Pathophysiology and etiology of the syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Richard H Sterns, MD
Richard H Sterns, MD
- Editor-in-Chief — Nephrology
- Section Editor — Fluid and Electrolytes
- Professor of Medicine
- University of Rochester School of Medicine and Dentistry
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) . 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 . (See "Diagnostic 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".)
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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- Pathogenesis of hyponatremia
- Patterns of ADH secretion
- Determinants of urine output
- - Solute excretion
- - Escape from the effect of ADH
- CNS disturbances
- Pulmonary disease
- Hormone deficiency
- Hormone administration
- HIV infection
- Hereditary SIADH
- CEREBRAL SALT WASTING
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