IGF-1 (insulin-like growth factor 1)

It peaks in the teens and falls steadily with age (for example roughly 15 to 45 nmol/L in the twenties, 12 to 33 in the late forties and 9 to 26 in the late sixties), so labs print age-banded ranges and the printed one is the one to use.

IGF-1 is a hormone made mainly by the liver in response to growth hormone. Unlike growth hormone itself, which comes in short bursts, IGF-1 stays steady through the day, so it is a much better single measure of how much growth hormone the body is making overall. It supports growth in children and the upkeep of muscle and bone in adults.

It is the usual first test when too much growth hormone (acromegaly) or too little (growth hormone deficiency, pituitary failure) is being looked into, and it is used to monitor treatment of both. It is also used in children with short stature and appears on some private 'wellness' panels.

• Acromegaly, where a pituitary growth makes too much growth hormone
• Puberty and pregnancy
• Growth hormone injections used in treatment or misused in sport
• Higher protein and calorie intake, slightly
• Younger age, in the general population

• Older age, steadily, in the general population
• Growth hormone deficiency and pituitary failure
• Poor nutrition, low body weight, eating disorders and fasting
• Liver disease, poorly controlled diabetes, kidney disease and an underactive thyroid
• Oestrogen tablets (the combined pill and oral HRT), which reduce liver production
• Acute illness and long-term inflammatory disease

• Age explains most of the variation, so a value has to be read against the age band
• Oral oestrogen (the combined pill or tablet HRT) lowers it by a third or more
• Poor nutrition or a recent illness lowers it temporarily
• Different assays give different numbers

No fasting needed, any time of day. Doctors take the age band and any oral oestrogen into account when reading it. Repeat tests are best done in the same laboratory.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.