Figure 6. GH is produced in the pituitary gland. In the periphery, GH acts directly and indirectly through stimulation of IGF-I production. In the circulation, the liver is the most important source of IGF-I (75%) but other tissues (e.g. brain, adipose tissue, kidney, bone, and muscles) may contribute. Under GH stimulation the muscle, adipose tissue, and bone have been shown to secrete IGF-I that has a paracrine/autocrine effect.