The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of hyponatremia. The syndrome manifests as an inability to excrete a free water load, with inappropriately concentrated urine and resultant hyponatremia, hypoosmolality, and natriuresis. SIADH occurs in a setting of normal blood volume, without evidence of renal disease or deficiency of thyroxine or cortisol. Although usually transient, SIADH may be chronic; it is often associated with drug use or a lesion in the central nervous system or lung. When the cardinal features of SIADH were defined by Bartter and Schwartz (1967), levels of AVP could not be measured. Subsequently, radioimmunoassays revealed that SIADH is usually associated with measurably elevated serum levels of AVP. Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is characterized by a clinical picture similar to SIADH, but is associated with undetectable levels of AVP (Feldman et al., 2005).

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal (or neonatal) forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal (or neonatal) forms of Bartter syndrome (e.g., BARTS1, 601678) typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.