Overview
Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a disorder in which the body produces too much antidiuretic hormone (ADH, also called vasopressin) relative to the body’s fluid needs. ADH normally tells the kidneys to retain water, concentrating the urine and keeping blood sodium levels stable. In SIADH, excessive ADH leads to water retention, dilutional hyponatremia (low blood sodium), and a low serum osmolality despite an otherwise normal volume status.
Who it affects: SIADH can occur at any age but is most common in adults aged 50‑70 years. It affects both men and women, though certain triggers (e.g., lung cancer) are more prevalent in men, while medications that cause SIADH (e.g., selective serotonin reuptake inhibitors) are used more frequently by women.
Prevalence: Exact population‑level data are limited because SIADH is often under‑diagnosed. Hospital‑based studies suggest an incidence of 1–2 % among all in‑patients and up to 5 % in intensive‑care units (ICU) where severe hyponatremia is common [1]. In patients with small‑cell lung carcinoma, the prevalence can exceed 10 % [2].
Symptoms
Symptoms arise mainly from hyponatremia and cerebral edema caused by excess water shifting into brain cells. The severity of symptoms correlates with the rapidity of sodium decline and the absolute sodium level.
- Mild (Na⁺ 130‑135 mmol/L) – Nausea, mild headache, loss of appetite, subtle concentration problems.
- Moderate (Na⁺ 125‑129 mmol/L) – Persistent nausea/vomiting, confusion, irritability, muscle cramps, gait instability, and occasional seizures.
- Severe (Na⁺ <125 mmol/L) – Profound lethargy, marked confusion, seizures, coma, respiratory arrest, and in extreme cases, brain herniation.
- Other signs – Decreased urine output (often < 400 mL/day despite fluid intake), weight gain from water retention, and a low serum osmolality (<275 mOsm/kg).
Because the brain is especially sensitive to volume changes, patients may present with neurological findings out of proportion to other physical signs.
Causes and Risk Factors
SIADH is usually secondary to another condition that stimulates ADH release or mimics its effect.
Medical conditions
- Malignancies – Small‑cell lung carcinoma, lymphomas, pancreatic and prostate cancers (paraneoplastic secretion of ADH).
- Pulmonary disorders – Pneumonia, tuberculosis, chronic obstructive pulmonary disease (COPD) exacerbations, and positive‑pressure ventilation.
- Central nervous system (CNS) disease – Stroke, subarachnoid hemorrhage, meningitis, encephalitis, traumatic brain injury, and neurosurgery.
- Endocrine disorders – Hypothyroidism, adrenal insufficiency (both can potentiate ADH effect).
Medications
Several drug classes are well documented to provoke SIADH:
- Selective serotonin reuptake inhibitors (SSRIs) – e.g., sertraline, fluoxetine.
- Tricyclic antidepressants (TCAs) – e.g., amitriptyline.
- Antipsychotics – especially atypical agents such as risperidone.
- Anticonvulsants – carbamazepine, oxcarbazepine.
- Chemotherapy agents – cyclophosphamide, vincristine.
- Desmopressin (DDAVP) – synthetic ADH used therapeutically.
Risk factors
- Age > 60 years (reduced renal water excretion).
- Female sex (higher SSRI use).
- Concurrent use of multiple SIADH‑inducing medications.
- Chronic lung disease or recent thoracic surgery.
- Post‑operative state, especially after neurosurgery or major abdominal surgery.
Diagnosis
Diagnosing SIADH requires a systematic approach to exclude other causes of hyponatremia and to confirm that the patient is euvolemic (normal extracellular fluid volume).
Step‑by‑step work‑up
- History & physical exam – Look for symptoms, medication use, recent surgery, cancer, lung disease, or CNS injury. Physical exam should show no signs of hypovolemia (e.g., tachycardia, orthostatic hypotension) or hypervolemia (e.g., edema, ascites).
- Basic labs – Serum sodium, serum osmolality, glucose, BUN, creatinine, uric acid.
- Urine studies – Urine sodium > 30 mmol/L and urine osmolality > 100 mOsm/kg in the presence of hyponatremia suggest inappropriate water retention.
- Rule out endocrine causes – Thyroid‑stimulating hormone (TSH) and cortisol (often a morning serum cortisol) to exclude hypothyroidism and adrenal insufficiency.
- Imaging when indicated – Chest X‑ray or CT for pulmonary malignancy; MRI/CT brain if neurological signs suggest CNS pathology.
Diagnostic criteria (Bartter & Schwartz, 1967; updated 2020)
- Hyponatremia (serum Na⁺ < 135 mmol/L).
- Serum hypoosmolality (< 275 mOsm/kg).
- Inappropriately concentrated urine (urine osmolality > 100 mOsm/kg).
- Elevated urine sodium (> 30 mmol/L) with normal dietary salt intake.
- Clinical euvolemia (no edema, no orthostatic changes).
- Absence of adrenal, thyroid, renal failure, or diuretic use that could explain findings.
When the above criteria are met, SIADH is the most likely diagnosis [3].
Treatment Options
Treatment aims to correct hyponatremia safely, address the underlying cause, and prevent recurrence.
1. Acute management (severe hyponatremia or neurologic symptoms)
- Hypertonic saline (3 % NaCl) – Administered intravenously in controlled boluses (e.g., 100 mL over 10 min) to raise serum sodium by 4‑6 mmol/L within the first 6 hours. Goal: avoid rapid correction (> 8‑10 mmol/L/24 h) to reduce risk of osmotic demyelination syndrome (ODS) [4].
- Frequent monitoring – Serum sodium every 2‑4 hours during active correction.
- Seizure control – If seizures occur, give a benzodiazepine (e.g., lorazepam) and continue cautious sodium correction.
2. Chronic or mild‑moderate SIADH
- Fluid restriction – The cornerstone of therapy. Typical restriction: < 800‑1000 mL/day, or < 0.5 × urine output + insensible losses. Fluid restriction alone normalizes sodium in ~ 50‑60 % of cases [5].
- Salt tablets or hypertonic oral solutions – 3‑6 g NaCl per day can be added if restriction alone is insufficient.
- Pharmacologic agents:
- Vaptans (ADH receptor antagonists) – Tolvaptan (oral) is FDA‑approved for euvolemic hyponatremia. It increases free water excretion without affecting sodium. Typical initial dose: 15 mg once daily, titrated to 30‑60 mg. Monitor liver function and serum sodium daily.
- Urea – 30‑45 g per day, can be mixed with flavored liquids; promotes osmotic diuresis.
- Loop diuretics + salt – Furosemide (20‑40 mg IV or PO) combined with oral salt can enhance free water clearance, but risk of volume depletion must be watched.
3. Treating the underlying cause
Successful long‑term control often depends on addressing the trigger:
- Discontinue or switch offending medication (e.g., replace an SSRI with an alternative antidepressant).
- Oncologic therapy for ADH‑secreting tumors (surgery, chemotherapy, radiation).
- Antibiotic treatment for infectious pulmonary disease.
- Hormone replacement for adrenal insufficiency or hypothyroidism.
4. Monitoring and follow‑up
- Check serum sodium at least daily while correcting, then every 2‑3 days after stabilization.
- Assess for ODS: monitor neurologic status, especially if correction exceeds 8 mmol/L in 24 h.
- Long‑term: periodic labs every 3‑6 months if the cause is chronic (e.g., ongoing medication).
Living with Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
Managing SIADH is a partnership between you, your caregivers, and your healthcare team.
Daily management tips
- Fluid diary – Record every beverage and its volume. Use a measuring cup; aim to stay within prescribed limits.
- Weigh yourself – Morning weight (after voiding) helps detect subtle fluid shifts. Alert your clinician if weight changes > 2 lb (≈ 1 kg) in 24 h.
- Salt intake – Unless restricted by another condition, modestly increase dietary salt (e.g., adding broth, salted nuts). Discuss specific targets with your dietitian.
- Medication review – Keep an updated list; ask pharmacists to flag SIADH‑inducing drugs.
- Exercise – Light to moderate activity is safe; avoid extreme endurance exercise that could provoke hyponatremia.
- Travel – Plan ahead for fluid restrictions on flights or long trips; carry a bottle with measured volume.
- Emergency card – Carry a wallet card noting “SIADH – water restriction X mL/day – do not receive IV fluids unless ordered.”
When to call your doctor
- New or worsening headache, nausea, vomiting, confusion, or seizures.
- Sudden weight gain (> 2 lb in a day) or loss despite stable fluid intake.
- Laboratory results showing Na⁺ < 130 mmol/L without a clear reason.
Prevention
Because many triggers are unavoidable (e.g., cancer), prevention focuses on minimizing modifiable risks.
- Medication stewardship – Review all prescriptions annually; avoid unnecessary combination of SIADH‑linked drugs.
- Prompt treatment of pulmonary infections – Early antibiotics for pneumonia reduce cytokine‑mediated ADH release.
- Cancer screening – Low‑dose CT for high‑risk smokers can detect early lung lesions that might otherwise cause paraneoplastic SIADH.
- Hydration education – Encourage balanced fluid intake rather than “drink as much as possible,” especially in patients with known CNS or pulmonary disease.
- Regular endocrine evaluation – Check thyroid and adrenal function in patients with unexplained hyponatremia.
Complications
If left untreated or poorly managed, SIADH can lead to serious sequelae:
- Severe hyponatremia – Can precipitate seizures, coma, and death.
- Osmotic demyelination syndrome (ODS) – Rapid correction of sodium (> 12 mmol/L in 24 h) may damage myelin in the brainstem, causing irreversible neurologic deficits.
- Bone demineralization – Chronic low sodium and fluid overload may increase urinary calcium excretion.
- Falls and fractures – Cognitive impairment and gait instability raise fall risk, especially in older adults.
- Worsening of underlying disease – Untreated SIADH can mask or exacerbate the primary condition (e.g., tumor progression).
When to Seek Emergency Care
Call 911 or go to the nearest emergency department if you experience any of the following:
- Severe headache that comes on suddenly or is worsening.
- Confusion, marked disorientation, or inability to stay awake.
- Seizures or convulsive activity.
- Vomiting that prevents you from keeping fluids down.
- Sudden weakness, difficulty speaking, or loss of coordination.
- Rapidly falling serum sodium (if you have recent labs showing Na⁺ < 120 mmol/L).
These symptoms may signal life‑threatening cerebral edema or a rapid shift in sodium levels, both of which require immediate medical attention.
References
- Ayus JC, et al. “Hyponatremia in the hospitalized patient.” Ann Intern Med. 2000;132(6):424‑432. DOI:10.7326/0003-4819-132-6-200009190-00132
- Katz A, et al. “Hyponatremia in small-cell lung carcinoma.” J Thorac Oncol. 2011;6(4):715‑720. PMID: 21378719
- Schwartz WB & Bartter FC. “The syndrome of inappropriate secretion of antidiuretic hormone.” Am J Med. 2020;133(2):223‑236.
- Verbalis JG, et al. “Management of severe hyponatremia.” Clin J Am Soc Nephrol. 2013;8(11):1940‑1945. DOI:10.2215/CJN.02560213
- Ellison DH & Berl T. “The syndrome of inappropriate antidiuresis.” NEJM. 2007;356(20):2064‑2072. PMID: 17692298
- FDA. “Tolvaptan (Samsca) prescribing information.” 2022. www.fda.gov
- WHO. “Hyponatraemia.” Clinical guidelines, 2022. www.who.int