Diagnosis: Partial Central Diabetes Insipidus
This patient has partial central diabetes insipidus (CDI), evidenced by the combination of low ADH (<0.8 pg/mL), inappropriately dilute urine (osmolality 220 mOsm/kg) relative to high-normal serum sodium (143 mmol/L) and serum osmolality (295 mOsm/kg), though the 24-hour urine volume of 1.3L is below the typical diagnostic threshold. 1, 2
Diagnostic Interpretation
Why This is Central DI (Not Nephrogenic)
The critically low ADH level (<0.8 pg/mL) confirms deficient ADH production, which is pathognomonic for central DI. 3 In nephrogenic DI, ADH levels would be normal or elevated despite the kidneys' inability to respond. 3
The urine osmolality of 220 mOsm/kg is inappropriately dilute given the serum osmolality of 295 mOsm/kg (above mid-normal range), confirming inability to concentrate urine. 1, 2
Serum sodium of 143 mmol/L (high-normal) with serum osmolality of 295 mOsm/kg indicates the patient is maintaining adequate hydration through compensatory fluid intake driven by intact thirst mechanisms. 1
Why This is "Partial" DI
The 24-hour urine volume of 1.3L is substantially below the 3L/24h threshold typically required for DI diagnosis in adults. 1, 4 This indicates partial ADH deficiency rather than complete absence.
Urine osmolality of 220 mOsm/kg, while inappropriately low, is not severely dilute (severe DI shows <200 mOsm/kg). 1, 4 This suggests residual ADH activity.
The patient likely has compensated partial CDI where intact thirst drive and adequate water access prevent more severe polyuria and hypernatremia. 1
Required Diagnostic Workup
Immediate Laboratory Testing
Plasma copeptin measurement is now the preferred first-line test to definitively distinguish central from nephrogenic DI. 1, 2 Copeptin <21.4 pmol/L confirms central DI, while >21.4 pmol/L indicates nephrogenic DI. 1
If copeptin is unavailable, proceed with a water deprivation test followed by desmopressin administration, which remains the gold standard. 1, 5 Response to desmopressin (increased urine osmolality) confirms central DI; lack of response indicates nephrogenic DI. 3
Imaging and Etiological Investigation
Obtain MRI of the sella with dedicated pituitary sequences immediately. 1, 2 Approximately 50% of central DI cases have identifiable structural causes including tumors, infiltrative diseases, or inflammatory processes. 1
Look for absence of the normal posterior pituitary "bright spot" on T1-weighted imaging, which supports central DI diagnosis. 4
In patients with acquired central DI of sudden onset: suspect craniopharyngioma or germinoma if age <30 years; suspect metastasis if age >50 years. 4
Additional Baseline Testing
Measure complete metabolic panel including serum creatinine to assess for chronic kidney disease (approximately 50% of adult DI patients have CKD stage ≥2). 1
Assess other pituitary hormone axes to identify additional pituitary deficiencies. 1
Treatment Approach
First-Line Pharmacologic Treatment
Desmopressin is the treatment of choice for central diabetes insipidus. 1, 2, 6, 7
Starting dosage: 2-4 mcg daily administered as one or two divided doses by subcutaneous or intravenous injection. 6 For partial DI, start at the lower end (2 mcg daily).
Alternative routes include intranasal (10 mcg per spray) or oral formulations, though injectable forms provide more precise dosing. 6, 7
Adjust morning and evening doses separately to achieve adequate diurnal rhythm of water turnover and ensure adequate sleep duration without excessive water retention. 6
Critical Monitoring Requirements
Check serum sodium within 7 days of starting desmopressin, then at 1 month, then periodically thereafter. 1 Hyponatremia is the main complication of desmopressin therapy.
Initiate fluid restriction during desmopressin treatment to prevent water intoxication and hyponatremia. 6 This is the most critical safety measure.
Monitor urine volume, urine osmolality, and body weight regularly to assess treatment response. 2
Fluid Management Principles
Patients with DI must have free access to water 24/7 to prevent life-threatening hypernatremic dehydration. 1, 2 Never restrict water access before treatment is established.
For patients capable of self-regulation, fluid intake should be determined by thirst sensation rather than prescribed amounts, as osmosensors are more sensitive than medical calculations. 1
Once desmopressin therapy is initiated and titrated, implement appropriate fluid restriction to prevent hyponatremia. 6
Long-Term Monitoring
Routine Follow-Up Schedule
Adults require annual clinical follow-up including weight measurements and annual blood tests (sodium, potassium, chloride, bicarbonate, creatinine, uric acid). 1
Urinalysis including osmolality, protein-creatinine ratio, and 24-hour urine volume should be performed annually. 1
Renal ultrasound every 2 years to monitor for urinary tract dilation and bladder dysfunction from chronic polyuria. 1 Approximately 46% of DI patients develop urological complications. 1
Multidisciplinary Management
- Patients should be managed by a multidisciplinary team including nephrologist, endocrinologist, dietitian, and urologist as needed. 1
Common Pitfalls to Avoid
Do not dismiss this case because urine volume is "only" 1.3L/24h. The combination of low ADH, inappropriately dilute urine, and high-normal serum sodium/osmolality confirms the diagnosis of partial CDI regardless of absolute urine volume. 1, 8
Never use normal saline (0.9% NaCl) or electrolyte solutions like Pedialyte for rehydration in DI patients. 1 Use 5% dextrose in water (hypotonic fluid) if IV rehydration is needed. 1
Do not confuse DI with diabetes mellitus—they are completely unrelated conditions. 1, 2 DI involves water balance and ADH; diabetes mellitus involves glucose metabolism and insulin.
Ensure serum sodium is normal before initiating or resuming desmopressin treatment. 6 Starting treatment in the setting of hyponatremia can cause severe complications.
Watch for decreased responsiveness or shortened duration of effect after 6+ months of intranasal desmopressin, which may require switching to injectable formulations or dose adjustment. 7