IGF-1 (Insulin-like Growth Factor 1)

IGF-1 (Insulin-like Growth Factor 1), historically called somatomedin C, is a 70-amino-acid peptide hormone structurally homologous to proinsulin. It is produced primarily by hepatocytes under the transcriptional stimulus of growth hormone (GH) acting on GH receptors in the liver, and it mediates the majority of GH's systemic growth-promoting and anabolic effects via the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase.

Why it matters in peptide research

IGF-1 is the key readout of GHRH/GH axis activity and is therefore central to interpreting the downstream effects of any peptide that modulates growth hormone secretion. When a GHRH analogue such as tesamorelin stimulates pituitary somatotrophs to release GH, the liver responds within hours by upregulating IGF-1 secretion. Plasma IGF-1 levels thus serve as an integrated biomarker of GH exposure — more stable and easier to measure than GH itself, which has a short half-life and pulsatile secretion pattern.

IGF-1's anabolic actions are broad: it promotes protein synthesis via PI3K/Akt/mTOR signalling, stimulates satellite cell proliferation in skeletal muscle, drives chondrocyte differentiation in growth plates, and exerts anti-apoptotic effects in neurons. In adults, IGF-1 contributes to lean mass maintenance, bone mineral density, and potentially cognitive function, explaining why GH-deficient patients show improvements in these parameters when GH axis activity is restored.

Because IGF-1 signals through the same PI3K/Akt pathway as insulin, sustained supra-physiological IGF-1 levels are theorised to promote cell survival and proliferation in ways that could favour cancer growth — a key reason why GH secretagogue research in cancer settings requires careful interpretation. At physiological concentrations achieved by GHRH analogue-based protocols, however, IGF-1 levels typically remain within or slightly above normal reference ranges rather than reaching pharmacologically supraphysiological values.

Peptides / stacks that act on this

• Tesamorelin — synthetic GHRH analogue; raises GH pulse amplitude, increasing hepatic IGF-1 production; the primary validated clinical endpoint in tesamorelin trials is IGF-1 normalisation

Reading tip

IGF-1 reference ranges vary substantially by age, sex, and laboratory assay methodology. When evaluating a study reporting IGF-1 changes, always check whether the reported values are in nmol/L or ng/mL (conversion factor ≈ 7.65) and compare to the age-matched reference range used by that laboratory.