Definition: serum [Mg2+] <1.8 mg/dL.

Etiology/Risk Factors

• Malnutrition (especially with heavy/chronic alcohol use causing renal wasting).
• Malabsorption or diarrhea.
• PPI use.
• Renal losses:
  • Polyuria and high tubular flow rates (e.g. osmotic diuresis, post-ATN diuresis).
  • Hypercalcemia (e.g. in hyperparathyroidism).
  • Loop diuretics, thiazide diuretics.
  • Proximal tubule toxins (e.g. aminoglycosides, amphotericin, cisplatin).
  • Calcineurin inhibitors (tacrolimus more than cyclosporine).
  • Volume expansion (reduced Mg2+ reabsorption due to reduced Na+ and H2O reabsorption).
  • Gitelman & Bartter syndromes.
• Other:
  • Uncontrolled diabetes mellitus.
  • Post-parathyroidectomy (hungry bone syndrome).

Evaluation

• Review patient history, medication list, and clinical circumstances.
• Order chem 10.

Management

• Supplement to keep >1.8-2. Use 2 as the goal in patients being actively diuresed or with cardiac ectopy. Doses should be reduced in low GFR to reduce the risk of hypermagnesemia.
  • Oral formulations: many available. One common one is Mg oxide (400-800mg BID). Oral supplementation can cause diarrhea at larger doses. 
  • IV dosing: see section Appendix C: Sliding Scales. 
• Generally, oral supplementation is preferred over IV unless arrhythmias or neuromuscular symptoms are present. IV magnesium may be less efficient due to increased excretion after acutely raising serum levels. However, IV magnesium may be necessary to correct very low serum magnesium, particularly in the inpatient setting.

Key Points

• Magnesium may be low in patients with alcohol use, diarrhea, or diuretic use. 
• Generally, oral supplementation is preferred, though it can cause diarrhea. 
• Replete IV for patients with arrhythmias or neuromuscular symptoms.

Weisinger JR, Bellorin-Font E. Magnesium and phosphorus. Lancet 1998;352:391-396.

Agus ZS. Hypomagnesemia J Am Soc Nephrol 1999;10:1616-1622.