SIADH and Polyuria
No, a patient with SIADH would not have polyuria; instead, they typically present with concentrated urine and reduced urine output due to inappropriate water retention. 1, 2
Pathophysiology of Urine Output in SIADH
SIADH is fundamentally a disorder of water retention caused by persistent or elevated plasma arginine vasopressin (AVP) despite hyponatremia and low plasma osmolality. 3 This leads to:
- Impaired water excretion with retention of free water, which is the hallmark of the condition 4, 5
- Inappropriately concentrated urine with osmolality typically >500 mOsm/kg (often >300 mOsm/kg minimum), despite low serum osmolality 1, 2, 6
- Reduced urine volume rather than increased volume, as the kidneys cannot properly dilute and excrete water 3, 7
Clinical Presentation of Urine Characteristics
The urine in SIADH demonstrates specific characteristics that are opposite to polyuria:
- High urine osmolality (>500 mOsm/kg) indicating concentrated, not dilute, urine 2, 6
- Elevated urinary sodium (>20-40 mEq/L) due to compensatory natriuresis, but this occurs in the context of reduced total urine volume 1, 6
- Euvolemic state clinically, meaning no signs of volume depletion or overload 2, 6
The concentrated urine results from the kidney's inability to excrete free water due to persistent ADH activity, leading to water retention and subsequent physiologic natriuresis where fluid balance is maintained at the expense of plasma sodium. 2, 3
Key Diagnostic Distinction
A critical pitfall is confusing SIADH with conditions that do cause polyuria, such as:
- Diabetes insipidus (central or nephrogenic), which presents with dilute urine (osmolality <100 mOsm/kg) and large urine volumes 1
- Cerebral salt wasting (CSW), which can present with high urine sodium but is characterized by true hypovolemia and volume depletion, not the euvolemic state of SIADH 1, 2, 6
The diagnosis of SIADH requires demonstration of high urine osmolality with low plasma osmolality in the absence of diuretic use, which is fundamentally incompatible with polyuria. 4, 8