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SIADH – Causes, Symptoms & Treatment

SIADH is a condition in which the body produces too much antidiuretic hormone, causing dangerous low sodium levels in the blood.

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Things worth knowing about "SIADH"

SIADH is a condition in which the body produces too much antidiuretic hormone, causing dangerous low sodium levels in the blood.

What is SIADH?

SIADH stands for Syndrome of Inappropriate Antidiuretic Hormone Secretion. It is a metabolic disorder in which the body produces excessive amounts of ADH (antidiuretic hormone, also known as vasopressin) or responds to it abnormally. ADH normally regulates how much water the kidneys retain. In SIADH, too much water is held in the body, diluting the blood and causing dangerously low sodium levels – a condition known as hyponatremia.

Causes

SIADH can be triggered by a wide range of conditions and factors:

Central nervous system disorders: Meningitis, encephalitis, traumatic brain injury, stroke, or brain tumors
Lung conditions: Pneumonia, tuberculosis, asthma, or mechanical ventilation
Tumors: Certain cancers (e.g., small cell lung cancer) can produce ADH themselves (ectopic ADH production)
Medications: Antidepressants (SSRIs), carbamazepine, cyclophosphamide, certain pain relievers, and diuretics
Other causes: Hypothyroidism, adrenal insufficiency, major surgery, or severe emotional stress

Symptoms

The symptoms of SIADH depend largely on how severely and how quickly sodium levels drop:

Mild cases: Headache, nausea, fatigue, loss of appetite, and general malaise
Moderate cases: Confusion, memory problems, difficulty walking, and balance disturbances
Severe cases: Seizures, loss of consciousness, coma – this is a medical emergency

Because sodium is essential for normal brain function, severe hyponatremia can be life-threatening and requires immediate medical attention.

Diagnosis

The diagnosis of SIADH is based on a combination of clinical assessment and laboratory findings:

Blood tests: Sodium level below 135 mmol/l (hyponatremia), low serum osmolality (<275 mOsm/kg)
Urine tests: Elevated urine osmolality (>100 mOsm/kg) and high urinary sodium concentration (>40 mmol/l)
Exclusion of other conditions: Thyroid, adrenal, and kidney function must be normal; no signs of edema or dehydration
Imaging: Brain MRI or chest CT to identify the underlying cause

Treatment

Treatment depends on the severity of hyponatremia and the underlying cause:

General Measures

Fluid restriction: The primary approach for mild to moderate cases is limiting daily fluid intake to 800–1000 ml per day.
Treating the underlying cause: Discontinuing causative medications, treating infections or tumors

Medical Therapy

Saline infusion (NaCl): In severe, symptomatic hyponatremia, hypertonic saline (3%) is administered slowly intravenously. Important: Sodium levels must not be raised too quickly, as this can cause osmotic demyelination syndrome.
Vaptans (e.g., tolvaptan): These medications block the effect of ADH on the kidneys and promote selective water excretion. They are used in chronic SIADH.
Urea: Can be used as an alternative agent to promote water excretion in the urine.

Prognosis

The prognosis of SIADH depends primarily on the underlying cause. When identified early and managed appropriately, hyponatremia is generally treatable. In tumor-related SIADH, the outlook depends on the course of the cancer. Overly rapid correction of sodium levels must be avoided, as it can lead to severe and irreversible neurological damage.

References

Ellison, D. H. & Berl, T. (2007). The syndrome of inappropriate antidiuresis. New England Journal of Medicine, 356(20), 2064–2072.
Spasovski, G. et al. (2014). Clinical practice guideline on diagnosis and treatment of hyponatraemia. European Journal of Endocrinology, 170(3), G1–G47.
Fenske, W. & Allolio, B. (2012). Current state and future perspectives in the diagnosis of diabetes insipidus: a clinical review. Journal of Clinical Endocrinology & Metabolism, 97(10), 3426–3437.

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