Any deviation from the normal level of growth hormone (GH) in the blood circulation. GH or somatotropin is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration. Its secretion from the pituitary is regulated by the neurosecretory nuclei of the hypothalamus, which can release Growth hormone-releasing hormone (GHRH or somatocrinin) and Growth hormone-inhibiting hormone (GHIH or somatostatin) into the hypophyseal portal venous blood surrounding the pituitary. GH is secreted in a pulsatile manner, which is one of the reasons why an isolated measurement of its blood concentration is not meaningful.