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Syndrome of Inappropriate Antidiuretic Hormone Release

created May 18th, 19:24 by Divyansh Sharma


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SIADH consists of hyponatremia, inappropriately elevated urine osmolality (>100 mOsm/kg), and decreased serum osmolality in a euvolemic patient. SIADH should be diagnosed when these findings occur in the setting of otherwise normal cardiac, renal, adrenal, hepatic, and thyroid function; in the absence of diuretic therapy; and in absence of other factors known to stimulate ADH secretion, such as hypotension, severe pain, nausea, and stress.
 
In general, the plasma Na+ concentration is the primary osmotic determinant of AVP release. In persons with SIADH, the nonphysiological secretion of AVP results in enhanced water reabsorption, leading to dilutional hyponatremia. While a large fraction of this water is intracellular, the extracellular fraction causes volume expansion. Volume receptors are activated, and natriuretic peptides are secreted, which causes natriuresis and some degree of accompanying potassium excretion. Eventually, a steady state is reached and the amount of Na+ excreted in the urine matches Na intake.
 
Ingestion of water is an essential prerequisite to the development of the syndrome. Regardless of cause, hyponatremia does not occur if water intake is severely restricted.
 
In addition to the inappropriate AVP secretion, persons with this syndrome may also have an inappropriate thirst sensation, which leads to an intake of water that is in excess of free water excreted. This increase in water ingested may contribute to the maintenance of hyponatremia.

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