A number of conditions may present with serum hyponatremia and high urine osmolality.
Conditions with a low urine sodium concentration (sodium retention or severe sodium depletion):
(1) nonrenal sodium loss (with kidneys functioning to retain sodium)
(1b) hypovolemia (dehydration, other)
(2) heart failure
Conditions with a high urine sodium concentration (sodium wastage):
(1) diuretic use
(2) primary or secondary adrenal insufficiency
(3) cerebral salt wastage
(4) salt-losing nephropathy
(5) syndrome of inappropriate antidiuretic hormone (SIADH) secretion
The urine sodium may be normal if a condition associated with sodium wastage is mixed with a condition causing sodium retention.
The presence of a urine osmolality greater than serum osmolality suggests SIADH secretion, but this diagnosis requires consideration and exclusion of other conditions first.
Further evaluation using serum osmolality and extracellular fluid volume is discussed above in 13.15.05).
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Specialty: Endocrinology, Clinical Laboratory