What does urine osmolality indicate regarding hyponatremia?

Updated: Jun 17, 2019
  • Author: Eric E Simon, MD; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Urine osmolality helps to differentiate between conditions associated with impaired free water excretion and primary polydipsia, in which water excretion should be normal (provided intact kidney function). With primary polydipsia, as with malnutrition (severe decreased solids intake) and a reset osmostat, the urine osmolality is maximally dilute, generally less than 100 mOsm/kg. A urine osmolality greater than 100 mOsm/kg indicates impaired ability of the kidneys to dilute the urine. This usually is secondary to elevated vasopressin (antidiuretic hormone; ADH) levels, appropriate or inappropriate.


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