What can cause elevated urinary calcium and sodium with low serum sodium and normal serum calcium on a comprehensive metabolic panel?

Elevated Urinary Calcium and Sodium with Low Serum Sodium and Normal Serum Calcium

The most likely cause is loop diuretic therapy (furosemide or bumetanide), which simultaneously increases urinary excretion of both calcium and sodium while depleting serum sodium through volume loss. 1, 2

Primary Mechanism: Loop Diuretics

Loop diuretics are the classic cause of this electrolyte pattern because they:

• Increase urinary calcium excretion by inhibiting calcium reabsorption in the thick ascending limb of Henle, leading to hypercalciuria despite normal serum calcium 1, 2
• Increase urinary sodium excretion (natriuresis) while simultaneously causing volume depletion that lowers serum sodium 1, 2
• Cause hyponatremia through excessive sodium losses and volume depletion, particularly with inadequate salt intake 1

The FDA labels for both furosemide and bumetanide explicitly warn that these drugs "may increase urinary calcium excretion" and cause "hyponatremia" as part of electrolyte depletion 1, 2. This distinguishes loop diuretics from thiazide diuretics, which decrease urinary calcium excretion 3, 4.

Secondary Considerations

High Dietary Sodium Intake with Volume Depletion

If the patient is not on diuretics, consider:

• Excessive sodium intake combined with volume depletion from other causes (vomiting, diarrhea, third-spacing) can produce this pattern 5, 6
• Sodium drives calcium excretion at a ratio of approximately 100 mmol sodium per 1 mmol calcium lost in urine 5
• However, this typically maintains or elevates serum sodium unless volume depletion is severe 6

Hypercalcemia-Induced Natriuresis (Less Likely Here)

• When serum calcium is elevated, each millimole of urinary calcium carries 10-20 mmol of sodium with it 5
• This scenario is excluded because your patient has normal serum calcium 5

Chronic Lung Disease in Premature Infants (Pediatric Context Only)

• Furosemide use for chronic lung disease of infancy causes hypercalciuria, hyponatremia, and volume depletion 3
• This combination increases risk of nephrocalcinosis in very low birth weight infants 3

Diagnostic Algorithm

1. Review medication list immediately for loop diuretics (furosemide, bumetanide, torsemide, ethacrynic acid) 1, 2

2. If on loop diuretics:

   • Measure serum potassium, magnesium, and chloride (expect hypokalemia, hypomagnesemia, hypochloremic alkalosis) 1
   • Check volume status clinically (orthostatic hypotension, tachycardia, dry mucous membranes) 1
   • Consider dose reduction or switching to thiazide diuretic if hypercalciuria is problematic 3

3. If NOT on loop diuretics:

   • Assess dietary sodium intake (24-hour urine sodium >200 mEq suggests high intake) 5, 6
   • Evaluate for volume depletion sources (GI losses, third-spacing, inadequate intake) 1
   • Measure PTH and 25-hydroxyvitamin D to exclude primary hyperparathyroidism (though serum calcium would typically be elevated) 3

4. Check for nephrocalcinosis risk:

   • Obtain renal ultrasound if hypercalciuria persists, especially in premature infants or patients with chronic diuretic use 3, 1
   • Calculate urine calcium/creatinine ratio on spot urine 3

Critical Pitfalls to Avoid

• Do not assume normal serum calcium excludes a calcium-related problem—the kidneys are wasting calcium despite normal serum levels, indicating renal tubular dysfunction 1, 2
• Do not confuse loop diuretics with thiazides—thiazides cause hypocalciuria (decreased urinary calcium) and are actually used to treat hypercalciuria 3, 4
• Do not overlook volume status—hyponatremia in this context is typically hypovolemic, requiring sodium and volume replacement, not fluid restriction 1
• Do not ignore magnesium and potassium—loop diuretics cause concurrent losses of these electrolytes, which must be repleted 1, 2

Management Approach

If loop diuretic-induced:

• Reduce diuretic dose or switch to thiazide diuretic (which will lower urinary calcium) 3
• Provide sodium chloride supplementation to correct hyponatremia 1
• Add potassium citrate or potassium chloride supplementation 3, 1
• Monitor serum electrolytes frequently during initial therapy 1

If dietary sodium excess with volume depletion:

• Restrict sodium intake to <2,000 mg/day 3
• Restore volume with isotonic fluids 1
• Increase fluid intake to dilute urinary calcium concentration 3