IGF-1: How to Interpret Your Result

What IGF-1 Actually Measures

IGF-1 is a 70-amino-acid peptide produced primarily by the liver in response to growth hormone stimulation. When your clinician orders an IGF-1 test, they are measuring the integrated output of your GH axis over the previous 24 to 48 hours, rather than a single GH pulse that lasts only minutes [1]. This makes IGF-1 far more practical than random GH sampling.

GH itself is secreted in a pulsatile pattern, with the largest bursts occurring during slow-wave sleep. A single GH blood draw can return anywhere from undetectable to 20 ng/mL in the same healthy person within a few hours. IGF-1 sidesteps that volatility. The Endocrine Society's 2011 clinical practice guideline on GH deficiency in adults states that "serum IGF-1 measurement is the most useful initial screening test" for suspected GH disorders [2]. The test is typically reported in ng/mL (nanograms per milliliter), though some labs use nmol/L. One ng/mL equals approximately 0.131 nmol/L.

Your result only means something when compared to the right reference population. Age matters enormously. Sex matters modestly. And the assay platform your lab uses can shift absolute numbers by 10 to 20%, which is why switching labs mid-treatment can create false alarms [3].

Normal IGF-1 Ranges by Age

The reference range for IGF-1 is not a single number. It is a moving target that peaks in adolescence and declines steadily through adulthood, dropping roughly 14% per decade after age 30 [4].

Here are approximate adult reference intervals used by major commercial laboratories (Quest, Labcorp, Mayo) for serum IGF-1 in ng/mL:

• Ages 18 to 25: 116 to 358 ng/mL
• Ages 26 to 35: 117 to 329 ng/mL
• Ages 36 to 45: 101 to 267 ng/mL
• Ages 46 to 55: 87 to 238 ng/mL
• Ages 56 to 65: 75 to 212 ng/mL
• Ages 66 to 75: 64 to 188 ng/mL
• Ages 76 and older: 48 to 166 ng/mL

These ranges represent the 2.5th to 97.5th percentiles of healthy populations. A 55-year-old with an IGF-1 of 95 ng/mL is within normal limits, even though that same value in a 20-year-old would warrant further evaluation. Pubescent adolescents can have values exceeding 500 ng/mL during peak growth velocity, which is entirely physiologic [5].

Sex-based differences exist but are smaller than age effects. Premenopausal women tend to run 10 to 15% lower than age-matched men, partly because oral estrogen suppresses hepatic IGF-1 production [6]. This is clinically relevant: a woman on oral contraceptives or oral hormone replacement may show a suppressed IGF-1 that does not reflect true GH status. Transdermal estrogen avoids this effect.

What a High IGF-1 Means

An IGF-1 above the age-adjusted upper limit raises three main diagnostic possibilities: acromegaly, exogenous GH or peptide use, or assay interference.

Acromegaly, caused by a GH-secreting pituitary adenoma, is the most important diagnosis to exclude. The disease affects roughly 3 to 14 per 100,000 people, and the average delay from symptom onset to diagnosis is 7 to 10 years [7]. The Endocrine Society's 2014 acromegaly guideline recommends measuring IGF-1 as the initial biochemical test in patients with clinical features such as enlarged hands, jaw prognathism, or new-onset sleep apnea. If IGF-1 is elevated, the next step is a 75-gram oral glucose tolerance test (OGTT) to confirm autonomous GH secretion. In healthy individuals, GH suppresses to <0.4 ng/mL after glucose loading. Failure to suppress confirms the diagnosis [8].

For patients on GH replacement or GH-releasing peptide therapy (CJC-1295, ipamorelin, tesamorelin), a high IGF-1 indicates the dose may need reduction. The Endocrine Society recommends targeting IGF-1 in the mid-normal range for age, specifically the upper half of the normal range without exceeding it [2]. Overshooting carries theoretical risks: insulin resistance, fluid retention, joint pain, and in some observational cohorts, increased cancer signaling.

The relationship between IGF-1 and cancer risk is nuanced. A pooled analysis of 17 prospective studies (N=12,319 cases) published in the Annals of Internal Medicine found that individuals in the highest quartile of circulating IGF-1 had a modest but statistically significant increase in colorectal cancer risk (OR 1.07 per 1 SD increase, 95% CI 1.01 to 1.14) [9]. Prostate and premenopausal breast cancer showed similar small associations. These are population-level correlations, not proof that therapeutic IGF-1 optimization causes cancer. But they explain why clinicians avoid pushing IGF-1 above the upper limit of normal.

What a Low IGF-1 Means

A low IGF-1, defined as below the age-adjusted lower limit, suggests reduced GH secretion, poor nutritional status, or liver dysfunction. Sometimes all three overlap.

GH deficiency (GHD) in adults most commonly results from pituitary surgery, radiation, traumatic brain injury, or idiopathic causes. The AACE 2019 guidelines note that "an IGF-1 level below the lower limit of normal for age and sex has high specificity (over 95%) for severe GHD when pituitary disease is already established" [10]. In patients without known pituitary disease, a low IGF-1 is less diagnostic on its own and typically requires confirmatory stimulation testing with insulin tolerance test (ITT), glucagon, or macimorelin.

Malnutrition suppresses IGF-1 because the liver requires adequate protein and caloric intake to produce it. Patients with anorexia nervosa frequently show IGF-1 levels below 80 ng/mL regardless of age. Chronic caloric restriction in otherwise healthy adults also lowers IGF-1, which some longevity researchers interpret as potentially protective. The CALERIE trial (N=218) showed that 2 years of 25% caloric restriction reduced IGF-1 by approximately 15% in non-obese adults [11].

Liver disease suppresses IGF-1 because hepatocytes are the primary production site. In patients with cirrhosis, IGF-1 levels correlate inversely with the severity of liver dysfunction and can fall below 40 ng/mL in advanced disease [12]. An unexpectedly low IGF-1 in a patient without known pituitary pathology should prompt liver function testing.

Hypothyroidism, poorly controlled type 1 diabetes, chronic kidney disease, and high-dose glucocorticoid use also suppress IGF-1 and should be considered as confounders before diagnosing GH deficiency [2].

How to Raise IGF-1

Addressing a low IGF-1 depends on its cause. If the underlying issue is nutritional, GH axis interventions will not help.

Optimize protein intake. IGF-1 production requires adequate amino acid delivery to the liver. A study in the Journal of Clinical Endocrinology and Metabolism demonstrated that protein intake below 0.8 g/kg/day was independently associated with lower IGF-1 levels in adults over 50, even after controlling for total caloric intake [13]. Increasing protein to 1.0 to 1.2 g/kg/day can raise IGF-1 by 10 to 20% in protein-deficient individuals.

Improve sleep quality. Because the largest GH pulses occur during slow-wave sleep, disrupted sleep architecture directly suppresses IGF-1 over time. One night of total sleep deprivation reduces the next-day GH secretory burst by roughly 70% [14].

Resistance training. Acute bouts of resistance exercise stimulate GH release, and chronic resistance training is associated with modestly higher baseline IGF-1 in older adults. The effect is real but modest, typically a 5 to 15% increase over 12 to 16 weeks of training [15].

GH secretagogue therapy. For patients with confirmed GH deficiency or age-related GH decline who want pharmacologic intervention, GH-releasing peptides such as ipamorelin, CJC-1295, or tesamorelin stimulate endogenous GH secretion and reliably raise IGF-1. Tesamorelin, the only FDA-approved GH-releasing hormone analog (approved for HIV-associated lipodystrophy), increased IGF-1 by an average of 81% from baseline in its key trial (N=412) [16]. Dr. Steven Grinspoon of Massachusetts General Hospital, the trial's lead investigator, noted that "tesamorelin restores physiologic GH pulsatility rather than providing continuous exogenous GH, which may carry a more favorable metabolic profile."

Direct GH replacement. Recombinant human GH (somatropin) remains the standard treatment for diagnosed adult GH deficiency. Typical starting doses are 0.1 to 0.3 mg/day, titrated to bring IGF-1 into the mid-normal range for age [2]. The goal is not maximal IGF-1. It is optimization within reference limits.

How to Lower IGF-1

Reducing an elevated IGF-1 matters in two situations: treating acromegaly and mitigating excess from exogenous GH or peptide use.

In acromegaly, first-line medical therapy after surgery includes somatostatin analogs (octreotide LAR, lanreotide). Octreotide LAR normalizes IGF-1 in approximately 55 to 65% of patients at conventional doses [8]. Pegvisomant, a GH receptor antagonist, normalizes IGF-1 in over 90% of cases but does not shrink the tumor. The ACROSTUDY registry (N=2,090), the largest prospective observational study of pegvisomant, confirmed IGF-1 normalization in 73% of patients in real-world use, with a median dose of 15 mg/day [17].

For patients on GH or peptide therapy whose IGF-1 exceeds the upper limit, the answer is dose reduction. Reduce the dose by 25 to 50%, recheck IGF-1 in 4 to 6 weeks, and repeat until the level falls within the upper half of the age-matched range.

Dietary approaches can modestly lower IGF-1 in non-acromegalic individuals. The CALERIE trial data showed that sustained caloric restriction reduces IGF-1 [11]. Reducing dairy protein intake may also lower IGF-1, as dairy consumption is positively associated with circulating IGF-1 in multiple epidemiologic studies, likely because of its high casein and whey content stimulating hepatic IGF-1 synthesis [18]. Dr. Walter Willett of the Harvard T.H. Chan School of Public Health has noted that "the IGF-1 response to dairy is one of the most consistent diet-hormone associations in nutritional epidemiology."

Intermittent fasting and time-restricted feeding can acutely suppress IGF-1, though the chronic effect depends on overall caloric and protein balance.

IGF-1 in Peptide and GH Therapy Monitoring

For patients using GH-releasing peptides or direct GH replacement, IGF-1 is the primary monitoring biomarker. It tells the clinician whether the dose is producing the intended physiologic effect without overshooting.

The standard monitoring protocol involves checking a baseline IGF-1 before starting therapy, then rechecking at 4 to 6 weeks after dose initiation or any dose change. Once stable, every 3 to 6 months is sufficient [2]. The target for most adults on GH replacement is an IGF-1 in the upper half of the age-adjusted normal range. Going above the upper limit is not "better." It introduces fluid retention, carpal tunnel symptoms, arthralgias, and potential insulin resistance.

IGFBP-3, the major binding protein for IGF-1, is sometimes ordered alongside IGF-1 but adds limited clinical value in adults. It can be helpful in pediatric GH deficiency where IGF-1 levels are very low and less reliable, but in adult practice, IGF-1 alone is the standard [10].

Timing of the blood draw matters modestly. IGF-1 does not fluctuate as dramatically as GH across the day, but for consistency, most clinicians recommend a morning fasting draw. If the patient injects GH or a secretagogue at night, a morning draw captures the steady-state IGF-1 response from the prior evening dose without acute post-injection interference.

Patients switching between GH replacement and GH-releasing peptides (ipamorelin, CJC-1295 with DAC, sermorelin) should expect different IGF-1 kinetics. Direct GH replacement produces a dose-dependent, predictable IGF-1 rise. Peptide secretagogues produce a more variable response because they rely on the patient's remaining pituitary GH reserve. A patient with significant pituitary damage may see little IGF-1 response to secretagogues and require direct GH instead.

IGF-1 and Longevity: The U-Shaped Curve

The relationship between IGF-1 and mortality follows a U-shaped pattern in large population studies. Both the lowest and highest quintiles of circulating IGF-1 are associated with increased all-cause mortality.

A meta-analysis published in the Journal of Clinical Endocrinology and Metabolism (N=30,876 participants across 12 studies) found that individuals with IGF-1 in the lowest quintile had a 27% increased risk of cardiovascular mortality (HR 1.27 to 95% CI 1.09 to 1.48), while those in the highest quintile showed a 19% increase in cancer mortality (HR 1.19 to 95% CI 1.04 to 1.36) [19]. The middle tertile had the lowest overall risk.

This U-shaped curve has practical implications. Clinicians aiming for longevity optimization typically target IGF-1 in the 40th to 60th percentile for age, avoiding both deficiency and excess. For a 50-year-old man, that translates to roughly 140 to 190 ng/mL using most commercial assays.

The centenarian data adds another layer. Female offspring of Ashkenazi Jewish centenarians in the Longevity Genes Project showed lower IGF-1 levels compared to controls, and specific IGF-1 receptor mutations associated with reduced IGF-1 signaling were overrepresented in this long-lived cohort [20]. Whether pharmacologically lowering IGF-1 replicates this genetic advantage remains unproven.

Common Pitfalls in IGF-1 Interpretation

Several factors can produce misleading results. Recognizing them prevents unnecessary workups and inappropriate therapy changes.

Assay variability. Different immunoassay platforms (Siemens Immulite, DiaSorin Liaison, LC-MS/MS) produce systematically different IGF-1 values. A patient whose IGF-1 reads 195 ng/mL on one platform might read 165 ng/mL on another. The 2011 international consensus on IGF-1 standardization acknowledged that "lack of method harmonization remains the most significant barrier to consistent clinical decision-making based on IGF-1 values" [3]. Always use the same lab and the same assay platform for serial monitoring.

Oral estrogen effect. As noted, oral estrogen (but not transdermal) suppresses hepatic IGF-1 production by 20 to 30% through first-pass liver effects [6]. A woman on oral estradiol who shows a "low" IGF-1 does not necessarily have GH deficiency. Switching to transdermal estrogen and retesting in 6 weeks can clarify.

Acute illness. Critical illness, major surgery, and acute inflammation suppress IGF-1 transiently. Do not diagnose GH deficiency based on an IGF-1 drawn during hospitalization.

BMI extremes. Obesity paradoxically lowers IGF-1 relative to GH secretion (GH resistance), while also suppressing GH itself. The net effect in obesity is often a low-normal IGF-1 that rises with weight loss, not because GH status improved but because hepatic sensitivity to GH recovered [4].

Repeat testing before acting on any single abnormal result. One value is a data point. Two values in the same direction, drawn on the same assay, are a pattern worth investigating.