Abstract
Hyponatremia (serum sodium < 134 mmol/L) is a common electrolyte disturbance. A history of concurrent illness and medication use as well as the assessment of extracellular volume status on physical examination may provide useful clues as to the pathogenesis of hyponatremia. Measurement of the effective serum tonicity is the first step in the laboratory evaluation. In patients with normal or elevated effective serum osmolality (280 mOsm/kg or greater) pseudohyponatremia should be excluded. In the hypo‐osmolar state, urine osmolality is used to determine whether water excretion is normal or impaired. A urine osmolality value of <100 mOsm/kg indicates complete and appropriate suppression of antidiuretic hormone secretion. A urine sodium level <20 mmol/L is indicative of hypovolemia, whereas a level >40 mmol/L is suggestive of the syndrome of inappropriate antidiuretic hormone secretion. Levels of hormones (thyroid‐stimulaing hormone and cortisol) and arterial blood gases should be determined in difficult cases ofhyponatremia. A clinical diagnostic algorithm for hyponatremia is included in this review article.
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