Basic, Organ-Based and Clinical Sciences
The Syndrome of Inappropriate Secretion Anti-Diuretic Hormone (SIADH) is characterized by excessive ADH secreted from the posterior pituitary (neurohypophysis) leading to free water retention and excretion of concentrated urine. Despite volume expansion, there is no edema in SIADH due to natriuresis secondary to atrial natriuretic peptide (ANP) release, inhibition of the renin-angiotensin-aldosterone system, and decrease in proximal tubular sodium absorption. This results in a euvolemic hypotonic hyponatremia. Although it is the most common cause of euvolemic hyponatremia, SIADH remains a diagnosis of exclusion after other causes of hypotonic hyponatremia have been ruled out. After careful history is taken, work up of hyponatremia involves assessing serum osmolality (normal 275-290 mosm/kg), extracellular fluid volume status (vital signs including orthostatic blood pressure, JVP, skin turgor, mucous membranes, peripheral edema, BUN, Creatinine, uric acid), and urine sodium concentration. Serum sodium concentration in SIADH is typically > 40 mEq/L and serum osmolality is low (<280 mosmol/kg) with a high urine osmolality (> 300 mosmol/kg) and UNa>20mEq/L.