Insulin-like growth factor (IGF) is a hormone that plays a central role in growth, tissue repair, and cell development throughout your life. The most well-known form, IGF-1, acts as the main messenger for growth hormone, carrying out much of the actual work that growth hormone gets credit for. Your liver produces most of the IGF-1 circulating in your blood, though other tissues make smaller amounts for local use.
How IGF-1 Works in Your Body
When your pituitary gland releases growth hormone, it travels to the liver and activates receptors there, triggering the production of IGF-1. This chain of events is called the GH/IGF-1 axis, and it’s the primary way your body translates growth hormone signals into actual tissue growth. IGF-1 then enters the bloodstream and reaches cells throughout the body, where it stimulates protein building, cell division, and growth.
IGF-1 doesn’t only work as a long-distance hormone traveling through the blood. Certain tissues, including cartilage cells, produce their own IGF-1 that acts locally without entering general circulation. This means IGF-1 operates on two levels: as a systemic hormone produced by the liver and as a locally acting signal made by individual tissues to fine-tune their own growth and repair.
IGF-1 vs. IGF-2
There are two main forms of insulin-like growth factor. IGF-1 is the dominant form after birth and throughout adult life, widely active in the brain, muscles, bones, and other organs. IGF-2 is more prominent during embryonic development, particularly in connective tissues. In rodents, IGF-2 expression drops sharply after birth and becomes restricted to a few specific areas. In humans, IGF-2 remains active in the adult brain, where it appears to help maintain the production of new nerve cells. The two forms bind to receptors differently, suggesting they have distinct roles even in tissues where both are present.
Effects on Muscle and Bone
IGF-1 is one of the most potent natural signals for building muscle. It activates a cellular pathway that ramps up protein production inside muscle fibers while simultaneously slowing down protein breakdown. This dual action makes IGF-1 essential for muscle growth and for preventing muscle wasting. It also activates satellite cells, the stem cells that sit dormant alongside muscle fibers and spring into action to repair and rebuild damaged tissue after exercise or injury.
For bones, IGF-1 mediates much of the length and density gains that growth hormone promotes, particularly during childhood and adolescence. It stimulates cartilage cells at growth plates, driving the elongation of long bones. In adults, it continues to support bone density by promoting the activity of bone-building cells.
How Levels Change With Age
IGF-1 levels rise during childhood, peak during puberty, and decline steadily with age. In young adults aged 18 to 20, typical blood levels range roughly from 155 to 535 ng/mL depending on the assay used and sex, with males and females showing broadly similar ranges. By middle age, levels have dropped considerably, and this decline continues into old age. The age-related drop in IGF-1 may be worsened by low protein intake or poor nutrition.
Several factors beyond age affect your levels. Nutritional status has a powerful influence. Fasting for 10 days can cause a fourfold drop in IGF-1, bringing levels down to ranges seen in people with growth hormone deficiency. This happens because prolonged fasting makes the liver resistant to growth hormone, so even though fasting actually increases growth hormone output, the liver stops responding to it. Severe calorie restriction or lack of essential amino acids has a similarly strong suppressive effect.
Diet and IGF-1 Levels
Among the macronutrients, protein has the strongest influence on IGF-1. In a study of middle-aged to elderly men, those in the top fifth of protein intake had IGF-1 levels about 15% higher than those in the bottom fifth. Both animal and plant protein showed this association. Carbohydrates have a slight positive effect on IGF-1, possibly by keeping the liver sensitive to growth hormone. Fat intake showed no meaningful connection to IGF-1 levels.
Overeating can raise IGF-1 somewhat, but the effect is modest compared to the dramatic drop caused by undernutrition. In practical terms, getting enough protein and total calories matters far more for maintaining healthy IGF-1 levels than any specific dietary strategy aimed at boosting them.
The Link to Cancer Risk
Higher circulating IGF-1 levels have been modestly associated with increased risk of prostate, breast, colorectal, and ovarian cancer. This makes biological sense: IGF-1 promotes cell growth and division, and it can also act as a signal that encourages cells to keep growing when they normally would not. People with acromegaly, a condition involving chronic overproduction of growth hormone and consequently elevated IGF-1, face roughly double the risk of gastrointestinal cancers compared to the general population.
This doesn’t mean normal, healthy IGF-1 levels are dangerous. The association involves levels that are persistently elevated above the normal range, or levels at the higher end of normal sustained over many years. It’s one reason why the use of growth hormone and IGF-1 as performance-enhancing or anti-aging supplements raises concerns beyond their immediate effects.
When IGF-1 Is Too Low
In children, low IGF-1 typically signals a growth hormone deficiency or an insensitivity to growth hormone. Signs include a slower growth rate compared to peers, shorter stature, and in some cases thin hair and poor nail growth. In adults, low IGF-1 is associated with fatigue, reduced bone density, and loss of muscle mass.
Several medical conditions can push IGF-1 levels down even when growth hormone production is normal or high. Chronic liver disease, kidney failure, diabetes, anorexia, and critical illness all create a state of growth hormone resistance where the liver stops converting growth hormone into IGF-1 effectively. Oral estrogen and certain estrogen-related medications can also reduce the liver’s responsiveness to growth hormone, lowering IGF-1 output.
When IGF-1 Is Too High
Excess IGF-1 in children causes gigantism, marked by excessive growth, an overly large head, and unusually large hands and feet. In adults, the same overproduction leads to acromegaly, which develops more gradually and involves thickening of the bones, enlarged facial features (particularly the nose, lips, and tongue), deepened voice, excessive sweating, and coarsened skin. Acromegaly also raises the risk of sleep apnea, high blood pressure, type 2 diabetes, and joint problems.
Diagnosing these conditions relies heavily on measuring IGF-1 in the blood. Unlike growth hormone, which pulses throughout the day and is difficult to measure reliably with a single blood draw, IGF-1 remains relatively stable and gives a more accurate picture of long-term growth hormone activity. This is why doctors use IGF-1 testing as the primary screening tool for both growth hormone deficiency and excess.
Why Doctors Order an IGF-1 Test
An IGF-1 blood test is typically ordered when a child is growing unusually slowly or quickly for their age, or when an adult shows signs of either growth hormone deficiency or acromegaly. The test measures total IGF-1 in the blood and compares it to age- and sex-specific reference ranges. Because IGF-1 levels vary so much by age, a result that’s normal for a teenager would be abnormally high for a 60-year-old.
The test is also used to monitor treatment. For people receiving growth hormone therapy, periodic IGF-1 checks help ensure the dose is producing adequate levels without pushing them too high. For acromegaly patients who’ve had surgery or are on medication, IGF-1 levels falling back into the normal range indicate successful treatment.