Causes and features of true hyponatraemia (serum osmolality is < 280 mOsmol/kg) with abnormal water excretion (urine osmolality > 100 mOsmol/kg)

 MRCP  renal

SIADHhypovolaemiarenal salt wasting
clinical cluesmass on CXR, known drug causesigns of oedema / dehydration, heart failure, chronic liver disease, nephrotic syndrome
urine sodium> 20 mmol/L (often > 40 mmol/L)< 40 mmol/L> 40 mmol/L
normal saline challenge< 5 mmol/L change in urine sodium> 5 mmol/L change in urine sodium
uric acidlownormal or slightly increasedlow
fractional excretion of urateraised (normalises with sodium correction)raised (does not normalise with sodium correction)
renin / aldosteronenormal or lowraised *renin (normal or raised), aldosterone raised
extracellular volumehigh normal or raisedreduced *reduced
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Hypothyroidism and adrenal insufficiency need to be excluded. Also, if water excretion is normal (urine osmolality < 100 mOsmol/kg) then primary polydipsia may be the cause.

Normal saline challenge is 2 L/day of IV normal saline in those cases with indeterminate urinary sodium (20-40 mmol/L).

* Renin / aldosterone levels and extracellular volume (ECV) are not usually measured in hypovolaemia as the cause should be obvious. However, they may help differentiate SIADH from renal salt wasting.


Milionis HJ, Liamis GL, Elisaf MS. The hyponatremic patient: a systematic approach to laboratory diagnosis. CMAJ 2002 Apr;166(8):1056–1062. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC100882/

O'Donoghue D, Trehan A. SIADH and hyponatraemia: foreword. NDT Plus 2009 Nov;2(suppl_3):iii1–iii4. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762825/

Maesaka JK, Miyawaki N, Palaia T, Fishbane S, Durham JHC. Renal salt wasting without cerebral disease: Diagnostic value of urate determinations in hyponatremia. Kidney International 2007;71(8):822–826. Available from: http://www.nature.com/ki/journal/v71/n8/full/5002093a.html


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