AVP and Hyponatremia: Vasopressin, also known as antidiuretic hormone (ADH), is synthesized in the hypothalamus and stored in the neurohypophysis, which keeps plasma osmolality within a narrow range by regulating the reabsorption of water in the kidneys.[5] It has been hypothesized that an inappropriate release of ADH or an increased renal response to ADH results in the syndrome of inappropriate antidiuresis (SIAD),[6] which manifests as excessive reabsorption of water and dilutional hyponatremia.