Diagnosis of SIADH

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.

Note:

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

Reference:

• (1) Thompson CJ, Crowley RK. Hyponatraemia. J R Coll Physicians Edinb2009; 39:154–7
• (2) Hannon MJ, Thompson CJ. The syndrome of inappropriate antidiuretic hormone: prevalence, causes and consequences. Eur J Endocrinol. 2010;162 Suppl 1:S5-12
• (3) Hoorn EJ, van der Lubbe N, Zietse R. SIADH and hyponatraemia: why does it matter?NDT Plus. 2009;2(Suppl_3):iii5-iii11.