Hyponatraemia Cause Algorithm

Maximally dilute urine: primary polydipsia, excess oral or IV water, or low solute intake (e.g. beer potomania, 'tea and toast' diet).

→ Restrict water intake and increase solute intake; review IV fluids.

Reduced ECF: diarrhoea, vomiting, third spacing, remote diuretic use. Expanded ECF: heart failure, cirrhosis, nephrotic syndrome.

→ Reduced ECF: restore volume with IV 0.9% saline or a balanced crystalloid. Expanded ECF: fluid restriction and treat the underlying condition.

With diuretic use or kidney disease, these are likely causes — but all other causes remain possible (ESE 2014).

→ Review and stop thiazide diuretics where possible; assess volume; renal advice for kidney disease.

Vomiting, primary adrenal insufficiency, renal or cerebral salt wasting, occult diuretics.

→ Check cortisol (exclude adrenal insufficiency); restore volume with IV 0.9% saline or a balanced crystalloid; treat the cause.

SIAD — after excluding secondary adrenal insufficiency, hypothyroidism and occult diuretics.

→ Check TSH and cortisol; review drugs; fluid restriction first line, then increased solute intake (urea) or low-dose loop diuretic with oral sodium chloride (ESE 2014); treat the cause. Correct by no more than 10 mmol/L in the first 24 h and 8 mmol/L in each 24 h after.

The ESE 2014 algorithm places expanded ECF under urine sodium ≤ 30 mmol/L; a urine sodium above 30 mmol/L without diuretics or kidney disease does not fit it.

→ Re-check diuretic use, kidney function and the volume assessment; seek senior or specialist advice.