diagnosis of SIADH

Last reviewed 04/2022

The following criteria should be fulfilled for a diagnosis of SIADH to be made:

  • plasma sodium concentration <135 mmol/l
  • plasma osmolality <280 mOsmol/kg
  • urine osmolality > 100 mOsmol/kg
  • urinary sodium concentration >30mmol/L
  • patient clinically euvolaemic
  • absence of clinical or biochemical features of adrenal and thyroid dysfunction.
  • no diuretic use (recent or past) (1,2)

The following investigations could be carried out in SIADH

  • urea and electrolytes
  • plasma and urine osmolality
  • urinary sodium
  • thyroid function tests
  • a short synacthen test (1)
  • chest and skull radiographs may be useful in excluding other causes of SIADH.


  • it is important to exclude diuretic use (especially thiazides), hypothyroidism and adrenal insufficiency before the diagnosis of SIADH is made (3)