IGF-1

Blood test

Also known as: Insulin-like growth factor 1, Somatomedin C

There is no single guideline cutoff for IGF-1; your result is read against age- and sex-specific ranges for the lab's own method, with a clinician.

IGF-1 is a hormone the liver makes in response to growth hormone. Growth hormone comes in bursts, but IGF-1 stays fairly steady, so one result reflects average growth hormone activity. It is used to check for too much or too little growth hormone, and to track growth hormone treatment.

What it tells you

A single growth hormone level is not useful because it swings through the day. IGF-1 rides on carrier proteins, which smooth out those swings.
It depends strongly on age. It peaks around age 15 and falls steadily after that. Women run slightly lower than men.
It is the first test when doctors suspect acromegaly, a condition of too much growth hormone. A high or borderline result is checked with a glucose drink test.
For growth hormone deficiency in adults, it helps in one direction only. A low result without another cause is strong evidence, but a normal result does not rule it out.
It also senses nutrition. It drops sharply during fasting and recovers with eating.
On growth hormone treatment, doctors aim to keep IGF-1 within the normal range for age. The guideline notes that no published studies say where in that range to aim.

What can throw it off

Age is the biggest factor. A result only makes sense against people of the same age.
Puberty and pregnancy raise it and can mimic growth hormone excess.
Fasting, undereating and calorie restriction lower it. In one small study, a 10-day fast cut it by about three-quarters.
Estrogen taken by mouth lowers it, while estrogen through the skin raises it. The difference comes from how the liver handles the pill.
Liver or kidney failure, an underactive thyroid, poorly controlled diabetes and severe infection make it unreliable, often low.
Growth hormone and medicines that boost it raise it. In one trial, tesamorelin more than doubled IGF-1.
Methods differ a lot between labs. A result from one method cannot be read against another lab's range.
Time of day matters little, so a draw at any hour is acceptable.

Evidence

Diagnosing growth hormone excess or deficiency

Early human evidenceCEBM Level 2b-3b

Guidelines make IGF-1 the first test for acromegaly, followed by a glucose test. For adult deficiency, a normal IGF-1 does not rule it out, so a stimulation test is needed. A low result helps only when other causes are excluded.

Predicting death

Some human evidenceCEBM Level 2a

Across 12 population studies of about 15,000 people, both low and high IGF-1 went with higher death risk than mid-range levels. The same pattern held for deaths from cancer and heart disease. These are links, not proof of cause.

Predicting cancer

Strong human evidenceCEBM Level 1a-1b

In pooled data, people in the top fifth of IGF-1 had about 30% higher odds of prostate cancer than the bottom fifth. The same held for hormone-sensitive breast cancer. A study of nearly 400,000 people linked higher IGF-1 to slightly more colorectal cancer. These links are modest and do not make IGF-1 a cancer test.

Limits

Every result belongs to its lab method. Changing labs or methods breaks a trend line.
A normal IGF-1 does not rule out growth hormone deficiency. A stimulation test is the standard.
Nutrition, liver disease, thyroid, blood sugar control and oral estrogen all distort it. A low result during a diet says little about the pituitary.
There is no outcome-based target or safe upper level on treatment. The links with death and cancer are observational.
It is not a cancer screen. The cancer links are small and seen across populations, not in one person.
No healthy number exists for people without pituitary disease. The range printed on a lab report is not a goal.

References

Reviewed by Shubham Gupta, MD — Founder & President · November 5, 2026

This page is for education only. It is not medical advice. Decisions about any treatment belong with a physician who knows your history.