A naturally occurring regulatory peptide that carries out numerous functions in the human body, including the inhibition of growth hormone secretion from the anterior pituitary gland. Somatostatin consists of 14 amino acids (see illus.); the two cysteine residues are joined by a disulfide bond so that the peptide forms a ring structure. A larger variant of this peptide, called somatostatin-28, is produced in some cells and has an additional 14 amino acids attached at the amino-terminal end of normal somatostatin (somatostatin-14).