What Is IGF-1? Definition and What Research Reports

Plain-language definition

IGF-1 stands for insulin-like growth factor 1. It is a small protein hormone, about 70 amino acids long, that the body produces — mostly in the liver — after growth hormone signals it to do so. Its name comes from the fact that its structure resembles insulin closely enough that the two molecules can weakly cross-react with each other's receptors. In everyday physiological terms, IGF-1 is the messenger that carries out many of the growth-promoting instructions attributed to growth hormone: it acts on cells throughout the body to influence cell division, protein synthesis, tissue maintenance and metabolism. Blood IGF-1 concentration is routinely measured in clinical endocrinology because it is far more stable across the day than growth hormone itself.

This page is for educational purposes only and is not medical advice; consult a licensed physician about any medical question. Nothing here describes how any substance should be used.

IGF-1 in biochemical and regulatory terms

Biochemically, IGF-1 is a single-chain polypeptide with three disulfide bridges, structurally homologous to proinsulin. It signals through the IGF-1 receptor (IGF-1R), a transmembrane tyrosine kinase. Receptor binding triggers autophosphorylation and recruitment of adaptor proteins, feeding into two principal intracellular cascades: the PI3K/Akt pathway and the Ras/MAPK (ERK) pathway. Experimental work in chicken embryonic myotubes reported that knocking down IGF-1 reduced phosphorylation of Akt and ERK and affected myotube development, illustrating that these two arms are downstream of IGF-1 in muscle cells (PMID 35717687).

Regulation is layered. Growth hormone released from the pituitary stimulates hepatic IGF-1 production; circulating IGF-1 then feeds back to restrain growth hormone secretion. Most IGF-1 in blood is not free but bound to IGF-binding proteins (principally IGFBP-3) and an acid-labile subunit, which extends its half-life and buffers its activity. Nutrition, age, sex steroids, liver function and inflammation all shift IGF-1 levels. Less obviously, the intestinal microbiota has been described as an influence on the growth hormone/IGF-1 axis and on skeletal outcomes, a relationship reviewed in the bone and mineral literature (PMID 29362822).

Terminology quick reference

How the term is used in peptide research

In peptide science, "IGF-1" appears in several distinct contexts, and readers encountering the term should note which one applies.

• As a biomarker. Because IGF-1 integrates growth hormone exposure over time, it is used as a surrogate measure of GH activity. In acromegaly, where a pituitary tumour drives excess GH, researchers examined the correlation between GH and IGF-1 during treatment and reported that the two measures did not always move in parallel, complicating assessment of disease control (PMID 27858572).
• As an anti-doping target. Reviews of sports endocrinology discuss growth hormone, IGF-1, insulin and erythropoietin together as substances subject to doping control, describing detection challenges and the endocrine consequences reported in that literature (PMID 36736729).
• As a signalling node in ageing research. Reduced insulin/IGF-1 signalling has been a long-running theme in longevity biology. A review of insulin, IGF-1 and longevity summarised evidence linking attenuated signalling through this pathway to extended lifespan in model organisms (PMID 22396862). More recent work reported that the longevity effects of reduced IGF-1 signalling depended on the stability of the mitochondrial genome, indicating the relationship is conditional rather than absolute (PMID 41931604; preprint version PMID 40501628). A separate study reported that the compound chrysophanol delayed ageing phenotypes in a model system via the insulin/IGF-1 signalling pathway (PMID 40086491).
• As a neurobiological signal. A physiology review described growth hormone and IGF-1 actions in the brain and their relevance to neuropsychiatric disease (PMID 40623083). In rodents, researchers reported that IGF-1 facilitated the extinction of conditioned fear, implicating the hormone in hippocampal plasticity relevant to fear learning (PMID 33792539).
• As a target of pathogens. A 2025 report described viral insulin/IGF-like peptides that inhibited IGF-1 receptor signalling in a way that enhanced viral replication, showing that the pathway is exploitable by viruses (PMID 40829596).

Where the term is commonly misused

Three misuses recur in online peptide discussion.

1. Treating IGF-1 as interchangeable with growth hormone. They are distinct molecules with distinct receptors. The acromegaly literature is explicit that GH and IGF-1 measurements can diverge during treatment, which is precisely why both are tracked (PMID 27858572).
2. Treating analogues as "IGF-1". Modified constructs such as IGF-1 LR3 differ from the endogenous hormone in binding-protein affinity and are research reagents, not the natural peptide. Published findings about endogenous IGF-1 do not automatically transfer to analogues.
3. Assuming "more is better" or "less is better". The ageing literature complicates both directions: reduced signalling has been associated with longevity in models (PMID 22396862), yet the same effect was reported to depend on mitochondrial genome stability (PMID 41931604), while brain and repair literature describes contexts where IGF-1 signalling supports function (PMID 40623083). The picture is context-dependent, not directional.

Local IGF-1: not only a circulating hormone

Beyond liver-derived circulating IGF-1, many tissues produce IGF-1 locally, where it acts in autocrine and paracrine fashion. Hair follicle biology is one example: a study of androgenetic alopecia described an AR/miR-221/IGF-1 pathway in dermal papilla cells and reported that this axis contributed to the disease mechanism (PMID 37496996). Muscle is another, where IGF-1 knockdown altered Akt and ERK phosphorylation in embryonic myotubes (PMID 35717687). Local IGF-1 activity is one reason serum IGF-1 concentration is an incomplete proxy for tissue-level signalling.

Safety and Adverse Findings: What Studies Report

The verified literature summarised here is largely mechanistic, clinical-biomarker or review-based rather than safety-focused, so it does not support a general adverse-event profile. The sports endocrinology review discussed growth hormone, IGF-1, insulin and erythropoietin in the context of doping and described the endocrine and detection issues raised by their non-medical use (PMID 36736729). Clinically, excess GH/IGF-1 signalling defines acromegaly, and researchers tracked both hormones because discordant values complicated judgements about disease control after treatment (PMID 27858572). The finding that viral peptides manipulate IGF-1 receptor signalling to favour replication further indicates the pathway interacts with host defence (PMID 40829596). None of this constitutes a risk assessment for any individual.

Related terms

• Growth hormone (GH, somatropin) — the pituitary hormone upstream of hepatic IGF-1.
• Insulin — structurally related; discussed alongside IGF-1 in both longevity and doping literature (PMID 22396862).
• IGF-2 — a related growth factor with a distinct expression pattern.
• IGFBP-3 — the main circulating binding protein for IGF-1.
• PI3K/Akt and MAPK/ERK — downstream cascades reported in IGF-1 knockdown work (PMID 35717687).
• GH/IGF-1 axis — the feedback system as a whole, reviewed for brain actions in 2026 (PMID 40623083).

How to read IGF-1 claims critically

Much of what circulates about IGF-1 comes from model organisms (worms, flies, mice, chicken myotubes) or from cell culture. Those findings describe mechanisms; they do not establish outcomes in humans. When a claim is encountered, useful questions include: was the species human? Was IGF-1 measured or manipulated? Was the endpoint a biomarker or a clinical outcome? Was the molecule endogenous IGF-1 or an analogue? The papers cited on this page span all of these categories, and the differences between them explain most of the apparent contradictions in popular summaries.

References

• Doping and sports endocrinology: growth hormone, IGF-1, insulin, and erythropoietin (Revista clinica espanola, 2023)
• The AR/miR-221/IGF-1 pathway mediates the pathogenesis of androgenetic alopecia (International Journal of Biological Sciences, 2023)
• Growth Hormone and IGF-1 Actions in the Brain and Neuropsychiatric Diseases (Physiology, 2026)
• IGF-1 facilitates extinction of conditioned fear (eLife, 2021)
• Insulin, IGF-1 and longevity (Aging and Disease, 2010)
• Gut Microbiota and IGF-1 (Calcified Tissue International, 2018)
• Viral insulin/IGF-like peptides inhibit IGF-1 receptor signaling to enhance viral replication (Cell Reports, 2025)
• Chrysophanol delays aging via insulin/IGF-1 signaling pathway (Free Radical Biology & Medicine, 2025)
• The longevity effects of reduced IGF-1 signaling depend on the stability of the mitochondrial genome (Science Advances, 2026)
• The longevity effects of reduced IGF-1 signaling depend on the stability of the mitochondrial genome (bioRxiv, 2025)
• Correlation between GH and IGF-1 during treatment for acromegaly (Journal of Neurosurgery, 2017)
• IGF-1 knockdown inhibits phosphorylation of Akt and ERK in chicken embryonic myotubes (Growth Hormone & IGF Research, 2022)