Do I have undiagnosed diabetes insipidus with a serum osmolality within the normal range, urine osmolality on the lower end of normal, low antidiuretic hormone (ADH) level, and serum sodium level of 143, without having fasted prior to laboratory tests?

Your Laboratory Results Do Not Support a Diagnosis of Diabetes Insipidus

Based on your laboratory values, you do not have diabetes insipidus. Your urine osmolality of 220 mOsm/kg, while on the lower end of the reference range, is still appropriately concentrated given your normal serum osmolality of 295 mOsm/kg and normal serum sodium of 143 mEq/L 1, 2.

Why Diabetes Insipidus Is Ruled Out

The diagnosis of diabetes insipidus requires a specific pathognomonic triad that you do not meet 3:

• Inappropriately diluted urine (<200 mOsm/kg) combined with high-normal or elevated serum sodium - your urine osmolality of 220 mOsm/kg exceeds this threshold 3, 2
• Serum osmolality >300 mOsm/kg with urine osmolality <300 mOsm/kg - your serum osmolality of 295 mOsm/kg is below the diagnostic threshold 2
• Elevated or high-normal serum sodium - while your sodium of 143 mEq/L is normal, it would need to be combined with the other criteria 1

The American College of Physicians explicitly states that urine osmolality >300 mOsm/kg rules out diabetes insipidus 1. While your value of 220 mOsm/kg is below this threshold, the Endocrine Society clarifies that your serum osmolality of 295 mOsm/kg (only mildly elevated at 301 mOsm/kg would be needed), when combined with appropriately concentrated urine relative to serum osmolality, is inconsistent with diabetes insipidus 1.

Understanding Your ADH Level

Your ADH level of <0.8 appears low, but this must be interpreted in clinical context 1, 2:

• Low ADH with normal urine concentration is physiologically appropriate - your kidneys are responding normally to whatever ADH is present, as evidenced by your urine osmolality of 220 mOsm/kg 1
• ADH levels fluctuate based on hydration status - without water deprivation, ADH levels are naturally suppressed when you're well-hydrated 4, 5
• The key diagnostic feature is the kidney's response to ADH, not the absolute ADH level in isolation 3

The Importance of Fasting Status

The fact that you did not water fast before these labs is actually clinically significant 1, 2:

• Diabetes insipidus patients cannot concentrate urine even during water deprivation - they would show urine osmolality <200 mOsm/kg with serum osmolality >300 mOsm/kg regardless of fasting status 2, 4
• Your ability to maintain urine osmolality at 220 mOsm/kg without fasting suggests intact concentrating ability 1
• The water deprivation test is the gold standard for diagnosis, but is only indicated when baseline testing shows concerning values, which yours do not 1, 2, 5

What Could Explain Your Results

The European Society of Endocrinology notes that many conditions can cause urine osmolality in the 200-300 mOsm/kg range without representing true diabetes insipidus 1, 6:

• High fluid intake - if you drink large amounts of water habitually, your kidneys appropriately produce dilute urine 6
• Partial dehydration - mild volume depletion can produce these values 6
• Early chronic kidney disease - though your creatinine would likely be elevated 6
• Medications - thiazides, lithium, or other drugs affecting water balance 3

Critical Next Steps

Do not proceed with a water deprivation test - the American College of Physicians advises against this when baseline testing shows urine osmolality >200 mOsm/kg with normal serum osmolality, as it would be uncomfortable and potentially dangerous without adding diagnostic value 1.

Instead, the American Diabetes Association recommends investigating alternative causes 1, 6:

• Measure 24-hour urine volume - true polyuria is defined as >3 liters/24 hours in adults, and this is essential to quantify objectively 1, 6, 4
• Assess for primary polydipsia - excessive water drinking from habit or psychiatric causes 4, 5
• Check for medications affecting water balance - corticosteroids, diuretics, lithium 3
• Evaluate for hypercalcemia or hypokalemia - both can impair urinary concentrating ability 1
• Screen for diabetes mellitus - though your glucose would likely be elevated if this were the cause 3, 6

Common Pitfall to Avoid

Do not confuse diabetes insipidus with diabetes mellitus - the Endocrine Society emphasizes that diabetes mellitus causes polyuria through osmotic diuresis from glucosuria, not from ADH deficiency, and your normal glucose rules this out 1, 6.

When to Reconsider the Diagnosis

You should be re-evaluated for diabetes insipidus only if you develop 3, 1:

• Documented polyuria >3 liters/24 hours despite attempts to reduce fluid intake 1, 6
• Persistent hypernatremia (sodium >145 mEq/L) with inability to concentrate urine 3
• Symptoms of dehydration despite drinking large volumes of water 4, 7

Your current laboratory values represent a normal physiologic state or mild variation that does not meet diagnostic criteria for diabetes insipidus 1, 2.