Could I have undiagnosed Diabetes Insipidus (DI) with a normal serum sodium level and symptoms of polyuria?

Can You Have Undiagnosed Diabetes Insipidus with Normal Serum Sodium?

Yes, you can absolutely have undiagnosed diabetes insipidus (DI) with a serum sodium of 141 mEq/L, as patients with DI commonly maintain normal serum sodium when they have free access to water and an intact thirst mechanism that drives them to drink enough to compensate for urinary losses. 1

Why Normal Sodium Doesn't Rule Out DI

• Patients with DI typically present with normal serum sodium at steady state when they can drink freely, precisely because their intact thirst mechanism drives adequate fluid replacement to compensate for massive urinary water losses 1
• The osmosensors that trigger thirst sensation in DI patients are typically more sensitive and accurate than any medical calculation, driving them to drink large volumes of fluid (often several liters daily) to maintain normal sodium levels 1
• The pathognomonic triad for DI is polyuria + polydipsia + inappropriately dilute urine (osmolality <200 mOsm/kg H₂O) combined with high-normal or elevated serum sodium—your sodium of 141 mEq/L qualifies as high-normal 1, 2

What You Need to Do Next

The diagnosis requires simultaneous measurement of serum sodium, serum osmolality, urine osmolality, and 24-hour urine volume to establish whether you truly have DI 1, 3

Specific diagnostic steps:

• Collect a proper 24-hour urine sample while maintaining your usual fluid intake based on thirst (do not artificially restrict or increase fluids, and avoid electrolyte-containing solutions like Pedialyte during collection) 1
• Measure urine osmolality from this collection—if it's <200 mOsm/kg H₂O in the presence of your high-normal sodium, this confirms DI 1, 2
• If urine osmolality falls between 200-300 mOsm/kg, you'll need a water deprivation test followed by desmopressin administration, which remains the gold standard for diagnosis 1
• Plasma copeptin measurement can serve as the primary differentiating test to distinguish between central DI (low copeptin <21.4 pmol/L) and nephrogenic DI (high copeptin >21.4 pmol/L) 1, 3

Critical Distinction from Diabetes Mellitus

First, ensure your blood glucose is normal to exclude diabetes mellitus, as the two conditions are completely unrelated despite sharing the word "diabetes" 4, 2

• Diabetes mellitus causes polyuria through osmotic diuresis from glucose spilling into urine, whereas DI causes polyuria from inability to concentrate urine due to ADH deficiency or resistance 1, 2
• Diabetes mellitus would show fasting glucose ≥126 mg/dL or random glucose ≥200 mg/dL with symptoms, plus high urine osmolality from glucose 4, 1

Common Pitfall to Avoid

Never restrict water access while investigating DI—this is a life-threatening error that leads to severe hypernatremic dehydration, as your body cannot concentrate urine and will rapidly become dangerously dehydrated 1, 3. Continue drinking to thirst throughout the diagnostic workup.

If DI Is Confirmed

• For central DI: Desmopressin is the treatment of choice, administered intranasally, orally, or by injection, with serum sodium checked within 7 days and at 1 month after starting treatment 1, 5
• For nephrogenic DI: Combination therapy with thiazide diuretics plus NSAIDs, along with low-salt diet (≤6 g/day) and protein restriction (<1 g/kg/day), can reduce urine output by up to 50% 1, 3
• MRI of the sella with dedicated pituitary sequences is recommended if central DI is suspected, as approximately 50% of cases have identifiable structural causes including tumors or infiltrative diseases 1, 3