The syndrome of inappropriate ADH secretion (SIADH) is a condition that is characterised by levels of arginine vasopressin (AVP)/ antidiuretic hormone (ADH) that are inappropriate for the given blood osmolality. The ADH receptor is found in the collecting ducts of the kidney where it plays a vital role in the reabsorption of water. The ADH (vasopressin) acts on the v2 receptors in the collecting ducts and stimulates the up-regulation of water channels (aquaporins) in the membrane. When there are high levels of vasopressin in the blood, the volume of urine produced will be small and highly concentrated. It is for this reason that vasopressin is referred to as 'anti-diuretic hormone'. In this syndrome, the inappropriate levels of ADH result in the production of small volumes and highly concentrated urine even in times of fluid excess.