Synopsis
Diagnose syndrome of inappropriate antidiuresis only after excluding mimics, treat neurological emergencies safely, and target the underlying driver of water retention.
- SIAD is a diagnosis of hypotonic hyponatraemia with inappropriately concentrated urine after effective-volume depletion, adrenal insufficiency, severe hypothyroidism, kidney failure and diuretics are addressed.
- Confirm true hypotonicity because hyperglycaemia, exogenous osmoles and pseudohyponatraemia create different physiology and treatment.
- Urine osmolality shows whether AVP activity is present; maximally dilute urine points towards excess intake rather than persistent antidiuresis.
Investigation priorities
Confirm that low sodium represents hypotonic hyponatraemia rather than a non-hypotonic state.
Management branches
Hypotonic hyponatraemia accompanies seizure, reduced consciousness or another severe neurological syndrome.
- Begin monitored ABCDE care, check glucose, stop hypotonic fluid and obtain serum and urine samples if this causes no treatment delay.
- Give protocol-defined boluses of hypertonic saline, reassessing symptoms and sodium after each rather than prescribing an open-ended infusion.
Stable hypotonic hyponatraemia has no immediately obvious volume-depleted or oedematous cause.
Key medicines
Hypertonic sodium chlorideAdminister the current Society for Endocrinology protocol-defined bolus in a monitored acute setting, repeat only after symptom and sodium reassessment, and stop when the limited initial correction goal is achieved.
TolvaptanInitiate only under an experienced specialist and local commissioned protocol at the approved starting regimen, with free access to water and closely timed sodium checks; adjust according to response rather than routine escalation.