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Endocrinology & Metabolic

SIADH (Syndrome of Inappropriate Antidiuresis)

SIADH is inappropriate antidiuresis causing water retention and euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine; it is a diagnosis of exclusion and management depends first on symptoms, then on the cause and risk of over-correction.

In a nutshell

SIADH is inappropriate antidiuresis causing water retention and euvolaemic hypotonic hyponatraemia. Expect plasma osmolality below 275 mOsm/kg, urine osmolality above 100 mOsm/kg and urine sodium usually above 30 mmol/L, but diagnose only after excluding hypovolaemia, adrenal insufficiency, severe hypothyroidism, renal/medication causes and other mimics. Treat severe or moderately severe symptoms immediately with monitored hypertonic saline; otherwise use fluid restriction, treat the cause and reserve urea or tolvaptan for specialist care.

Classic presentation

A patient with pneumonia, nausea, CNS disease, malignancy or a new SSRI is found to have euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine and normal adrenal and thyroid assessment.

Key points

  • SIADH is a diagnosis of exclusion, not a diagnosis from a low sodium and a high urine sodium alone.
  • The diagnostic pattern is hypotonic hyponatraemia, urine osmolality above 100 mOsm/kg, urine sodium usually above 30 mmol/L and clinical euvolaemia.
  • Seizures, coma, persistent vomiting, confusion or severe headache plausibly due to hyponatraemia need emergency hypertonic saline; do not wait for the cause.
  • For non-severe persistent SIADH, fluid restriction and treatment of the cause come first.
  • Avoid over-correction: Society for Endocrinology limits the rise to 10 mmol/L in the first 24 hours and 8 mmol/L in each following 24 hours.
  • Tolvaptan is specialist hospital treatment with close sodium and volume monitoring, not a routine emergency drug.

First-line investigation

Paired serum and urine osmolality with urine sodium, glucose, renal function, thyroid function, cortisol assessment, volume status and medication review.

Management

Treat neurological symptoms first

  • Give monitored 3% hypertonic saline boluses for severe or moderately severe symptoms, aiming for a 5 mmol/L sodium rise or symptom improvement rather than normalisation.3

Confirm the water-balance pattern

  • Measure plasma osmolality, paired urine osmolality and urine sodium, assess volume status and exclude adrenal, thyroid, renal, medication and glucose-related mimics.3,1,5

Restrict fluid and treat the cause

  • For non-severe persistent SIADH, use individualised fluid restriction and stop or treat the precipitant, with daily sodium and fluid-balance review.1,5

Avoid iatrogenic worsening

  • Avoid uncritical 0.9% saline in euvolaemic SIADH, monitor urine output and keep the sodium rise within safe limits; over-correction needs urgent specialist management.3,4

Use specialist second-line treatment

  • If fluid restriction fails, consider oral urea or loop diuretic plus oral sodium chloride with endocrine/nephrology advice; reserve hospital-initiated tolvaptan for selected persistent SIADH.2,7,8

Monitor and find the trigger

  • Recheck sodium after treatment changes, investigate persistent unexplained SIADH for pulmonary, CNS or malignant causes and safety-net for recurrent neurological symptoms.3,1,5

Exam traps

  • Do not call a patient SIADH before excluding adrenal insufficiency, severe hypothyroidism, hypovolaemia, renal/diuretic effects and primary polydipsia.
  • A patient with SIADH is usually euvolaemic; dehydration or oedema suggests another water-balance disorder.
  • Do not use fluid restriction alone for severe symptomatic hyponatraemia.
  • Do not give repeated 0.9% saline empirically to euvolaemic SIADH; clarify whether the patient is actually hypovolaemic.
  • Tolvaptan can over-correct sodium and must be initiated in hospital with close monitoring.
  • A rapid rise in urine output can precede a dangerous spontaneous sodium rise.

Key sources

  1. Grant et al.: The diagnosis and management of inpatient hyponatraemia and SIADH (UK clinician consensus algorithm from senior UK endocrine and medical teams; practical diagnosis, cause-directed treatment and inpatient management; published 2015.)Updated 1 Aug 2015
  2. European clinical practice guideline on diagnosis and treatment of hyponatraemia (Joint ESE/ESICM/ERBP guideline used for formal SIAD diagnostic criteria, urea/solute second-line options and correction safety; accessed 4 August 2026.)Updated 1 Mar 2014
  3. Society for Endocrinology: Emergency management of severe and moderately severely symptomatic hyponatraemia in adult patients (Current UK emergency guidance revised in 2022; symptom-led hypertonic-saline bolus treatment, correction limits, monitoring, over-correction management and diagnostic algorithm.)Updated 1 Jan 2022
  4. NICE CG174: Intravenous fluid therapy in adults in hospital (NICE principles for fluid assessment, monitoring, complex hyponatraemia and expert fluid/electrolyte input; accessed 4 August 2026.)
  5. Royal Cornwall Hospitals NHS Trust: Management of Hyponatraemia Clinical Guideline (Adult NHS-trust guideline version 3.0, July 2024; diagnostic urine studies, SIADH causes, fluid restriction, specialist tolvaptan initiation and monitoring principles.)Updated 1 Jul 2024
  6. NICE NG243: Adrenal insufficiency: identification and management (Current NICE adrenal-insufficiency pathway used to ensure adrenal insufficiency is excluded or treated rather than mislabelled as SIADH; accessed 4 August 2026.)
  7. Samsca (tolvaptan) 15 mg tablets: UK Summary of Product Characteristics (UK product information confirming hospital initiation for SIADH, dose titration, water access, sodium/volume monitoring and over-correction precautions; accessed 4 August 2026.)
  8. British National Formulary (BNF) (Current UK prescribing information for electrolyte replacement, sodium chloride, loop diuretics, demeclocycline and tolvaptan; check product-specific dose, contraindication, interaction and monitoring details at prescribing; accessed 4 August 2026.)

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.