IGF-1
Also: insulin-like growth factor 1
In plain language
A hormone, mostly made by the liver in response to growth hormone, that drives tissue growth and repair.
Technical definition
A peptide hormone produced primarily in the liver under growth hormone stimulation that mediates many of growth hormone's anabolic and growth-promoting effects; serum IGF-1 is used as a marker of GH activity.
Because IGF-1 levels reflect average growth hormone activity, they are often measured to gauge the effect of growth hormone secretagogues, which otherwise raise GH in short pulses that are hard to capture with a single blood draw.