How can the urine electrolytes and urine osmolality help diagnose the etiology of hyponatremia?
Hyponatremia is generally a result of impaired renal excretion of water due to high levels of anti-diuretic hormone (ADH) in addition to a source of water intake.
ADH can be elevated in the setting of hypovolemia. Urine electrolytes can provide a biochemical clue to the presence of hypovolemia with the presence of a low urine Na.
The normal physiologic response to hyponatremia would be to suppress ADH and excrete a maximally dilute urine. In the setting of hyponatremia, measuring the urine osmolality and finding a non-maximally dilute urine will confirm the presence of elevated ADH. It will not, however, provide the etiology of the elevated ADH. This is useful to distinguish from the admittedly uncommon condition of primary polydipsia.
In this condition, water excretion is normal but intake is so high that it exceeds excretory capacity – in this condition, urine osmolality is appropriately low.
For a more detailed review on hyponatremia, try the following links:
Adrogue, Horacio J., Madias, Nicolaos E. Hyponatremia. N Engl J Med 2000 342: 1581-1589.
Ellison, DH. Disorders of Sodium and Water. American Journal of Kidney Diseases. August 2005. Vol. 46, Issue 2, Pages 356-361