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Magnesium - Hypomagnesaemia


The normal plasma magnesium (Mg) concentration is maintained (primarily) by the kidney in the narrow range of 0.8 to 1.10 mmol/L. Hypomagnesaemia (Mg level < 0.8 mmol/L) usually remains asymptomatic until the Mg levels drop below 0.5mmol/L and is commonly associated with other metabolic abnormalities such as hypokalaemia, hypocalcaemia, and metabolic acidosis. A level <0.4mmol/L indicates severe deficiency.


Hypomagnesaemia mechanism


Severe malnutrition

Poor oral intake

Gastrointestinal loss



Primary intestinal hypomagnesaemia
Intestinal fistula

Extensive bowel resection
Nasogastric suction

Renal loss

Hypercalcaemia and hypercalciuria

Osmotic diuresis

Chronic parenteral fluid therapy



Proton pump inhibitors


Aminoglycoside antibiotics


Amphotericin B





Diuretic phase of acute kidney injury

Post-obstructive nephropathy

Phosphate depletion

Hungry bone syndrome

Gitelman syndrome

(Adapted from eTG table hypomagnesaemia)


History and examination

Neuromuscular symptoms:

  • Weakness and apathy
  • Tremor
  • Paraesthesia
  • Tetany
  • Muscle fasciculations
  • Seizures, drowsiness, confusion, and coma.

Cardiovascular features:

  • Arrhythmias (Torsades de Pointes and hypomagnesaemic hypokalaemia, digoxin toxicity with tachyarrhythmias).
  • ECG signs including wide QRS, prolonged AT flattened T waves and presence of U waves.

Associated metabolic abnormalities:

  • Resistant hypocalcaemia
  • Resistant hypokalaemia
  • PTH resistance and impaired PTH release.



Serum Mg levels (ionised more accurate)

Serum protein levels (Mg is protein-bound extracellularly)

Other Electrolytes: Calcium phosphate and potassium levels

Glucose (association with diabetes)

ECG (interval changes and exclude arrhythmias consistent with torsade de pointes or hypomagnesaemic hypokalaemia)

Other test sometimes used are 24-hour Mg urinary excretion (to check for renal wasting)


Includes replacing the Mg in conjunction with correction of other underlying electrolyte abnormalities and other underlying disease process, including improving renal impairment.

The severity of hypomagnesaemia is based primarily on the symptoms as opposed to the laboratory values.

The severity of hypomagnesaemia is based primarily on the symptoms as opposed to the laboratory values.

Mild to moderate hypomagnesaemia and asymptomatic: Mg <0.8 mmol/L

  • Oral magnesium supplements
    • magnesium aspartate 1000 to 3000 mg (elemental magnesium 74.8 to 224.4 mg) orally, daily in divided doses, with food.

Severe hypomagnesaemia and symptomatic: <0.4mmol/L

The rate of infusion depends on the extent of the deficit and the clinical features.

  • magnesium 25 to 50 mmol IV in sodium chloride 0.9% 500 mL to 1000 mL over 12 to 24 hours initially; aim to achieve and maintain serum magnesium concentration above 0.4 mmol/L.


  • magnesium 10 mmol IV in sodium chloride 0.9% 100 mL over 60 minutes. Repeat if needed, titrate to effect and serum magnesium concentration.

In the presence of life-threatening cardiac arrhythmia, 4 to 8 mmol magnesium can be given over 5 to 10 minutes.

During intravenous therapy, the serum magnesium concentration should be monitored every 1 to 2 hours initially.

Reduce the magnesium doses by 50% in renal impairment.

Consider referral to critical care services when

Life threatening cardiac arrythmia

Neurological symptoms




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