Which electrolyte should be replaced first in a patient with hypomagnesemia and hypocalcemia?

Replace Magnesium First

In patients with concurrent hypomagnesemia and hypocalcemia, magnesium must be replaced first because hypocalcemia will be refractory to calcium supplementation until magnesium levels are corrected. 1

Why Magnesium Takes Priority

Severe hypomagnesemia induces secondary hypocalcemia through two critical mechanisms 2:

• Functional hypoparathyroidism: Magnesium deficiency impairs parathyroid hormone (PTH) secretion, preventing the body's normal response to low calcium 2
• End-organ resistance: Even when PTH is secreted, target tissues become resistant to its effects in the absence of adequate magnesium 3

The clinical implication is straightforward: attempting to correct hypocalcemia without first addressing magnesium deficiency will fail because the underlying hormonal dysfunction persists 1.

Treatment Algorithm

Step 1: Assess Severity and Correct Volume Status First

Before any electrolyte replacement 4:

• Correct water and sodium depletion with IV saline to address secondary hyperaldosteronism, which increases renal magnesium and calcium losses 4
• Check renal function—avoid magnesium supplementation if creatinine clearance <20 mL/min due to hypermagnesemia risk 4

Step 2: Replace Magnesium

For severe symptomatic hypomagnesemia (tetany, seizures, cardiac arrhythmias) 5:

• Give 1-2 g magnesium sulfate IV bolus over 5-15 minutes 6, 1
• For life-threatening presentations like torsades de pointes: 1-2 g IV push over 5 minutes 1
• Follow with continuous infusion: 5 g (40 mEq) in 1 L of saline over 3 hours 5

For mild-moderate hypomagnesemia 1, 5:

• Oral magnesium oxide 12-24 mmol daily (480-960 mg elemental magnesium), preferably at night when intestinal transit is slowest 4, 1
• IM administration: 1 g (8.12 mEq) every 6 hours for 4 doses if oral route unavailable 5

Step 3: Replace Calcium Only After Magnesium

Once magnesium replacement is initiated 1:

• Begin calcium supplementation with calcium carbonate plus vitamin D3 3
• Add calcitriol 0.25 μg twice daily for severe cases 3
• Monitor both magnesium and calcium levels closely—expect calcium to normalize as magnesium is corrected 3

Critical Pitfalls to Avoid

Do not give calcium first 1:

• Calcium supplementation will be ineffective and wasteful until magnesium is repleted
• You risk overcorrecting calcium once magnesium normalizes and PTH function returns

Watch for concurrent hypokalemia 4, 7:

• Hypomagnesemia occurs in 42% of patients with hypokalemia 7
• Hypokalemia is also refractory to potassium replacement until magnesium is corrected, as magnesium deficiency causes dysfunction of multiple potassium transport systems 4
• Replace magnesium first, then potassium will respond to supplementation 4

Monitor for magnesium toxicity during IV replacement 5:

• Check for loss of patellar reflexes (first sign of toxicity)
• Watch for respiratory depression, hypotension, and bradycardia
• Have calcium chloride 5-10 mL (10%) or calcium gluconate 15-30 mL (10%) available to reverse toxicity 6

Adjust for renal function 4, 5:

• Maximum dose is 20 g/48 hours in severe renal insufficiency with frequent serum monitoring 5
• Use dialysis solutions containing magnesium for patients on continuous renal replacement therapy 4

Expected Timeline

• IV magnesium: Therapeutic levels achieved almost immediately 5
• IM magnesium: Therapeutic levels in 60 minutes 5
• Oral magnesium: Several days to normalize total body stores 4
• Calcium normalization: Typically follows within 24-72 hours after magnesium repletion begins 3