What is consistent with Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

Increased Urine Sodium is Consistent with SIADH

Increased urine sodium (option B) is consistent with syndrome of inappropriate antidiuretic hormone (SIADH). This is a key diagnostic feature of SIADH, with urinary sodium typically exceeding 40 mEq/L despite hyponatremia 1.

Diagnostic Criteria for SIADH

SIADH is characterized by the following features:

• Hyponatremia (serum sodium < 134 mEq/L)
• Hypoosmolality (plasma osmolality < 275 mOsm/kg)
• Inappropriately high urine osmolality (> 500 mOsm/kg)
• Inappropriately high urinary sodium concentration (> 20-40 mEq/L)
• Clinical euvolemia
• Absence of hypothyroidism, adrenal insufficiency, or volume depletion 1, 2

Why Other Options Are Incorrect

A. Hypovolemia

Hypovolemia is inconsistent with SIADH. Patients with SIADH are typically euvolemic despite increased total body water 2, 3. The pathophysiology involves persistent ADH secretion causing water retention without significant changes in sodium balance, resulting in dilutional hyponatremia with preserved volume status 4.

C. Hypernatremia

Hypernatremia directly contradicts the defining feature of SIADH, which is hypotonic hyponatremia. SIADH is characterized by excessive water retention due to inappropriate ADH secretion, leading to dilution of serum sodium 1, 4.

D. Plasma Hyperosmolality

Plasma hyperosmolality is inconsistent with SIADH. The syndrome is defined by plasma hypo-osmolality (< 275 mOsm/kg) resulting from excessive water retention 1, 2. This hypo-osmolality is a cardinal diagnostic criterion for SIADH.

Clinical Significance of Increased Urine Sodium in SIADH

The elevated urine sodium in SIADH occurs because:

1. Persistent ADH action promotes water reabsorption in the collecting ducts
2. The resulting volume expansion leads to decreased proximal tubular sodium reabsorption
3. This causes increased sodium delivery to the distal nephron
4. Despite the hyponatremia, the kidneys continue to excrete sodium in the urine

This paradoxical natriuresis in the face of hyponatremia is a hallmark of SIADH that distinguishes it from other hyponatremic states like hypovolemic hyponatremia, where urine sodium is typically low due to appropriate sodium conservation 4, 3.

Common Pitfalls in SIADH Diagnosis

• Failing to distinguish between hypovolemic and euvolemic hyponatremia
• Not measuring urine sodium and osmolality simultaneously with serum values
• Overlooking medications that can cause SIADH (e.g., certain antidepressants, antipsychotics, antineoplastics)
• Not excluding other causes of euvolemic hyponatremia (hypothyroidism, adrenal insufficiency)

The diagnosis of SIADH requires a systematic approach to exclude other causes of hyponatremia while confirming the presence of inappropriate antidiuresis with elevated urine sodium despite hyponatremia 1, 2.