Conditions · Metabolism (biochemistry)
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) !
Antidiuretic hormone is released when the body does not need it; the kidney holds water and blood sodium falls.
Updated 2026-11-01
Structures involved
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In physiology
- Osmosis, osmolarity and tonicity
- Cell volume regulation
- The posterior pituitary: ADH and oxytocin
- Functions of the hypothalamus
- The countercurrent mechanism, urea recycling and urine concentration
- ADH, thirst and water balance
- Sodium and extracellular volume regulation (with natriuretic peptides)
- Reasoning through fluid and electrolyte imbalances
- Fluid shifts between compartments
- Urine dilution, free water and osmotic diuresis
Why it happens
What it is
ADH release or action not suppressed by low osmolality, causing euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine.
Who gets it
It is the most common cause of euvolaemic hyponatraemia in hospital; it is linked to lung and central nervous system disease, cancer (especially small cell lung cancer), drugs and the postoperative period.
Causes
- Tumours: small cell lung carcinoma (ectopic ADH), head and neck, others
- Central nervous system: meningitis, encephalitis, head trauma, subarachnoid haemorrhage, stroke
- Lung: pneumonia, tuberculosis, mechanical ventilation
- Drugs: SSRIs, carbamazepine, oxcarbazepine, cyclophosphamide, vincristine, opioids, MDMA
- Others: pain, nausea, after surgery, HIV; genetic forms (activating V2 receptor mutation)
Risk factors
- Lung cancer and smoking
- Pneumonia or CNS infection in older adults
- Antidepressants or antiepileptics
- After surgery with pain and nausea
Pathophysiology
ADH acts on V2 receptors in the collecting duct, raises cAMP and inserts aquaporin 2 into the apical membrane: free water is reabsorbed. In SIADH ADH stays active despite low osmolality; the retained water dilutes sodium. Extracellular volume barely rises because natriuretic peptide and lower aldosterone cause urinary sodium loss (escape natriuresis), so the patient is euvolaemic, with high urine sodium and low uric acid and BUN. Symptoms depend on the speed and depth of hyponatraemia (brain oedema).
Classification
- By cause: tumour, neurological, pulmonary, drug, idiopathic
- Variant: nephrogenic syndrome of inappropriate antidiuresis (activated V2 receptor, low ADH)
Symptoms and signs
- Those of hyponatraemia: nausea, headache, confusion, falls
- Seizures and coma if severe or rapid
- No oedema or signs of dehydration
Diagnosis
- Clinical
- Euvolaemic patient, no recent diuretics, normal thyroid and adrenal function; search for the cause (lung, nervous system, drugs).
- Tests
- Low serum sodium, low serum osmolality, inappropriately high urine osmolality, high urine sodium on normal salt intake, low uric acid and BUN, normal TSH and cortisol; serum ADH is rarely needed.
- Imaging
- Chest X-ray or CT for a tumour or infection; brain CT or MRI if the cause is neurological.
Treatment
- Medical
- Fluid restriction as the basis of treatment in mild and chronic forms
- Medical
- Increase the solute load (oral salt, urea) so the kidney excretes more water
- Acute
- Hypertonic saline by protocol if symptoms are severe
- Medical
- Vaptans (V2 receptor antagonists) in selected cases; treat the cause (stop the drug, treat the tumour or infection)
- Follow-up
- Monitor sodium to keep within the safe correction limit
Drugs for this condition
Complications
- Brain oedema and seizures
- Falls and fractures
- Osmotic demyelination from rapid correction
Prognosis
Depends on the cause: it resolves when a drug is stopped or pneumonia cured; in cancer it follows the tumour's course.
Prevention
- Checking sodium when starting linked drugs in older adults
- Avoiding hypotonic fluids after surgery and in pneumonia
Test yourself
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Why does a patient with SIADH have no oedema?
Volume excess triggers natriuresis (natriuretic peptide, less aldosterone) and sodium is lost in the urine, so extracellular volume barely rises.
Which urine findings support SIADH?
Urine inappropriately concentrated for the low serum osmolality and high urine sodium on normal salt intake.
Which tumour most often makes ectopic ADH?
Small cell lung carcinoma.
Why can isotonic saline worsen the hyponatraemia of SIADH?
The kidney excretes the sodium in very concentrated urine and keeps the water, so serum sodium can fall further.
Related conditions
- Hyponatraemia
- Lung cancer
- Bacterial meningitis
- Traumatic brain injury
- Community-acquired pneumonia
- Subarachnoid haemorrhage
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Syndrome of inappropriate antidiuretic hormone secretion: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Antidiuretic hormone blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Low blood sodium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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