Does Growth Hormone directly affect peripheral tissues?
No. Most (although not all) effects of GH are mediated by another hormone called insulin-like growth factor-1 (IGF-1). IGF-1 is made by the liver and other organs in response to stimulation by GH. IGF-1 feeds back to the pituitary gland and suppresses GH secretion. Unlike GH, IGF-1 has a long half-life in plasma; thus, plasma IGF-1 levels are generally more helpful than GH levels in the diagnosis of GH abnormalities.