Treatment of Hypomagnesemia (Low Magnesium)
For mild hypomagnesemia, start with oral magnesium oxide 12 mmol (approximately 400 mg) at night, increasing to 24 mmol daily if needed; reserve IV magnesium sulfate 1-2 g for severe symptomatic cases (<1.2 mg/dL) or cardiac arrhythmias. 1
Initial Assessment and Correction
First, correct water and sodium depletion before treating magnesium deficiency. 1, 2
- Secondary hyperaldosteronism from volume depletion worsens renal magnesium wasting 2
- This is particularly critical in patients with high-output stomas, diarrhea, or gastrointestinal losses where each liter of jejunostomy fluid contains ~100 mmol/L sodium 2
- Administer IV saline to eliminate secondary hyperaldosteronism 2
Oral Magnesium Therapy (First-Line for Mild Cases)
Magnesium oxide is the preferred oral supplement because it contains more elemental magnesium than other salts and converts to magnesium chloride in the stomach. 1
Dosing Algorithm:
- Initial dose: 12 mmol magnesium oxide at night 1
- Total daily dose range: 12-24 mmol depending on severity and response 1
- Timing: Administer at night when intestinal transit is slowest to maximize absorption 1, 2
- Alternative salts: Organic magnesium salts (aspartate, citrate, lactate) have higher bioavailability than magnesium oxide or hydroxide and can be considered as alternatives 1, 2
Important Caveat:
Most magnesium salts are poorly absorbed and may worsen diarrhea or stomal output in patients with gastrointestinal disorders. 1, 2 Reducing excess dietary lipids can help improve magnesium absorption. 1
Parenteral Magnesium Therapy (For Severe or Symptomatic Cases)
Reserve IV magnesium for:
- Severe hypomagnesemia (<1.2 mg/dL or <0.5 mmol/L) with symptoms 1, 3
- Cardiac arrhythmias associated with hypomagnesemia regardless of measured serum levels 1, 4
- Torsades de pointes-type ventricular tachycardia with prolonged QT interval 1, 2
IV Dosing:
- Standard dose: 1-2 g magnesium sulfate IV bolus over 5-15 minutes 2
- For torsades de pointes: 1-2 g magnesium as IV bolus over 5 minutes 1, 2
- Alternative: Magnesium sulfate added to saline infusion bags 1
- Subcutaneous option: 4-12 mmol magnesium sulfate added to saline bags, administered 1-3 times weekly for patients requiring chronic supplementation 1, 2
Monitoring for Toxicity:
Watch for loss of patellar reflexes, respiratory depression, hypotension, and bradycardia during IV replacement. 2 Establish adequate renal function before administering any magnesium supplementation. 3
Refractory Cases
If oral therapy fails after adequate trial:
- Consider oral 1-alpha hydroxy-cholecalciferol (0.25-9.00 μg daily) in gradually increasing doses to improve magnesium balance 1, 2
- Monitor serum calcium regularly to avoid hypercalcemia 1, 2
Special Populations
Short Bowel Syndrome/Malabsorption:
- Higher doses of oral magnesium or parenteral supplementation may be required 1, 2
- Initially use IV magnesium sulfate, then transition to oral magnesium oxide and/or 1-alpha cholecalciferol 1
- Supplementation may be required despite normal serum magnesium concentration 4
Cardiac Patients:
- Maintain magnesium >2 mg/dL to prevent torsades de pointes and drug-induced arrhythmias in patients with cardiac arrhythmias or QT prolongation 4
- Patients with prolonged QTc interval (>500 ms) receiving QT-prolonging medications should target magnesium >2 mg/dL 4
Critical Associated Electrolyte Abnormalities
Hypomagnesemia causes dysfunction of potassium transport systems and increases renal potassium excretion, making hypokalemia resistant to potassium treatment alone. 2
- Correct magnesium deficiency before treating hypocalcemia or hypokalemia 2
- For hypomagnesemia-induced hypocalcemia, magnesium replacement should precede calcium supplementation 2
- Calcium normalization typically follows within 24-72 hours after magnesium repletion begins 2
- Monitor for secondary electrolyte abnormalities, particularly potassium and calcium, which often accompany hypomagnesemia 2
Target Levels
- Minimum target: >0.6 mmol/L (>1.46 mg/dL) 1, 4
- Normal range: 1.8-2.2 mEq/L 4
- For cardiac arrhythmias: >2 mg/dL 4