Insulin-like Growth Factor 1 (IGF-1) is a protein hormone that shares a structural resemblance to insulin, playing a widespread role in growth and tissue maintenance. It is a component of the endocrine system that regulates physical development, particularly during childhood and adolescence. The concentration of IGF-1 in the blood is often measured by healthcare providers to assess the function of the growth hormone system. Because its levels are relatively stable throughout the day, IGF-1 serves as a reliable marker reflecting the average production of growth hormone over time.
The Role and Regulation of IGF-1
The production of IGF-1 is tightly controlled by the Growth Hormone (GH) axis, an intricate communication system involving the brain and the liver. Growth hormone is secreted by the pituitary gland. GH travels through the bloodstream to the liver, which is the primary site of IGF-1 synthesis.
Upon receiving the signal from GH, the liver releases IGF-1 into the circulation. IGF-1 then acts as the main effector, mediating most of the growth-promoting effects of growth hormone on various tissues. Its functions include stimulating the proliferation of cells, supporting the growth of bone and cartilage, and regulating metabolism.
In the bloodstream, most IGF-1 is bound to specific proteins, with Insulin-like Growth Factor Binding Protein-3 (IGFBP-3) being the most abundant. These binding proteins transport IGF-1 and extend its half-life, ensuring a steady supply to the body’s tissues. This complex regulatory loop is essential for coordinating systemic body growth and maintaining anabolic processes in adults.
Age-Related Patterns and Reference Ranges
The circulating concentration of IGF-1 changes dramatically over a person’s lifespan, making age the most important factor in determining the reference range. Levels are relatively low during infancy, gradually increasing throughout early childhood. This baseline level then rises sharply during the pubertal years, reflecting the intense growth spurt of adolescence.
The peak concentration of IGF-1 is typically observed during the mid-to-late teenage years (ages 13 to 17), where values can sometimes exceed 700 nanograms per milliliter (ng/mL). Following the completion of growth and the end of puberty, IGF-1 levels begin a progressive and steady decline. This decrease mirrors the natural reduction in growth hormone production that occurs as the body transitions into adulthood.
For young adults (ages 20 to 30), a typical reference range might be approximately 150 to 450 ng/mL, though this varies considerably by laboratory assay. By middle age (40 to 50 years old), the upper limit of the normal range often decreases to around 300 ng/mL. In the elderly population (over 60), levels continue to fall, with reference ranges commonly dropping below 100 ng/mL at the lower end.
No single universal number exists for normal IGF-1 levels, as ranges vary depending on the specific laboratory, testing method, and population studied. Test results are always interpreted by comparing the measured value to the age-specific reference interval provided by the performing laboratory. A result is often expressed as a standard deviation score (SDS) to indicate how far it deviates from the average for that specific age group.
Clinical Significance of Abnormal Levels
When IGF-1 levels fall outside the established age-appropriate reference range, it can indicate a disorder in the body’s growth hormone axis. Elevated levels of IGF-1 are often a sign of excessive growth hormone activity. In children, this condition is known as gigantism, leading to excessive linear growth and an unusually tall stature.
In adults, the same excess GH production causes acromegaly, a disorder where bones thicken and tissues swell, resulting in enlarged hands, feet, and facial features. Both gigantism and acromegaly are frequently caused by a benign tumor in the pituitary gland that secretes too much growth hormone. Monitoring IGF-1 is important for diagnosing these conditions and for tracking the effectiveness of treatment.
Conversely, abnormally low IGF-1 concentrations can point to a growth hormone deficiency (GHD). In children, GHD results in slowed growth and short stature, often requiring treatment with synthetic growth hormone. Adults with GHD may experience reduced muscle mass, decreased bone density, and persistent fatigue.
Low IGF-1 can also signal Laron syndrome, a rare condition where the body produces GH but cannot properly respond to it due to a problem with the GH receptor. This insensitivity prevents the liver from producing adequate IGF-1, leading to severe short stature. In all cases of abnormal IGF-1, the result provides a crucial starting point for further diagnostic testing to confirm the underlying cause.
Non-Disease Factors Affecting IGF-1 Measurement
Several factors unrelated to a primary pituitary or growth disorder can influence an IGF-1 measurement. Because the liver is the main source of circulating IGF-1, significant liver dysfunction can suppress the hormone’s production and lead to a falsely low test result. Similarly, severe malnutrition or prolonged fasting reduces IGF-1 levels because the body prioritizes conserving energy over growth when nutrient stores are depleted.
Chronic illnesses, such as kidney failure or poorly controlled diabetes, can also interfere with IGF-1 regulation. These conditions may alter the concentration of the binding proteins, which in turn affects the measurable amount of IGF-1. The reliability of the measurement is subject to variability depending on the specific laboratory assay used, as different commercial tests may yield slightly different numbers.
Clinicians must consider these confounding variables when interpreting a single IGF-1 result, often opting for a repeat test to confirm a true abnormality before pursuing extensive investigations.