IGF-1: what it is, what it tells you and when it’s tested

IGF-1, or somatomedin C, is a protein made mostly by the liver when it gets the signal from growth hormone (GH). Growth hormone rises and falls in waves through the day; IGF-1 much less. That is why doctors measure IGF-1 to judge how much growth hormone you make.

When it’s tested

Doctors mostly order it when they suspect the pituitary makes too much growth hormone (acromegaly): hands and feet that grow in adulthood, facial features that change, heavy sweating. Or when there is a lump in the pituitary.

It is also used when too little is suspected, in children who grow slowly and in adults with pituitary conditions, and to follow people treated with growth hormone.

It is not a prevention test for people who feel well: it depends a lot on age, and a single result is only read in a clinical picture. Rosa does not test it. It does look at blood sugar, HbA1c and TSH, which shift with acromegaly or lower IGF-1.

What it tells you, and what it does not

If it is high for your age and there are signs of acromegaly, your doctor goes on to other tests, such as a glucose tolerance test, and a pituitary MRI scan.

If it is low, growth hormone may be short, but other causes are more common: eating too little, a liver or kidneys that work poorly, an underactive thyroid, poorly controlled diabetes, or oestrogen taken by mouth.

A normal IGF-1 does not rule out growth hormone deficiency in an adult: that needs a stimulation test. And IGF-1 does not measure how fit you are or how much muscle you have.

That is why it is read together with other tests:

Normal IGF-1 levels

There is no single normal range: it depends on age and the method. IGF-1 rises until puberty, peaks around 15 and then slowly falls throughout life. Women are a little lower on average.

That is why your report gives a range for your age. For example, a large Italian hospital lab gives 84 to 259 ng/mL between 26 and 30 and 45 to 220 ng/mL between 51 and 70. Other labs use different limits. Some reports also give a score compared with the average for your age.

It changes little through the day, so the time of the test matters less than for other hormones.

If your IGF-1 is high or low

What can skew the result

Many things shift IGF-1 without any pituitary problem. Tell your doctor about them.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal IGF-1 levels? They depend on age. For example, a large Italian hospital lab gives 84 to 259 ng/mL between 26 and 30. The range for your age on your own report is what counts.

What does high IGF-1 mean? In puberty and pregnancy it is normal. Otherwise, if there are signs of acromegaly, your doctor goes on to other tests. A result just above the range, with no symptoms, is often simply repeated.

What does low IGF-1 mean? More often it comes from diet, the liver, the thyroid or oestrogen by mouth. Growth hormone deficiency is less common and is confirmed with a stimulation test.

What is the difference between IGF-1 and growth hormone? Growth hormone comes from the pituitary and changes a lot through the day. IGF-1 is made by the liver on its signal and is more stable, which is why it is the one measured.

Do I need to fast for an IGF-1 test? It depends on the lab and the other tests on the form. Follow the instructions you are given.

Does Rosa test IGF-1? No. Rosa measures blood sugar, HbA1c, TSH and albumin, which help read an IGF-1 that is out of place. If your doctor wants IGF-1, they order it separately.

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