IGF-1 is produced primarily by the liver in response to growth hormone (GH) stimulation. It mediates many of the anabolic and metabolic effects of growth hormone, including muscle growth, bone density, and tissue repair. IGF-1 levels decline with age and are a proxy for growth hormone status (since GH is pulsatile and hard to measure directly).
TRT can modestly increase IGF-1 levels in some men, but significant elevations should prompt evaluation of growth hormone status.