GHN is synthesized within the somatotropes of the anterior pituitary gland and is secreted into circulation under tripartite control, stimulated by growth hormone releasing hormone (GHRH) and the Growth Hormone Secretagogue, Ghrelin (GHS) and inhibited by somatostatin (SST) (Figure 9). GHRH, GHS and SST secretion are themselves regulated by numerous central nervous system neurotransmitters (Table 5). GH exerts direct metabolic effects on target tissues and exerts many of its growth effects through IGF-1, which is produced by the liver and the target tissues (e.g. bones). Additional regulation of GH secretion is achieved through feedback control by IGF-1 and GH at the pituitary and at the hypothalamus.