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Pathophysiology and etiology of the syndrome of inappropriate antidiuretic hormone secretion (SIADH)

INTRODUCTION

The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is a disorder of impaired water excretion caused by the inability to suppress the secretion of antidiuretic hormone (ADH) [1]. If water intake exceeds the reduced urine output, the ensuing water retention leads to the development of hyponatremia.

The SIADH should be suspected in any patient with hyponatremia, hypoosmolality, and a urine osmolality above 100 mosmol/kg. In SIADH, the urine sodium concentration is usually above 40 meq/L, the serum potassium concentration is normal, there is no acid-base disturbance, and the serum uric acid concentration is frequently low [1]. (See "Evaluation of adults with hyponatremia".)

The pathophysiology and etiology of SIADH will be reviewed here. The treatment of this disorder is discussed separately. (See "Treatment of hyponatremia: Syndrome of inappropriate antidiuretic hormone secretion (SIADH) and reset osmostat".)

PATHOPHYSIOLOGY

Pathogenesis of hyponatremia — The plasma sodium concentration (PNa) is a function of the ratio of the body's content of exchangeable sodium and potassium (NaE and KE) and total body water (TBW) as described by Edelman's classic equation:

 PNa  ≈  NaE + KE/Total body water

                  

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Literature review current through: Mar 2014. | This topic last updated: Nov 20, 2012.
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