Insulin-like growth factor 1 is produced primarily by the liver in response to GH stimulation.
Mechanism: Mediates most of GH's anabolic effects. Driving cell proliferation, muscle protein synthesis, and tissue repair. Has a much longer half-life than GH (hours vs. minutes), making it the better lab marker for GH-axis status.
Lab range: 100–300 ng/mL adult; declines with age. Z-scores by age are more useful than absolute values.
Caveats: GH-axis protocols (peptides, secretagogues) target IGF-1 elevation. But chronically high IGF-1 has cancer-progression concerns; modest, age-appropriate elevation is the goal. Not maximizing.