Hato and Ng used a number of laboratory findings to distinguish the syndrome of inappropriate antidiuretic hormone (SIADH) secretion from hypovolemia in a patient with hyponatremia. The authors are from Indiana University and the University of Hawai'i.


Patient selection: hyponatremia from SIADH secretion vs hypovolemia. Patients with other causes of hyponatremia were excluded (hypthyroidism, hypocorticolism, hypopituitarism, beer potomania, chronic renal disease, nephrotic syndrome, cirrhosis, congestive heart failure, polydipsia).



(1) urine sodium concentration in mmol/L

(2) serum BUN in mg/dL

(3) ratio of urine sodium to serum BUN


Probably Hypovolemia


Probably SIADH

urine sodium

< 30

30 to 50

> 50

serum BUN

> 20

10 to 20

< 10

ratio urine sodium to serum BUN

< 2

2 to 5.5

> 5.5



• SIADH secretion is associated with euvolemia if salt and water intake are normal but can show signs of hypovolemia if there is significant fluid and salt restriction.

• Hypovolemia may be associated with decreased skin turgor, hypotension, tachycardia, and/or dry mucous membranes.

• The urine sodium value was higher for a patient with hypovolemia if s/he were receiving diuretics.


The authors found that an infusion of isotonic saline had a beneficial effect on patients with a urine sodium up to 50 mmol/L. The increase in serum sodium was around 12 mmol/L vs 5 mmol/L for a patient with SIADH secretion.


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