What Hormone Causes Hyponatremia
Antidiuretic hormone (ADH), also called vasopressin, is the primary hormone that causes hyponatremia when secreted inappropriately or in excess. 1, 2
Mechanism of ADH-Induced Hyponatremia
ADH causes hyponatremia through the following mechanism:
- ADH promotes water retention by increasing water reabsorption in the kidney collecting ducts, leading to dilution of serum sodium 2, 3
- Persistent or elevated plasma ADH despite hyponatremia and low plasma osmolality results in continued water retention and physiologic natriuresis, where fluid balance is maintained at the expense of plasma sodium 2
- Impaired free water excretion occurs because the kidneys cannot properly dilute urine when ADH activity is inappropriately high 1, 3
Syndrome of Inappropriate ADH (SIADH)
SIADH is the most commonly recognized cause of hyponatremia among hospitalized patients and represents the classic example of ADH-induced hyponatremia 3:
Diagnostic Criteria for SIADH
- Hyponatremia (serum sodium <134-135 mEq/L) 2
- Hypoosmolality (plasma osmolality <275 mOsm/kg) 2
- Inappropriately high urine osmolality (>500 mOsm/kg) despite low serum osmolality 2
- Inappropriately high urinary sodium (>20-40 mEq/L) 2
- Euvolemic state (absence of clinical signs of hypovolemia or hypervolemia) 2
- Normal thyroid, adrenal, and renal function 2
Common Causes of SIADH
- Malignancies, particularly small cell lung cancer, which commonly produces vasopressin 2
- CNS disorders including subarachnoid hemorrhage, meningitis, and brain tumors 2, 3
- Pulmonary diseases such as pneumonia and tuberculosis 2, 3
- Medications including SSRIs, carbamazepine, cyclophosphamide, vincristine, and NSAIDs 2
- Postoperative states and conditions causing pain, nausea, and stress, which are nonosmotic stimuli for ADH release 1, 3
Other Hormonal Causes
While ADH is the primary hormone causing hyponatremia, other hormonal deficiencies can contribute:
- Hypothyroidism (low thyroid hormone) can impair free water excretion 1
- Adrenal insufficiency (low cortisol) can lead to hyponatremia 1
- Hypoaldosteronism can contribute to sodium wasting 1
However, these must be excluded to diagnose SIADH, as SIADH requires normal thyroid and adrenal function 2.
Clinical Significance
- SIADH affects 1-5% of lung cancer patients, particularly those with small cell lung cancer 1
- Hospital-acquired hyponatremia from hypotonic IV fluids in the setting of elevated ADH affects 15-30% of hospitalized patients 1
- Nonosmotic stimuli including pain, nausea, stress, and acute illness states like pancreatitis lead to ADH excess, placing patients at risk when electrolyte-free water is supplied 1