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IGF-1 (Insulin-like Growth Factor-1)

The downstream effector of growth hormone. Drives most of GH's anabolic effects.

IGF-1 is a peptide hormone produced primarily by the liver in response to growth hormone signaling. It mediates most of GH's anabolic effects — muscle growth, tissue repair, bone density — by binding the IGF-1 receptor. Synthetic analogs (IGF-1 LR3, PEG-MGF) extend half-life or target specific receptors. The cancer-concern flag on many GH-axis peptides comes from IGF-1's role in cell proliferation.