GH is a polypeptide hormone with a molecular weight of 23 kDa released from somatotropes of the anterior pituitary. It is regulated by several neurotransmitters and neuropeptides. Among other functions it plays an essential role in regulating body growth. Plasma GH levels in humans are markedly elevated in the perinatal period, in the range of 50 ng/ml, but decline to near-adult levels of 0.5-1.0 ng/ml within a few weeks. GH levels are slightly higher in women than in men, and are increased in both sexes in response to exercise.