A growth hormone produced mainly by the liver that drives most of GH’s tissue-building effects — the downstream mediator of the GH axis.Produced primarily in the liver in response to GH stimulation, but also locally in muscle, skin, bone, and scalp as a paracrine signal. Mediates GH’s anabolic effects via the IGF-1 receptor (IGF1R) through PI3K/Akt and MAPK signaling. Serum IGF-1 is the standard clinical biomarker for assessing GH axis activity and monitoring GH secretagogue effects. IGF-1-LR3 and IGF-1-DES are synthetic analogs with modified IGFBP binding (LR3) or truncation (DES) that extend activity duration or increase local potency respectively.