Unexplained hyponatremia: seek and you will find.

Background: Hyponatremia is a common diagnostic challenge.

Methods: An index case is presented to discuss the diagnostic approach to chronic and unexplained hyponatremia.

Results: The index case concerns a 60-year-old man with chronic hepatitis C and previous alcohol use who was referred because of weight loss, poor dietary intake, dizzy spells, and unexplained hyponatremia (serum sodium 124-129 mmol/l). A low urine sodium concentration (20 mmol/l) and a low fractional sodium excretion (0.07%) were observed repeatedly, while urine osmolality was high (>400 mosm/kg). The central questions in this case are: what is the differential diagnosis, which tests are needed to confirm or exclude a diagnosis, and how would you proceed if no obvious cause is found?

Conclusions: The diagnosis of this case of unexplained hyponatremia was unexpected, but important because it was treatable. The challenges and caveats of the diagnostic approach to hyponatremia are discussed. A diagnostic algorithm to guide clinicians who are confronted with similar cases is presented.