Syndrome of Inappropriate Antidiuretic Hormone (SIADH): Diagnostic Features
Increased urine sodium is consistent with syndrome of inappropriate antidiuretic hormone (SIADH).
Diagnostic Criteria for SIADH
SIADH is characterized by the following key features:
- Hyponatremia (serum sodium <134 mEq/L)
- Plasma hypoosmolality (<275 mOsm/kg), not hyperosmolality
- Inappropriately high urine osmolality (>500 mOsm/kg)
- Inappropriately high urinary sodium concentration (>20 mEq/L) 1
- Clinical euvolemia, not hypovolemia
- Normal adrenal and thyroid function
Distinguishing SIADH from Other Causes of Hyponatremia
When evaluating the options provided in the question:
Hypovolemia: Inconsistent with SIADH. Patients with SIADH are clinically euvolemic, not hypovolemic. Hypovolemia is characteristic of Cerebral Salt Wasting (CSW), which is an important differential diagnosis for SIADH 1.
Increased urine sodium: Consistent with SIADH. Patients with SIADH have inappropriately high urinary sodium concentration (>20 mEq/L) despite hyponatremia 1.
Hypernatremia: Inconsistent with SIADH. SIADH is characterized by hyponatremia (serum sodium <134 mEq/L), not hypernatremia 1.
Plasma hyperosmolarity: Inconsistent with SIADH. SIADH is characterized by plasma hypoosmolality (<275 mOsm/kg), not hyperosmolarity 1.
Excessive diuresis: Inconsistent with SIADH. Patients with SIADH typically have normal or low urine output due to water retention caused by inappropriate ADH secretion 1.
Pathophysiology of SIADH
In SIADH, there is persistent detectable or elevated plasma arginine vasopressin (AVP) concentrations despite hypoosmolality. This leads to:
- Water retention via stimulation of V2 receptors in the kidney 2
- Decreased free water clearance
- Dilutional hyponatremia
- Increased urinary sodium excretion (natriuresis)
Common Causes of SIADH
- Malignancies (especially small cell lung cancer)
- CNS disorders (stroke, hemorrhage, trauma, infection)
- Pulmonary diseases (pneumonia, tuberculosis, asthma, COPD)
- Medications (antidepressants, antipsychotics, anticonvulsants, diuretics)
- Post-surgical states 1
Clinical Implications
The key to diagnosing SIADH is recognizing the paradoxical combination of:
- Hyponatremia
- Plasma hypoosmolality
- Inappropriately concentrated urine (high urine osmolality)
- Elevated urine sodium
- Clinical euvolemia
This pattern distinguishes SIADH from other causes of hyponatremia such as hypovolemia, where urine sodium would typically be low (<20 mEq/L) as the kidneys attempt to conserve sodium.
Therefore, among the options presented, increased urine sodium is the feature consistent with SIADH.