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For further information, see CMDT Part 21-13: Hypomagnesemia

Key Features

Essentials of Diagnosis

  • Serum magnesium < 1.8 mg/dL (< 0.75 mmol/L)

  • Causes neurologic symptoms and arrhythmias

  • Serum concentration may be normal even in the presence of magnesium depletion

  • Check urinary magnesium excretion if renal magnesium wasting is suspected

  • Associated with hypocalcemia

General Considerations

  • Normomagnesemia does not exclude magnesium depletion because only 1% of total body magnesium is in the extracellular fluid

  • Magnesium repletion should be considered in patients with risk factors for hypomagnesemia and refractory hypokalemia or hypocalcemia

  • Renal potassium wasting is refractory to potassium replacement until magnesium is repleted

  • Hypomagnesemia also suppresses PTH release and causes end-organ resistance to PTH and low 1,25-dihydroxyvitamin D3 levels and consequent hypocalcemia is refractory to calcium replacement until the magnesium is normalized

  • Molecular mechanisms of magnesium wasting have been revealed in some hereditary disorders

  • The potassium binder patiromer can cause hypomagnesemia by binding magnesium in the colon


  • Diminished absorption or intake (Table 21–10)

    • Malabsorption

    • Chronic diarrhea

    • Laxative abuse

    • Prolonged gastrointestinal suction

    • Small bowel bypass

    • Malnutrition

    • Alcohol use disorder

    • Total parenteral alimentation with inadequate Mg2+ content

  • Increased renal loss

    • Diuretic therapy

    • Hyperaldosteronism

    • Gitelman syndrome (a variant of Bartter syndrome)

    • Hyperparathyroidism

    • Hyperthyroidism

    • Hypercalcemia

    • Volume expansion

    • Tubulointerstitial diseases

    • Transplant kidney

    • Drugs (aminoglycoside, cisplatin, amphotericin B, pentamidine)

  • Others

    • Diabetes mellitus

    • Postparathyroidectomy (hungry bone syndrome)

    • Respiratory alkalosis

    • Pregnancy

Table 21–10.Causes of hypomagnesemia.

Clinical Findings

Symptoms and Signs

  • Because hypomagnesemia causes hypokalemia and hypocalcemia, it is difficult to determine if symptoms are from hypomagnesemia itself or from potassium and calcium depletion

  • Marked neuromuscular and central nervous system hyperirritability

    • Tremors

    • Cramps

    • Trousseau and Chvostek signs

    • Confusion

    • Disorientation

    • Coma

  • Weakness is common

  • Cardiovascular manifestations include

    • Hypertension

    • Tachycardia

    • Ventricular arrhythmias, including torsades de pointes


Laboratory Tests

  • Serum magnesium < 1.8 mg/dL (< 0.75 mmol/L)

  • Urinary excretion of magnesium exceeding 10–30 mg/day or a fractional excretion more than 3% indicates renal magnesium wasting

  • Hypokalemia and hypocalcemia are often present

  • PTH secretion is often suppressed

Diagnostic Procedures

  • ECG may show widening of the QRS complex, peaked T waves with ultimate diminution and a prolonged PR interval


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