IGF-1: How to Read It

IGF-1, or insulin-like growth factor 1, is a hormone produced mainly by the liver in response to growth hormone stimulation from the pituitary gland. Because it circulates at relatively stable levels compared to growth hormone itself, IGF-1 is used clinically as a proxy for average growth hormone activity over time rather than a single point-in-time growth hormone measurement.

This page is part of the site's marker-by-marker series; for the broader framework on reading a full panel, see the guide to reading a hormone panel.

What IGF-1 measures and why it's used

Growth hormone is released from the pituitary gland in short, pulsatile bursts — a pattern that makes a single growth hormone blood draw a poor snapshot of someone's overall growth hormone status, since the same person could test very differently depending on whether the sample happened to catch a pulse or a trough. IGF-1 solves this practical problem: growth hormone stimulates the liver to produce IGF-1, and IGF-1 itself circulates with only minor fluctuation across the day.

Research summarizing IGF-1's clinical use notes that it displays only minor circadian fluctuation, "negating the need for repetitive standardized sampling procedures" the way growth hormone testing requires, and that IGF-1 is stable in a stored blood sample for an extended period — both properties that make it a more practical single-draw indicator of average growth hormone exposure. IGF-1 also has broader physiological roles beyond reflecting growth hormone status: it acts on muscle, bone, cartilage, and other tissues and shares structural similarity with insulin, giving it some insulin-like metabolic effects as well as growth-related ones.

Reference ranges, and why they vary by age and assay

IGF-1 is unusual among common lab markers in how strongly it depends on age: levels rise through childhood, peak around puberty, and then decline steadily and predictably through the rest of adulthood. Because of this pattern, labs report age-banded reference ranges rather than a single adult range — a result that reads as unremarkable for one age band could sit outside the "typical" range for a different age band, without that difference by itself meaning anything is wrong.

IGF-1 reference ranges also vary meaningfully by the specific immunoassay platform a lab uses. A study establishing reference intervals across eight different IGF-1 assay methods in an adult population found that while the lower bounds of the "normal" range were fairly consistent between platforms, the upper bounds varied considerably — and overall agreement between assays, measured statistically, was only moderate. The researchers attributed much of this variation to differences in how each assay handles IGF-1's binding proteins (which normally carry the vast majority of circulating IGF-1 and must be adequately removed or accounted for during testing) rather than to differences in calibration standards alone.

The practical implication is that an IGF-1 number is only meaningful in the context of the specific lab and assay platform that produced it, and comparing an IGF-1 result against a number from a different lab, a different platform, or an internet reference chart can be genuinely misleading. The age- and assay-specific range printed on the actual lab report is the one that applies.

What commonly drives IGF-1 up or down

Because IGF-1 is produced by the liver in response to growth hormone, anything that changes growth hormone signaling or liver function can move IGF-1. Age is the dominant factor in a healthy population, with levels declining progressively from young adulthood onward. Research on IGF-1's clinical determinants also identifies nutritional status, body mass index at either extreme, pregnancy, and — to a smaller degree — sex and ethnicity as contributing factors, and notes that liver disease affects IGF-1 levels directly since the liver is its primary source.

Beyond these general population-level factors, IGF-1 is also a clinically important marker in the evaluation of specific growth hormone axis conditions, including growth hormone deficiency and, at the opposite end, acromegaly — but evaluating IGF-1 in that specific diagnostic context, or in the context of any therapy intended to influence the growth hormone axis, is squarely the role of a licensed physician working from an individual's full history and labs, not something this general reference page is positioned to interpret.

Why trends matter more than a single result

Even though IGF-1 is more stable across a single day than growth hormone itself, a single IGF-1 result still reflects one moment against a backdrop of longer-term factors — nutritional status, recent illness, and liver health among them — that can shift over weeks or months. And because different assay platforms can produce meaningfully different numbers for the same underlying physiology, comparing one IGF-1 draw against another is most reliable when both were run on the same lab's platform.

For these reasons, a trend built from repeated IGF-1 measurements taken through the same laboratory, tracked over time, gives a physician a more dependable picture of someone's growth hormone status than any single isolated value — particularly given how much IGF-1 is expected to decline with age on its own, independent of anything else going on physiologically.

As with every marker discussed on this site, interpreting a specific IGF-1 result — what it suggests, and whether it warrants any further evaluation — is the role of a licensed physician reviewing the complete clinical picture, not a general educational reference page.