Does SIADH Cause Frequent Urination?
No, SIADH does not cause frequent urination—in fact, it causes the opposite: water retention with inappropriately concentrated urine despite low serum sodium. 1
Pathophysiology of Urinary Output in SIADH
SIADH is characterized by persistent or elevated plasma arginine vasopressin (AVP) despite hyponatremia and low plasma osmolality, leading to water retention. 1 The kidneys cannot excrete free water appropriately, resulting in:
- Inappropriately concentrated urine (>500 mosm/kg) despite plasma hypoosmolality 1, 2
- Elevated urinary sodium concentration (>20-40 mEq/L) due to physiologic natriuresis that occurs to maintain fluid balance at the expense of plasma sodium 1, 2
- Reduced urine volume rather than increased frequency, as the body retains water 1
Key Diagnostic Features
The concentrated urine with high sodium content in SIADH results from the kidney's inability to dilute urine appropriately. A physiologic natriuresis occurs where fluid balance is maintained at the expense of plasma sodium—not through increased urination. 1
Diagnostic criteria include: 1, 2
- Hyponatremia (serum sodium <134 mEq/L)
- Hypoosmolality (plasma osmolality <275 mosm/kg)
- Urine osmolality >500 mosm/kg (inappropriately concentrated)
- Urinary sodium >20 mEq/L
- Euvolemic state (no edema, no orthostatic hypotension, normal skin turgor, moist mucous membranes) 3
Common Pitfall to Avoid
Do not confuse SIADH with cerebral salt wasting (CSW) or diabetes insipidus. 1, 3 CSW causes true hypovolemia with excessive urinary losses and may present with increased urination, but it requires volume replacement—the opposite treatment of SIADH. 1, 3 Diabetes insipidus causes polyuria with dilute urine, which is the complete opposite of SIADH's concentrated urine. 1
In SIADH, patients retain water and produce small volumes of concentrated urine—they do not experience frequent urination. 1, 4, 5