Do I have undiagnosed diabetes insipidus given my normal serum osmolality, normal serum sodium (Na+) levels, and variable urine osmolality results after a 12-hour water and food fast?

You Do Not Have Diabetes Insipidus

Based on your laboratory results, you definitively do not have diabetes insipidus—your ability to concentrate urine to 498 mOsm/kg after a 12-hour fast, combined with a copeptin level of 4.6 pmol/L and normal serum sodium, demonstrates completely normal ADH secretion and kidney function. 1

Why Your Results Rule Out Diabetes Insipidus

Your Fasting Results Are Normal

• Urine osmolality of 498 mOsm/kg is excellent concentration ability—diabetes insipidus is characterized by inability to concentrate urine above 200 mOsm/kg despite dehydration 1, 2
• Your serum sodium of 143 mEq/L is perfectly normal (not elevated), whereas diabetes insipidus typically presents with high-normal or elevated sodium (>145 mEq/L) when water access is restricted 1
• Your serum osmolality of 301 mOsm/kg is normal and not elevated 1
• Your copeptin level of 4.6 pmol/L indicates a normally functioning ADH system—this is less than one-quarter of the 21.4 pmol/L threshold used to diagnose nephrogenic diabetes insipidus 1

The Diagnostic Triad Is Absent

The pathognomonic triad of diabetes insipidus requires all three features simultaneously 1, 2:

1. Polyuria (>3 liters per 24 hours in adults)
2. Inappropriately dilute urine (osmolality <200 mOsm/kg)
3. High-normal or elevated serum sodium (especially >145 mEq/L with restricted water access)

You have none of these features—your urine is appropriately concentrated at 498 mOsm/kg, and your sodium is normal at 143 mEq/L 1.

Understanding Your Non-Fasting Results

Why Your Urine Was Dilute at 220 mOsm/kg

• When you're drinking fluids normally throughout the day, your kidneys appropriately dilute urine to excrete excess water—this is normal physiology, not disease 1
• The ADH level of <0.8 pg/mL during normal hydration simply reflects that your body correctly suppressed ADH when you were well-hydrated (no need to conserve water) 1
• The key distinction: In diabetes insipidus, urine remains dilute (<200 mOsm/kg) even during dehydration or fasting, which is not your case 1, 2

Your Fasting Test Proved Normal Function

• After 12 hours without water, your body appropriately secreted ADH (copeptin 4.6 pmol/L) and your kidneys responded correctly by concentrating urine to 498 mOsm/kg 1
• This demonstrates both intact central ADH production and intact kidney response to ADH 1, 3

What Diabetes Insipidus Actually Looks Like

Diagnostic Criteria You Don't Meet

• Baseline plasma copeptin >21.4 pmol/L is diagnostic for nephrogenic diabetes insipidus—yours is 4.6 pmol/L 1
• Inability to concentrate urine above 200 mOsm/kg despite dehydration—you concentrated to 498 mOsm/kg 1, 2
• Persistent polyuria with dilute urine even during water deprivation—not present in your case 2, 3

Clinical Presentation You Don't Have

• Patients with diabetes insipidus typically produce 5-20 liters of urine daily and experience extreme, unquenchable thirst driving them to drink massive volumes 1, 4
• They wake multiple times nightly to urinate and drink water (nocturnal polyuria and polydipsia) 2
• Without free access to water, they rapidly develop life-threatening hypernatremic dehydration with serum sodium >145-150 mEq/L 1, 4

Critical Pitfall to Avoid

• Many conditions can cause urine osmolality in the 200-300 mOsm/kg range during normal hydration without representing diabetes insipidus, including normal physiologic variation, partial dehydration, or early chronic kidney disease 1
• The diagnosis of diabetes insipidus requires urine osmolality definitively <200 mOsm/kg in the setting of serum hyperosmolality or inability to concentrate during formal water deprivation testing 1, 2
• Your ability to concentrate urine to 498 mOsm/kg after just an informal 12-hour fast completely excludes this diagnosis 1

What Your Results Actually Show

• Your ADH system functions perfectly: appropriate suppression when hydrated (ADH <0.8, urine osmolality 220) and appropriate activation when fasting (copeptin 4.6, urine osmolality 498) 1
• All other electrolytes are normal: calcium 9.8, uric acid 5.4, chloride 103, CO2 25 1
• These results indicate normal kidney concentrating ability and normal posterior pituitary function 1, 3