormone. Unlike growth hormone itself, which is released in pulses and fluctuates throughout the day, IGF-1 circulates in a more stable fashion with minimal variation tied to meals or time of day.1Nature. Normative data for insulin-like growth factor-1 and insulin-like growth factor binding protein-3 and their determinants in healthy adult males from india: the INDIIGo study That stability is what makes it a practical clinical test: a single blood draw gives a reasonably reliable snapshot.
In childhood, levels are relatively modest. They climb steeply once puberty begins, peaking around two years after the fastest phase of the adolescent growth spurt. Interestingly, IGF-1 stays elevated for years after height growth has largely finished, a timing pattern that suggests the hormone serves purposes beyond simply making bones longer, including maturing the reproductive system.2The Journal of Clinical Endocrinology & Metabolism. Why Do Normal Children Have Acromegalic Levels of IGF-I During Puberty? During those peak pubertal years, some healthy teenagers carry IGF-1 levels that would be flagged as abnormally high in an adult, a fact that can confuse interpretation if age-appropriate ranges are not applied.
From late adolescence onward, the trajectory reverses. Circulating IGF-1 declines progressively through the twenties, thirties, and beyond. Large studies in both Chinese and European populations confirm a steady, roughly linear drop from around age 18 to at least 70.3PubMed Central. Reference ranges for serum insulin-like growth factor I (IGF-I) in healthy Chinese adults One study comparing younger and older adults found the age-related decline to be statistically significant, with no meaningful difference between men and women in the rate of drop.4PubMed Central. Age-related decline in circulating IGF-1 associates with impaired neurovascular coupling responses in older adults By age 60 or 70, a person’s typical IGF-1 can be a fraction of what it was at 16.
Why Your Lab’s Numbers Might Not Match Another Lab’s
This is one of the more frustrating aspects of IGF-1 testing: two different laboratories can draw blood from the same person and return meaningfully different numbers. The root cause is that several distinct assay technologies are in use, and they do not always agree.
Most commercial IGF-1 tests are immunoassays, which use antibodies to detect the hormone. Different manufacturers build these tests with different antibody pairs and different calibration curves. Even though all major commercial IGF-1 immunoassays are calibrated to the same international standard (WHO IS 02/254), substantial variability between assays persists, leading to inconsistent patient classification.5PubMed Central. Harmonization of IGF1 immunoassays towards a higher-order LC-MS/MS reference anchor In plain terms, the ruler is supposed to be the same, but the readings still drift apart.
A head-to-head comparison of two popular immunoassay platforms (DiaSorin Liaison XL and Roche Elecsys) and a mass-spectrometry-based method found a constant positive bias in one immunoassay compared to the other, averaging about 42 micrograms per liter or roughly a quarter higher. When compared against the mass spectrometry method, the gap widened to about 29 percent. In a real-world check of 43 patient samples flagged as discrepant, nine were classified as elevated by the biased immunoassay while the mass spectrometry result and clinical diagnosis said they were normal.6PubMed. Bias in IGF-1 concentrations and interpretation across three different clinical laboratory assays A separate prospective study comparing mass spectrometry with immunoassay found that about a quarter of samples were classified differently depending on which test was used, with immunoassays generally reporting higher values.7PubMed. Discordance between mass spectrometry and immunometric IGF-1 assay in pituitary disease: a prospective study
A French study evaluating two newer immunoassays against carefully screened healthy adults found that while the lower ends of the reference intervals were fairly similar across platforms, the upper limits varied markedly between the two.8PubMed Central. Reference values for IGF-I serum concentration in an adult population: use of the VARIETE cohort for two new immunoassays That means the ceiling for “normal” depends partly on which machine ran your blood. The practical takeaway: if your doctor is tracking your IGF-1 over time, the same lab and the same assay platform should be used for each draw. Comparing a result from Lab A’s immunoassay against a previous result from Lab B’s mass spectrometry method is not apples to apples.
Do Men and Women Need Separate Reference Ranges?
You might expect a sex-based difference given how many other hormones diverge between men and women, but the evidence on IGF-1 is surprisingly mixed. A Serbian study that examined the question directly found that sex differences in IGF-1 were not significant overall and concluded that separate reference ranges for men and women are not necessary.9PubMed Central. Serum Insulin-Like Growth Factor-1 (IGF-1) Age-Specific Reference Values for Healthy Adult Population of Serbia The study on age-related decline cited earlier also found no sex-related differences in circulating IGF-1.4PubMed Central. Age-related decline in circulating IGF-1 associates with impaired neurovascular coupling responses in older adults
Some labs do publish separate male and female ranges anyway, partly because small sex differences can appear in specific age windows, particularly during and just after puberty when the tempo of maturation differs between boys and girls. During childhood and most of adulthood, though, the overlap between sexes is large enough that many researchers argue combined ranges work well. The bottom line: if your lab report shows sex-specific ranges, those are valid to use, but a result compared against a combined age-matched range is generally just as informative.
Ethnic and Genetic Variation
Reference ranges are typically built from local or regional populations, which raises the question of whether they translate well across ethnic groups. A large U.S. study using NHANES data found that IGF-1 levels differed significantly by race and ethnicity after adjusting for age and BMI. Among men, non-Hispanic white males had the highest age-adjusted levels (around 289 ng/mL), followed by non-Hispanic Black men (about 279 ng/mL), with Mexican-American men lowest (around 244 ng/mL). Among women, the pattern shifted: non-Hispanic Black women had the highest levels (about 275 ng/mL), non-Hispanic white women were in the middle (around 258 ng/mL), and Mexican-American women had the lowest (roughly 226 ng/mL).10PubMed Central. Race/ethnic Variation in Serum Levels of IGF-I and IGFBP-3 in US Adults
These differences appear early in life. Studies of prepubertal children have found that African-American children tend to have higher IGF-1 than European-American children, a pattern seen in both boys and girls.11Journal of Pediatric Endocrinology & Metabolism. Ethnic differences in androgens, IGF-I and body fat in healthy prepubertal girls A study using ancestry-informative genetic markers found that the degree of African genetic admixture was an independent predictor of IGF-1 levels in children, suggesting the difference has a genetic basis rather than being purely driven by diet or socioeconomic factors.12Pediatric Research. Early Ethnic Difference in Insulin-Like Growth Factor-1 Is Associated with African Genetic Admixture
The practical implication is that a reference range derived from one ethnic population may not perfectly fit another. Most commercial lab ranges are drawn from predominantly white populations, which could lead to subtle misclassification for patients of different backgrounds. Clinicians working with diverse populations are generally aware of this, but it is worth keeping in mind if your results land near the edge of a reference interval.
What Else Shifts IGF-1 Besides Age
Age is the biggest single driver of your IGF-1 level, but several other factors can push it meaningfully higher or lower.
Body Weight
The relationship between body mass and IGF-1 is not a straight line. In a study of over 3,200 Chinese children aged 2 to 18, IGF-1 levels rose as BMI increased up to a point, then dropped again in the most obese children, creating an inverted-U shape.13PubMed Central. Association of Body Mass Index with Insulin-like Growth Factor-1 Levels among 3227 Chinese Children Aged 2–18 Years Both underweight and severely obese children had higher odds of low IGF-1 compared to children in the middle of the weight spectrum. In adults, the pattern is broadly similar: moderate weight tends to support normal levels, while malnutrition and extreme obesity can both suppress them.
Protein Intake
Dietary protein is one of the strongest nutritional regulators of IGF-1. A study tracking adults of different ages found that mean IGF-1 levels were positively associated with protein consumption in both younger and older age groups.14PubMed Central. Low Protein Intake is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population People eating high-protein diets tended to have higher IGF-1, and those eating low-protein diets had substantially lower levels. This link is robust enough that clinicians evaluating an unexpectedly low IGF-1 result will often ask about dietary habits before ordering further workup.
Exercise
Both endurance and resistance exercise can acutely raise total circulating IGF-1, though the effect on free IGF-1 (the unbound fraction) is less clear. A meta-analysis pooling multiple exercise studies found that endurance exercise had a larger acute effect on total IGF-1 than resistance exercise did, but neither type significantly moved free IGF-1 levels.15PubMed. Can IGF-1 Serum Levels Really be Changed by Acute Physical Exercise? A Systematic Review and Meta-Analysis The researchers noted that the conditions under which exercise reliably changes IGF-1 are not well defined, so a single workout is unlikely to meaningfully shift your lab result.
Pregnancy
Pregnant women experience a significant rise in IGF-1, particularly in the third trimester. One study establishing trimester-specific ranges found third-trimester levels roughly double the upper end seen in age-matched non-pregnant women (up to about 344 ng/mL versus about 188 ng/mL).16PubMed Central. MON-283 IGF-1 Levels During Normal Pregnancy First and second trimester values were closer to non-pregnant ranges, with the sharp rise appearing in the final months. Standard reference ranges for IGF-1 do not account for pregnancy, so a third-trimester level compared against a general adult range will look falsely high.
Liver Disease, Kidney Disease, and Misleading Results
Because the liver is the main production site for circulating IGF-1, any condition that damages the liver can suppress levels. In patients with cirrhosis, IGF-1 deficiency is a common finding regardless of what caused the liver disease, and the resulting hormone shortfall contributes to the malnutrition and muscle wasting often seen in advanced liver failure.17PubMed Central. Insulin-Like Growth Factor-1 Deficiency and Cirrhosis Establishment A low IGF-1 in someone with known liver disease does not necessarily point to a pituitary or growth hormone problem; it may simply reflect the liver’s inability to produce the hormone.
Kidney disease creates a different kind of interpretive trap. In children with chronic kidney disease, total IGF-1 in the blood can look stable or even mildly elevated while the bioavailable fraction (the portion actually free to act on tissues) drops progressively as kidney function worsens.18Child Kidney Dis. The kidney and IGF-1 axis as a central regulator of linear growth and adult height in pediatric health and disease This happens because the binding proteins that carry IGF-1 in the blood accumulate when the kidneys cannot clear them, trapping more of the hormone in an inactive bound state. A standard total-IGF-1 test can look reassuring while the patient is effectively IGF-1 deficient at the tissue level. In adults on hemodialysis, bioactive IGF-1 was found to decline by roughly 50 percent during a dialysis session even while total IGF-1 dropped only marginally.19PubMed. Marked reductions in bioactive insulin-like growth factor I (IGF-I) during hemodialysis
How IGF-1 Is Used to Diagnose Growth Hormone Disorders
IGF-1 is the workhorse lab test for two opposite clinical problems: too little growth hormone and too much.
In children being evaluated for short stature, a low IGF-1 raises suspicion for growth hormone deficiency and often triggers further testing with growth hormone stimulation. However, IGF-1 alone is not a great standalone screening test for this purpose. A large reappraisal found that the best diagnostic cutoff for IGF-1 in detecting growth hormone deficiency was about 1.5 standard deviations below the mean for age, with a sensitivity of roughly 68 percent and specificity around 63 percent. Accuracy was better in pubertal children than in younger, prepubertal ones.20PubMed Central. IGF1 for the diagnosis of growth hormone deficiency in children and adolescents: a reappraisal In other words, a normal IGF-1 does not fully rule out growth hormone deficiency, and a low result does not confirm it. The test is a useful piece of the puzzle, not the whole picture.
On the other end, IGF-1 is central to diagnosing and monitoring acromegaly, a condition caused by excess growth hormone, usually from a pituitary tumor. Updated consensus guidelines have elevated IGF-1’s role: in a patient with physical features suggestive of acromegaly, an IGF-1 concentration greater than 1.3 times the upper limit of normal for age is now considered confirmatory, without necessarily requiring additional testing. And when monitoring treatment response in acromegaly, current guidelines recommend tracking IGF-1 as the main measure of whether therapy is working.21Endocrinology and Metabolism. Insulin-Like Growth Factor 1 as a Pillar in Acromegaly: From Diagnosis to Long-Term Management
Binding Proteins and What “Total IGF-1” Actually Measures
When you see an IGF-1 result on a lab report, you are almost always looking at total IGF-1, meaning the combined amount of hormone that is free in the blood plus the much larger fraction bound to carrier proteins. The most abundant of these carriers is IGFBP-3, which forms a large complex that transports IGF-1, extends its half-life in circulation, and controls how much free hormone is available to interact with receptors on cells.22PubMed Central. Establishment of IGF-1 and IGFBP-3 continuous reference percentiles from data of healthy children using three kinds of immunoassay systems
This distinction between total and bioavailable IGF-1 matters in certain clinical settings, as the kidney disease example illustrates. A total IGF-1 level within the reference range can coexist with a functionally low level of the biologically active form if binding protein levels are elevated. Routine lab testing does not usually measure free or bioactive IGF-1 because the assays are less standardized and more expensive. But when a total IGF-1 result does not seem to match the clinical picture, particularly in patients with kidney disease or severe illness, clinicians may need to consider that the number on the page may overstate how much IGF-1 is actually doing its job.
How to Read Your Own Results
Most lab reports will show your measured IGF-1 value alongside the reference range for your age group (and sometimes sex). Some labs also convert the raw number into a standard deviation score (SDS), which tells you where your result sits relative to the population average for your age: a score near zero is typical, negative scores are below average, and positive scores are above. This standardized score can be more useful than the raw number because it accounts for the expected age-related decline and makes it easier to track changes over time, even if the raw number naturally drops as you get older.
If your result is near the boundaries of the reference range, the assay variability discussed earlier becomes relevant. A value just above the upper limit on one platform could test within range on another. Similarly, a result at the low end of normal in someone with significant liver or kidney disease may understate the problem. Context, including your medical history, symptoms, and sometimes repeat testing, matters far more than any single number in isolation. Your doctor should interpret your IGF-1 alongside other clinical information rather than treating the reference range as a rigid pass/fail cutoff.
Efforts to Standardize Reference Ranges
The inconsistency between assay platforms has been recognized as a real clinical problem, and work is underway to fix it. Mass spectrometry-based methods, which measure IGF-1 through its molecular weight rather than relying on antibody binding, are increasingly being used as the anchor for harmonization efforts. The goal is to align immunoassay results across manufacturers so that a given patient’s blood would return the same classification regardless of which test was used.5PubMed Central. Harmonization of IGF1 immunoassays towards a higher-order LC-MS/MS reference anchor
Progress has been incremental. All major assay makers now calibrate to the same WHO international standard, but as the data show, this has not eliminated the between-assay bias. Newer harmonization work aims to go further by using mass spectrometry as a higher-order reference method that immunoassays can be corrected against. Until that standardization is fully implemented across clinical labs, the practical advice remains the same: use the same lab for serial measurements, compare your result only against the reference range printed on your own report, and treat borderline results with appropriate caution.