Somatostatin [SST, GHIH (growth hormone-inhibiting hormone) or SRIF (somatotropin release-inhibiting factor)] is a cyclic peptide hormone existing as two isoforms and produced in the pancreas islet, GI tract and the central nervous system. Tetradecapeptide Somatostatin-14 (SRIF-14, the originally identified somatostatin) and the 28-amino acid Somatostatin-28 (SRIF-28) have similar biological activities but differ in their potency. Somatostatins regulate the endocrine system: they inhibit the release of growth hormone in contrast to Growth Hormone Factor (GRF), which stimulates the release of growth hormone. They also inhibit the release of prolactin and thyrotropin, peptide hormones from the pituitary gland and glucagon and insulin from the pancreas. They control the secretion of gut hormones and function as neurotransmitters.