Do You Have Diabetes Insipidus?
Your laboratory findings—serum osmolality of 300 mOsm/kg, urine osmolality of 170 mOsm/kg, serum sodium of 143 mmol/L, combined with polyuria and colorless urine—are highly suggestive of diabetes insipidus and warrant immediate further evaluation. 1
Why Your Results Point to Diabetes Insipidus
Your presentation demonstrates the pathognomonic triad for diabetes insipidus: 1
- Polyuria with inappropriately dilute urine (osmolality <200 mOsm/kg H₂O) 1, 2
- High-normal to elevated serum sodium (143 mmol/L is at the upper limit of normal) 1
- Elevated serum osmolality (300 mOsm/kg) with inability to concentrate urine 1
The specific gravity of 1.006 and colorless, clear urine further confirm that your kidneys are producing large volumes of dilute urine despite your body being in a relatively hyperosmolar state—exactly what happens in diabetes insipidus. 3, 4
Critical Distinction: This Is NOT Diabetes Mellitus
Your normal serum sodium (not glucose) and the absence of glucosuria distinguish this from diabetes mellitus, which causes polyuria through osmotic diuresis from glucose spilling into urine, not from antidiuretic hormone problems. 1 Diabetes mellitus would show fasting glucose ≥126 mg/dL or random glucose ≥200 mg/dL with symptoms, plus high urine osmolality from glucose. 5
Next Steps: Determining Central vs. Nephrogenic Type
You need plasma copeptin measurement as the primary test to distinguish between central diabetes insipidus (vasopressin deficiency) and nephrogenic diabetes insipidus (vasopressin resistance). 5, 1
Interpretation of Copeptin Levels:
- Copeptin >21.4 pmol/L = Nephrogenic diabetes insipidus (kidneys resistant to vasopressin) 5
- Copeptin <21.4 pmol/L = Central diabetes insipidus or primary polydipsia; requires additional testing with hypertonic saline or arginine stimulation 5
Alternative Diagnostic Approach:
If copeptin testing is unavailable, a desmopressin (DDAVP) trial can differentiate: 5, 6
- Response to DDAVP (decreased urine output, increased urine osmolality) = Central diabetes insipidus 5, 6
- No response to DDAVP = Nephrogenic diabetes insipidus 5, 2
Common Pitfalls to Avoid
Do not undergo water deprivation testing without proper medical supervision, as this can be dangerous and lead to severe hypernatremic dehydration, especially if you have true diabetes insipidus. 3, 4 The water deprivation test is only needed when copeptin levels are equivocal or when primary polydipsia cannot be excluded. 5
Urgent Considerations
You must maintain free access to fluids at all times to prevent life-threatening hypernatremic dehydration. 1, 7 Your thirst mechanism is your best guide—drink according to thirst, not prescribed amounts. 5, 1
Warning Signs Requiring Immediate Medical Attention:
- Inability to keep up with fluid losses 1
- Confusion, lethargy, or altered mental status 7
- Serum sodium rising above 145 mmol/L 1
- Signs of severe dehydration 3
What Caused This?
Investigate potential causes based on type: 5, 3
For Central Diabetes Insipidus:
- Recent head trauma or neurosurgery 3
- Pituitary/hypothalamic tumors (craniopharyngioma, germinoma, metastases) 3
- Autoimmune hypophysitis 3
- Genetic causes 5
For Nephrogenic Diabetes Insipidus:
- Lithium use (most common acquired cause) 5, 2
- Chronic kidney disease 5
- Hypercalcemia or hypokalemia 5
- Genetic mutations in AVPR2 or AQP2 genes 5
Required Workup
Obtain the following tests immediately: 1, 7
- Plasma copeptin level 5, 1
- Repeat serum sodium and osmolality 1, 7
- 24-hour urine volume measurement 1
- Serum creatinine and electrolytes 1
- Pituitary MRI with dedicated sella sequences (if central diabetes insipidus suspected) 1, 3
Genetic testing with a multigene panel including AVPR2, AQP2, and AVP genes is recommended if nephrogenic diabetes insipidus is confirmed, even in adults. 5
Treatment Will Depend on Final Diagnosis
If Central Diabetes Insipidus:
Desmopressin is the treatment of choice, administered intranasally, orally, or by injection. 1, 6, 7 Starting dose is typically 2-4 mcg subcutaneously or intravenously in divided doses. 7
If Nephrogenic Diabetes Insipidus:
Combination therapy with thiazide diuretics and NSAIDs (prostaglandin synthesis inhibitors), plus dietary modifications including low-salt diet (≤6 g/day) and protein restriction (<1 g/kg/day). 1 Desmopressin is ineffective for nephrogenic diabetes insipidus. 7
Critical Monitoring:
Serum sodium must be checked within 7 days and at 1 month after starting any treatment, then periodically, as hyponatremia is the main complication of desmopressin therapy. 7