Distinguishing Symptoms: Diabetes Insipidus versus SIADH
Diabetes insipidus and SIADH present with opposite clinical pictures: DI causes polyuria, polydipsia, and hypernatremia with dilute urine, while SIADH causes hyponatremia with inappropriately concentrated urine and euvolemia. 1
Core Diagnostic Features
Diabetes Insipidus Presentation
- Polyuria: Urine output >3 liters per 24 hours in adults, with passage of large volumes of dilute urine 2, 3
- Polydipsia: Excessive thirst driving patients to drink large volumes of fluid to compensate for urinary water losses 2
- Hypernatremia: Serum sodium typically >145 mEq/L with serum osmolality usually >300 mOsm/kg H₂O due to free water loss 1
- Dilute urine: Urine osmolality <200 mOsm/kg H₂O despite elevated serum osmolality—this is pathognomonic when combined with hypernatremia 2, 3, 4
- Volume depletion signs: Orthostatic hypotension, dry mucous membranes, decreased skin turgor 1
- Additional symptoms in children: Failure to thrive, hypernatremic dehydration, and in infants, inability to express thirst clearly 2
SIADH Presentation
- Hyponatremia: Serum sodium <134-135 mEq/L with plasma hypoosmolality <275 mOsm/kg 1, 5
- Inappropriately concentrated urine: Urine osmolality >500 mOsm/kg despite low serum sodium and osmolality 1, 6
- Elevated urinary sodium: Urine sodium >30 mEq/L with fractional excretion of sodium >0.5% in 70% of cases, despite hyponatremia 6
- Euvolemia to mild hypervolemia: No peripheral edema, no signs of volume depletion or overload 1, 5
- Neurological symptoms: Nausea, vomiting, confusion, seizures, altered mental status (severity correlates with absolute sodium level and rate of fall, particularly if >0.5 mmol/L/h) 1, 5
- Low serum urea and uric acid: Urea typically low (less specific in elderly), uric acid low in 70% of SIADH cases versus 40% in salt-depleted patients 6
Critical Distinguishing Laboratory Values
The Diagnostic Triad for DI
- Polyuria (>3 L/24h in adults) 3
- Urine osmolality <200 mOsm/kg 3
- High-normal or elevated serum sodium 3
The Cardinal Criteria for SIADH
- Hypotonic hyponatremia 5
- Natriuresis (urine sodium >30 mEq/L) 6
- Urine osmolality exceeding plasma osmolality 5
- Absence of edema and volume depletion 5
- Normal renal and adrenal function 5
Special Considerations in Impaired Renal Function
In patients with impaired renal function, DI diagnosis becomes more complex because chronic kidney disease can cause urine osmolality in the 200-300 mOsm/kg range without representing true DI. 2 The diagnosis requires urine osmolality definitively <200 mOsm/kg in the setting of serum hyperosmolality 2.
For elderly patients with renal impairment:
- The renal threshold for glycosuria increases with age, making polyuria less apparent 7
- Thirst mechanisms are more likely impaired, so elderly DI patients may present with weight loss, fatigue, and confusion rather than classic polydipsia 7
- Approximately 50% of adult DI patients have CKD stage ≥2, requiring more frequent monitoring 2
- Desmopressin is substantially excreted by the kidney, increasing risk of toxic reactions in impaired renal function 8
Treatment Approaches
Diabetes Insipidus Management
Desmopressin (DDAVP) is the treatment of choice for central DI, administered intranasally, orally, or by injection with starting dose typically 2-4 mcg subcutaneously or intravenously in divided doses 2, 9. For nephrogenic DI, combination therapy with thiazide diuretics plus NSAIDs, along with dietary modifications (low-salt diet ≤6 g/day and protein restriction <1 g/kg/day) is recommended 2, 3.
Critical universal principle: All DI patients must have free access to fluid 24/7—water restriction is a life-threatening error leading to severe hypernatremic dehydration 2, 3. Patients capable of self-regulation should determine fluid intake based on thirst sensation rather than prescribed amounts 2, 3.
SIADH Management
Fluid restriction remains the mainstay of therapy for chronic SIADH, with hypertonic saline reserved only for severely symptomatic patients 10. V2 receptor antagonists are candidates for patients with high urine osmolality (>600 mOsm/kg), while those with low urine osmolality benefit from water restriction or urea 6.
Monitoring Requirements
For DI Patients on Desmopressin
Serum sodium must be checked within 7 days and at 1 month after starting treatment, then periodically, as hyponatremia is the main complication of desmopressin therapy 2, 9. More frequent monitoring is required for patients ≥65 years and those at increased risk of hyponatremia 9.
Common Pitfall
Never confuse these conditions: SIADH presents with hyponatremia, low serum osmolality, and inappropriately high urine osmolality, while DI presents with hypernatremia, high serum osmolality, and inappropriately low urine osmolality 2. The volume status is also opposite: SIADH patients are euvolemic to mildly hypervolemic without edema, while DI patients show signs of volume depletion 1.