Do I have undiagnosed diabetes insipidus with serum sodium at hypernatremia level, elevated urine osmolality, and low antidiuretic hormone (ADH) levels?

Do You Have Undiagnosed Diabetes Insipidus?

Based on your laboratory values, you do NOT have diabetes insipidus. Your results actually demonstrate normal kidney concentrating ability and appropriate ADH function, which directly contradicts the diagnosis of diabetes insipidus.

Why Your Labs Rule Out Diabetes Insipidus

Your Fasting Labs Show Normal Kidney Function

Your 12-hour fasting results demonstrate exactly what should happen in a healthy person:

• Urine osmolality of 498 mOsm/kg is appropriately concentrated - This is well above the <200 mOsm/kg threshold required for diabetes insipidus diagnosis 1, 2
• Serum sodium of 143 mEq/L is normal - Not elevated as would be expected with untreated DI 1
• Copeptin of 4.6 pmol/L indicates normal ADH response - This level shows your body is producing and responding to ADH appropriately 1, 3
• Serum osmolality of 301 mOsm/kg with high urine osmolality - This combination proves your kidneys can concentrate urine normally 1, 2

Your Non-Fasting Labs Also Appear Normal

Your baseline measurements without water restriction show:

• Urine osmolality of 220 mOsm/kg is in the indeterminate range - This falls between 200-300 mOsm/kg, which can occur with many normal conditions including partial hydration states, and does not definitively establish DI 2
• ADH <0.8 pg/mL with normal serum sodium - Low ADH is expected when you're well-hydrated and don't need to concentrate urine 1
• Serum osmolality of 295 mOsm/kg is normal - This indicates appropriate hydration 1

What Diabetes Insipidus Actually Looks Like

The diagnostic criteria for DI require ALL of the following simultaneously 1, 2:

• Urine osmolality definitively <200 mOsm/kg (yours was 498 when fasting)
• Serum osmolality ≥300 mOsm/kg (yours was 301, but your urine was appropriately concentrated)
• High-normal or elevated serum sodium (yours was normal at 143)
• Polyuria >3 liters per 24 hours in adults (not mentioned in your case)

Your fasting test essentially performed a water deprivation test, and you passed it. When you restricted fluids overnight, your kidneys appropriately concentrated your urine to 498 mOsm/kg, which is exactly what healthy kidneys should do 1, 3.

Critical Distinction: ADH Levels and Hydration Status

The low ADH level (<0.8) in your non-fasting state is physiologically appropriate 1. When you're well-hydrated with normal serum osmolality (295), your body doesn't need to produce ADH to retain water. This is normal physiology, not diabetes insipidus.

In true diabetes insipidus, you would see:

• Central DI: Low ADH with inability to concentrate urine even when dehydrated 1, 4
• Nephrogenic DI: High ADH (or copeptin >21.4 pmol/L) but kidneys cannot respond, resulting in dilute urine 1, 5

Your copeptin of 4.6 pmol/L during fasting, combined with concentrated urine, proves you have neither form 1, 3.

What Your Results Actually Mean

Your laboratory values demonstrate:

• Normal kidney concentrating ability - Proven by urine osmolality of 498 during fasting 1, 2
• Intact thirst and ADH mechanisms - Your body appropriately regulates water balance 1
• Normal response to dehydration - Copeptin rose appropriately and kidneys concentrated urine 1, 3

If You're Still Concerned About Symptoms

If you're experiencing excessive thirst or urination, other conditions to consider include 6, 1:

• Diabetes mellitus - Check fasting glucose and HbA1c to rule this out first
• Primary polydipsia - Excessive water drinking from habit or psychological factors
• Chronic kidney disease - Though your results don't suggest this
• Medications - Certain drugs can affect urine concentration

The definitive test would be a formal water deprivation test followed by desmopressin administration, but your informal overnight fast already demonstrated normal kidney concentrating ability 1, 3.

Bottom Line

Your kidneys concentrated urine to 498 mOsm/kg when you fasted - this is incompatible with diabetes insipidus of any type 1, 2. Patients with DI cannot concentrate urine above 200 mOsm/kg even when severely dehydrated 1, 2, 3. Your results show the opposite: excellent concentrating ability when needed.