Hyponatremia secondary to the syndrome of inappropriate secretion of
antidiuretic hormone (
SIADH) is a frequent cause of hypotonicity. Although the differential diagnosis with other causes of hypotonicity such as
salt depletion is sometimes challenging, some simple and readily available
biologic parameters can be helpful in the diagnosis of
SIADH. In
SIADH,
urea is typically low; this is less specific for elderly patients, for whom lower clearance of
urea accounts for higher values. Low levels of
uric acid are more often seen in
SIADH (70%) compared with
salt-depleted patients (40%). Typically, patients with
SIADH will show a lower anion gap with nearly normal total CO2 and serum
potassium, this despite dilution. In patients with
hyponatremia secondary to hypocorticism, total CO2 is usually lower than in nonendocrine
SIADH despite low
urea and
uric acid levels. Urine biology can also be helpful in diagnosis of
SIADH because patients with
SIADH have high urine
sodium (Na; >30 mEq/L), and most of them will have a high fractional excretion of Na (>0.5% in 70% of cases), reflecting
salt intake. Conversely, low urine Na in patients with
SIADH and poor alimentation is not rare. Finally, measurement of urine osmolality is useful for the diagnosis of
polydipsia and reset osmostat and could further help in the choice of therapeutic strategy because patients with low urine osmolality will benefit from water restriction or
urea, whereas those with high urine osmolality (>600 mOsm/kg) would be good candidates for V2 antagonist.