The primary downstream mediator of growth hormone, and the main lab marker used to monitor GH secretagogue protocols.
IGF-1 (Insulin-like Growth Factor 1) is a protein hormone produced primarily in the liver in response to growth hormone (GH) signaling. Because pulsatile GH is difficult to measure directly — its levels fluctuate wildly over minutes and hours — IGF-1 serves as the stable, clinically useful readout of overall growth hormone activity. It is the single most important biomarker for anyone running GHRH analogs, GH secretagogues, or exogenous HGH.
IGF-1 levels decline steadily with age, with most people losing 20–40% of their peak IGF-1 between their 20s and 50s. Optimization protocols often target the upper quartile of the age-adjusted reference range rather than the "normal" range, which can span from genuinely deficient to peak optimization. This is one of the biomarkers where the reference range and the optimal range differ meaningfully.
IGF-1 is influenced by: baseline growth hormone production (declines with age), nutritional status (low protein depresses IGF-1), sleep quality (deep sleep drives GH pulses), liver function (IGF-1 is produced there), insulin resistance (alters GH-IGF signaling), and directly by pharmacologic GH secretagogues (tesamorelin, ipamorelin/CJC-1295, MK-677, HGH). Acute illness and inflammation suppress IGF-1. Laboratory assays can vary, so using the same lab for serial measurements is important.
For peptide and HGH users, IGF-1 is typically drawn at baseline, then at 6 weeks into a protocol (the standard response-assessment point), then every 3 months. A typical responder on tesamorelin 1–2 mg nightly will see IGF-1 rise 40–80% from baseline. Non-response at 6 weeks usually means compliance or technique issue (often an incomplete fasting window). Very high IGF-1 elevations warrant caution — above ~300 ng/mL in most adults suggests the dose is higher than needed. IGF-1 LR3 users will see suppressed native IGF-1 on standard assays (different epitope) — this is normal and expected.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.