IGF-1: a growth factor whose normal range depends on your age

Insulin-like growth factor 1 (IGF-1) is a growth factor made mainly by the liver, under signals from the pituitary gland, and also made locally in muscle, bone and other tissues. It drives growth in childhood and helps maintain muscle and bone in adults. Most IGF-1 in the blood travels bound to carrier proteins, the IGF binding proteins, and the bound pool changes slowly, so IGF-1 is fairly stable through the day. It is reported in ng/mL or nmol/L (1 ng/mL is about 0.13 nmol/L).

The ranges: always age-specific

There is no single normal range for adults. In the largest reference study (Journal of Clinical Endocrinology and Metabolism, 2014), which measured 15,014 people aged 0 to 94 in the US, Canada and Europe, IGF-1 fell after birth, rose to a peak at about age 15, then declined continuously. Women had slightly lower values on average. So labs report IGF-1 against a band for your age and sex, and often as a standard deviation score (a z-score) showing how far you are from the average for your age.

Because the assays differ, a result from one lab cannot be judged against another lab's range. Online claims about an "optimal" IGF-1 for longevity, high for muscle or low for lifespan, are opinion; the cohort data below point to the middle.

What raises and lowers it

• Age is the biggest factor: a healthy 70-year-old's value would be low for a 25-year-old.
• Lowers: undernutrition, severe calorie restriction and low protein intake, liver disease (the liver makes most of it), poorly controlled diabetes, an underactive thyroid, and serious illness.
• Raises: puberty, pregnancy, and a pituitary tumour that drives excess production (acromegaly), which is the main clinical reason the test is ordered.
• Some people who use research peptides track IGF-1 alongside their other labs; peptide blood testing covers timing and baselines.

What it predicts

A 2011 meta-analysis in the Journal of Clinical Endocrinology and Metabolism pooled 12 population studies with 14,906 people. Compared with mid-range levels, both low and high IGF-1 went with higher death from any cause: hazard ratio 1.27 for low and 1.18 for high. The dose-response curve was U-shaped. Comparing the 10th percentile with the 50th gave a predicted hazard ratio of 1.56, and the 90th against the 50th gave 1.29. The U shape held for both cancer and cardiovascular death.

For cancer specifically, a 2016 pooled analysis of individual data in Cancer Research, with up to 10,554 prostate cancer cases, found men in the highest fifth of IGF-1 had an odds ratio of 1.29 for prostate cancer compared with the lowest fifth, in prospective studies. In the UK Biobank, 206,263 women were followed for a median 7.1 years (Annals of Oncology, 2020): each 5 nmol/L higher IGF-1 was associated with an 11% higher risk of breast cancer (hazard ratio 1.11), and a Mendelian randomisation analysis in the same paper pointed the same way, more weakly (odds ratio 1.05). The cohort results are observational. Together they suggest that pushing IGF-1 well above the age range is not obviously a good idea, and that a low value deserves an explanation.

On GLP-1 drugs and weight loss

The pivotal semaglutide and tirzepatide trials do not report IGF-1 as an outcome, so there is no trial number to give. Because IGF-1 falls with sharp calorie and protein restriction, a large drop during rapid weight loss is plausible, and adequate protein is one reason the protein guide matters on these drugs. Treat a change as a question for the prescriber rather than a target.

How to test

• Morning draw, fasting preferred, to keep conditions consistent, although IGF-1 varies little across the day.
• Same lab every time, and record the reference range and z-score printed with each result.
• Note recent intake. A week of very low eating can lower it.
• Interpretation of an abnormal value belongs with a clinician, usually an endocrinologist.

Tracking it in Peptide AI

IGF-1 sits outside the metabolic and inflammatory panels. Peptide AI reads it from a lab PDF or manual entry, keeps the age-specific range your lab printed, and plots it over time against your dose timeline. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.