In a patient with normal serum sodium and low non‑fasting urine osmolality, what is the most likely diagnosis and what are the recommended next steps?

Normal Serum Sodium with Low Non-Fasting Urine Osmolality

The most likely diagnosis is primary polydipsia (psychogenic or habitual excessive water intake), and the next step is to measure paired serum and urine osmolality under conditions of ad libitum fluid intake, followed by a water deprivation test if the diagnosis remains unclear 1.

Diagnostic Reasoning

When serum sodium is normal but urine osmolality is inappropriately low (typically <100 mOsm/kg), this indicates complete suppression of antidiuretic hormone (ADH/vasopressin) secretion 2. This physiologic response occurs when:

• Primary polydipsia: Excessive water intake dilutes the urine, but the kidneys appropriately suppress ADH to maintain normal serum sodium
• Early or partial diabetes insipidus: Though this typically presents with hypernatremia or high-normal sodium

Algorithmic Approach to Diagnosis

Step 1: Confirm the Laboratory Pattern

• Verify serum sodium is truly normal (135-145 mmol/L)
• Confirm urine osmolality is <100 mOsm/kg (indicating maximal ADH suppression) 2
• Measure serum osmolality to ensure it's normal (280-295 mOsm/kg)

Step 2: Clinical Assessment

Look for specific features:

• Volume status: Assess for signs of hypovolemia (orthostatic hypotension, dry mucous membranes) vs euvolemia
• Polyuria/polydipsia history: Quantify fluid intake and urine output (>3L/day suggests pathologic polyuria) 3
• Psychiatric history: Primary polydipsia is common in psychiatric patients
• Medication review: Diuretics, lithium, or other nephrotoxic agents

Step 3: Water Deprivation Test (if diagnosis unclear)

If basal plasma osmolality and sodium fall within normal range, perform a standard dehydration test 1:

1. Monitor: Plasma osmolality, serum sodium, urine osmolality, and body weight hourly
2. Endpoint: Stop when urine osmolality plateaus or plasma osmolality exceeds 295 mOsm/kg
3. Interpretation:
   • Primary polydipsia: Urine osmolality will increase appropriately (>600 mOsm/kg) with dehydration
   • Neurogenic DI: Urine remains dilute despite dehydration; responds to DDAVP (>50% increase in urine osmolality) 1
   • Nephrogenic DI: Urine remains dilute; minimal response to DDAVP (<50% increase) 3, 1

Step 4: Additional Testing if Diabetes Insipidus Suspected

• Plasma vasopressin levels: Measured after dehydration to differentiate neurogenic from nephrogenic DI 1
• Genetic testing: If nephrogenic DI suspected, test AVPR2 and AQP2 genes 3
• Brain MRI: If neurogenic DI suspected, evaluate for pituitary/hypothalamic pathology

Critical Pitfalls to Avoid

Do not assume primary polydipsia without formal testing - partial DI can present similarly with compensatory increased water intake maintaining normal sodium levels 1.

Avoid salt-containing IV fluids in suspected DI - if the patient later proves to have nephrogenic DI, isotonic saline (300 mOsm/kg) will worsen hypernatremia since typical NDI urine osmolality is only ~100 mOsm/kg 3.

Monitor for psychiatric comorbidities - primary polydipsia often coexists with psychiatric conditions and may require behavioral interventions alongside medical management.

When to Suspect Diabetes Insipidus Despite Normal Sodium

Consider DI if the patient has 3:

• Polyuria (>3L/day in adults, >2L/m²/day in children)
• Polydipsia with preference for cold water
• Nocturia
• History of failure to thrive (in children)
• Urine osmolality persistently <200 mOsm/kg despite varying hydration states

The normal serum sodium in your scenario suggests either adequate compensatory water intake (masked DI) or true primary polydipsia with appropriate ADH suppression.