IGF-1.

Insulin-like growth factor 1, a mediator of growth-hormone effects.

IGF-1, insulin-like growth factor 1, is a hormone produced largely by the liver in response to growth hormone, and it mediates many of growth hormone's growth-promoting effects on muscle, bone, and other tissues. Because it is produced downstream and circulates at a relatively stable level rather than in sharp pulses, blood IGF-1 is commonly used as a practical readout of overall growth-hormone activity, which is far easier to measure than trying to catch growth-hormone pulses directly. It is also the feedback signal that tells the hypothalamus and pituitary to ease off, which is why the whole axis is self-limiting. Most circulating IGF-1 is bound to binding proteins rather than free, which affects how much is actually available to tissues and complicates interpretation of a single number. The structural similarity to insulin, which the name records, is why IGF-1 has some insulin-like effects on glucose. Two cautions belong here. IGF-1 is a marker of the axis, not a goal in itself, and a higher reading does not automatically translate into a benefit anyone would notice. And because IGF-1 signalling supports cell growth generally, its relationship with long-term risk is a serious research question rather than a settled one.