Antidiuretic hormone (ADH), also known as vasopressin or arginine vasopressin, is made in the hypothalamus, and is stored in the posterior pituitary. ADH maintains water homeostasis in the body by binding to receptors in the kidney and initiating water reabsorption. The synthesis and release of ADH is regulated by hypothalamus in response to changes in fluid osmolality and changes in blood volume via a negative feedback loop. ADH deficiency is called diabetes insipidus (DI) and central DI is caused when hypothalamic ADH production is decreased/when pituitary release is hindered. The ADH test alone is not diagnostic of a specific condition. Thus, ADH abnormalities are diagnosed by testing serum osmolality, urine osmolality, and serum sodium. However, serum ADH testing in addition to these laboratory tests is used in differential diagnosis of syndrome of inappropriate antidiuretic hormone (SIADH), DI, chronic hyponatremia, and psychogenic water intoxication and is performed as part of water deprivation and water loading tests.