21st May 2025

Diagnosis

The criteria for diagnosing SIADH include:

• Clinically euvolaemic
• Serum osmolality <275 mOsmol/kg 
• Urine Sodium >15 mmol/L
• Significantly raised urine osmolality >100 mOsmol/kg
• No evidence of other causes including adrenal, thyroid, pituitary or renal disease

Note: There is often (but not always) evidence of low serum uric acid/ urea

It is important to interpret these values with caution if a patient is on diuretic therapy.

The cause of hyponatraemia is often multifactorial and therefore it is important to thoroughly assess a patient and to rule out other potential causes.