How to Read an IGF-1 Result: Age-Adjusted Ranges and the GH Axis

This article is for research and educational purposes only and is not medical advice. It describes how laboratories and researchers interpret a marker, not how to diagnose or treat any individual.

Introduction

Insulin-like growth factor 1 (IGF-1) is one of the few practical windows into the growth hormone (GH) axis, but a raw IGF-1 number, on its own, tells you very little. The same value can be entirely typical for one person and unusually low for another, because IGF-1 is read against age- and sex-adjusted reference ranges, not a single fixed cut-off. This article covers how that interpretation works.

Quick answer: IGF-1 is GH-dependent and far more stable through the day than GH itself, which is why it is used as an integrated readout of GH activity. Because IGF-1 changes markedly with age and somewhat with sex, results are interpreted against age- and sex-specific normative ranges, often expressed as a standard-deviation score (SDS or z-score) rather than a raw concentration.

Why IGF-1, Not a Single GH Level

GH is secreted in pulses and has a short half-life, so a single blood GH measurement is episodic and largely uninformative about overall GH activity. IGF-1 is GH-dependent and much more consistent throughout the day, because its circulating level is determined by basal (interpulse) GH output rather than individual pulses, so it reflects integrated GH activity over time. (For a fuller treatment of why a one-off serum GH tells you little, see our companion piece, IGF-1 and GH interpretation.)

That stability has a structural basis: most circulating IGF-1 is not free. It travels bound in a high-molecular-weight ternary complex with IGFBP-3 and the acid-labile subunit (ALS), which acts as a circulating reservoir and extends IGF-1's half-life to roughly 16 hours, versus minutes for GH.

Age and Sex Dependence

The single most important thing about reading IGF-1 is that it is strongly age-dependent. IGF-1 rises through childhood, peaks around puberty and young adulthood, and declines progressively with age; it also differs somewhat by sex. Because of this, IGF-1 requires age- and sex-adjusted normative ranges, a value cannot be judged "normal" or "abnormal" without knowing the person's age and sex.

SDS / Z-Scores

To handle that dependence, laboratories increasingly report IGF-1 not just as a concentration but as a standard-deviation score (SDS, or z-score), how far a result sits from the age- and sex-matched mean. A value below the lower reference limit (commonly z below −2) is treated in the literature as low for that age and sex. This is why two identical raw numbers can carry opposite interpretations at different ages.

(We deliberately publish no specific ng/mL ranges here: reference intervals are age-, sex-, and assay-specific, and a number quoted out of that context is misleading.)

What Low and High Reflect in Research

IGF-1 is central to research and clinical assessment of the somatotropic axis. In that literature, low IGF-1 (for age and sex) is interpreted as consistent with reduced GH activity, as seen in studies of GH deficiency, while high IGF-1 is associated with GH excess, the hallmark of acromegaly research. IGF-1 is also used to monitor GH-axis status over time. These are descriptions of how the marker behaves in research populations, not a basis for self-diagnosis.

The Assay Caveat

One honest complication: different commercial IGF-1 assays can give substantially different results for the same sample, and harmonising methods and reference intervals is an ongoing challenge. Practically, that means an IGF-1 result should be interpreted against the issuing lab's reference range, and serial monitoring is most reliable when done on the same assay.

The GH to IGF-1 Axis

The reason IGF-1 works as a GH readout at all is the somatotropic axis: pituitary GH drives production of IGF-1, classically framed by the somatomedin hypothesis as liver-derived IGF-1 mediating GH's effects (modern work shows most tissues also produce IGF-1 locally). For a research-focused comparison of GH and IGF-1 as distinct molecules and signals, see the HGH vs IGF-1 research comparison.

Frequently Asked Questions

Why is IGF-1 measured instead of growth hormone? GH is pulsatile with a short half-life, so a single GH level is episodic and uninformative. IGF-1 is GH-dependent and stable through the day, bound in an IGFBP-3/ALS complex with a half-life of around 16 hours, so it reflects integrated GH activity rather than a momentary pulse.

Why does IGF-1 need an age-adjusted range? Because IGF-1 changes dramatically across life (high in adolescence, declining with age) and differs by sex. A result is only meaningful against age- and sex-specific normative data, often reported as an SDS/z-score.

What does a low or high IGF-1 mean? In research and clinical assessment, low IGF-1 for age/sex is interpreted as consistent with reduced GH activity (studied in GH deficiency), and high IGF-1 with GH excess (acromegaly). This is how the marker is read in the literature, not a personal diagnosis.

Can I compare IGF-1 results from two different labs? Cautiously. Assays are not fully standardised and can differ for the same sample, so compare against each lab's own reference range and track trends on a single assay where possible.

References

• Clemmons DR, Bidlingmaier M. Interpreting growth hormone and IGF-I results using modern assays and reference ranges... Front Endocrinol. 2023;14:1266339. PMID 38027199
• Huang R, Shi J, Wei R, Li J. Challenges of insulin-like growth factor-1 testing. Crit Rev Clin Lab Sci. 2024;61(5):388-403. PMID 38323343
• Faje AT, Barkan AL. Basal, but not pulsatile, growth hormone secretion determines the ambient circulating levels of IGF-I. J Clin Endocrinol Metab. 2010;95(5):2486-2491. PMID 20190159
• Allard JB, Duan C. IGF-Binding Proteins: Why Do They Exist and Why Are There So Many? Front Endocrinol. 2018;9:117. PMID 29686648
• Le Roith D, Bondy C, Yakar S, Liu JL, Butler A. The somatomedin hypothesis: 2001. Endocr Rev. 2001;22(1):53-74. PMID 11159816

Further Reading

• IGF-1 and GH interpretation, why a single serum GH tells you little.