• Normal range = 1.5 to 3.0 mEq/L
    • NB: This is a poor measure of total body Mg2+ as only a small fraction is within serum
  • 2nd most abundant cation in the body
  • Involved in >300 enzymatic reactions
    • Na+,K+-ATPase pump
  • Distribution:
    • 53% Bone
    • 27% Muscle
    • 19% Soft Tissues
    • 3% Serum


  • Relatively common, especially in the critically ill
    • Reported incidence as high as 65%
    • Doubling of mortality rates (Tong 2015, PMID: 15665255)
  • High prevalence in:
    • Alcoholic population 60-85% (Tso 1992, PMID: 1491157)
    • Patients with CHF due to loop diuretics
    • Patients on thiazides
    • DKA
      • Increased osmotic diuresis
      • Acidosis increases renal excretion of magnesium
      • Insulin shifts Mg2+ intracellularly


Medications Gastrointestinal Renal Endocrine Inflammatory Hepatic
Aminoglycosides Diarrhea Diuretic phase of ATN Diabetes Infection/sepsis Alcoholism
Amphotericin B Gastric suctioning Postobstructive diuresis Hyperparathyroidism Postoperative period Cirrhosis
Cisplatin Intestinal fistula Hyperthyroidism Trauma
Cyclosporin Malnutrition Hyperaldosteronism
Digoxin Pancreatitis
Proton pump inhibitors
(Table adapted from Chang 2014)

Clinical Manifestations


  • Hypokalemia
    • Renal K+ wasting
    • Decreased intracellular K+
  • Hypocalcemia
    • Impaired parathyroid hormone secretion
    • Renal and skeletal resistance to parathyroid hormone
    • Resistance to vitamin D


  • Tetany
  • Spontaneous carpal-pedal spasm
  • Seizures
  • Vertigo, ataxia, nystagmus, athetoid, and choreiform movements
  • Muscular weakness, tremor, fasciculation, and wasting
  • Psychiatric: depression, psychosis


  • Dysrhythmias
    • Ventricular tachycardia (torsade de pointes)
    • Atrial fibrillation
    • Supraventricular tachycardia
  • Hypertension
  • Vasospasm
  • Electrocardiographic changes
    • Prolonged QT interval
    • Prolonged PR interval
    • Wide QRS
    • Peaked T waves
    • ST depression

(Table adapted fromTong 2015)


  • Difficult to accurately determine total body Mg2+amount
    • Largely distributed in bone and intracellularly while only extracellular serum levels measured
    • Significant portion of extracellular Mg2+ is protein bound
      • Ionized Mg2+ biologically active
    • Patient’s may be intracellularly depleted with normal extracellular levels (Tso 1992, PMID: 1491157)


  • Serum magnesium level alone should not dictate therapy
  • Stop medication if iatrogenic
  • Dose reduction recommended if patient with renal failure
  • If concomitant hypokalemia is present, attempt to replete Mg2+ concurrently or before repleting K+
    • Hypomagnesemia is associated with enhanced renal K+ excretion and may result in refractory hypokalemia
  • For severe symptoms + hemodynamically unstable
    • Magnesium Sulfate 1 to 2 grams bolus
  • For severe symptoms + hemodynamically stable
    • Magnesium Sulfate 1 to 2 grams over 10 to 60 minutes
  • For concurrent repletion with potassium
    • 5 g/hr infusion (Rosen 2014)
  • NB: Increases in serum Mg2+ leads transient increase in renal excretion of magnesium
    • ~50% of IV Mg2+ given will be excreted

Take Home Points

  • <1% of total Mg2+ is extracellular, making accurate measurement difficult
  • Hypomagnesemia leads to increased K+ renal wasting and concurrent repletion of these electrolytes should be considered, especially if hypokalemia is refractory
  • The more you replete, the more you excrete (renally)


Chang WT, Radin B, McCurdy MT. Calcium, magnesium, and phosphate abnormalities in the emergency department. Emerg Med Clin North Am. 2014;32(2):349-66. PMID: 24766937

Marx JA, Rosen P. Rosen’s emergency medicine : concepts and clinical practice. 8th ed. Philadelphia, PA: Elsevier/Saunders; 2014.

Nelson L, Goldfrank LR. Goldfrank’s toxicologic emergencies. 9th ed. New York: McGraw-Hill Medical; 2011. xxviii, 1940 p.

Rubeiz GJ, Thill-Baharozian M, Hardie D, Carlson RW. Association of hypomagnesemia and mortality in acutely ill medical patients. Crit Care Med. 1993;21(2):203-9. PMID: 8428470

Tintinalli JE, Stapczynski JS, Ma OJ, Cline D, Meckler GD, Yealy DM. Tintinalli’s emergency medicine : a comprehensive study guide. Eight edition. ed. New York: McGraw-Hill Education; 2016. xliii, 2128 pages p.

Tong GM, Rude RK. Magnesium deficiency in critical illness. J Intensive Care Med. 2005;20(1):3-17. PMID: 15665255

Tso EL, Barish RA. Magnesium: clinical considerations. J Emerg Med. 1992;10(6):735-45. PMID: 1491157