AVP and Hyponatremia: [1] Hyponatremia has a high prevalence in acute and chronic hospitalized patients and may lead to increased length of stay and higher financial burden.[2–8] It is a marker of disease severity and is associated with increased mortality.[9] Syndrome of inappropriate antidiuresis (SIAD) is the most common cause of hyponatremia.[10] SIAD is considered to be a group of clinical syndromes of low serum sodium, high urinary sodium, and high urinary osmolality caused by sustained release or enhanced activity of endogenous antidiuretic hormone (ADH) or mutations in the ADH receptor gene.