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14 Magnesium (Mg)

Magnesium: normal 0.70–1.05 mmol/L; causes of deficiency (PPIs, diuretics, alcohol), links with potassium, calcium and arrhythmia.

Magnesium (Mg²⁺) is the second most abundant ion inside cells after potassium. It is a cofactor for more than 600 enzymes, including all reactions that use ATP. It is essential for energy production, DNA and protein synthesis, nerve and muscle function, heart rhythm and bone strength. Only about 1 % of the body's magnesium is in the blood, so a normal serum level does not exclude a deficiency in tissues. Low magnesium often accompanies, and maintains, low potassium and low calcium.

ElectrolytesHeart rhythmMuscle function

Normal serum level0.70–1.05 mmol/L (1.7–2.5 mg/dL)

Hypomagnesaemia< 0.70 mmol/L

Severe deficiency< 0.50 mmol/L

Hypermagnesaemia> 1.05 mmol/L

Figure 1. Serum magnesium zones. Because most magnesium is inside cells, a level in the low-normal range does not exclude depletion.

Low magnesium — hypomagnesaemia

Gut losses: chronic diarrhoea, malabsorption (coeliac disease, inflammatory bowel disease), after bariatric surgery
Medicines: long-term proton-pump inhibitors (omeprazole and others), loop and thiazide diuretics, some chemotherapy (cisplatin), certain antibiotics and immunosuppressants
Alcohol use disorder, poor diet
Poorly controlled diabetes (urinary losses), hungry-bone syndrome after parathyroid surgery, rare inherited tubular disorders

Symptoms include tiredness, muscle cramps, tremor and twitching, and numbness. In severe cases there may be seizures and arrhythmias, including torsade de pointes, for which intravenous magnesium is the first treatment. Hypokalaemia and hypocalcaemia often do not correct until magnesium is replaced.

High magnesium — hypermagnesaemia

Hypermagnesaemia is uncommon and usually caused by reduced kidney function combined with magnesium intake: magnesium-containing antacids, laxatives or enemas, or intravenous magnesium in pre-eclampsia. Symptoms appear at higher levels: nausea, flushing, low blood pressure, loss of tendon reflexes, muscle weakness, slow heart rate and, in extreme cases, respiratory failure and cardiac arrest.

Seek emergency care

if a very low or high magnesium level comes with palpitations, fainting, seizures, severe muscle weakness or difficulty breathing.

Planned evaluation

Low magnesium should prompt a review of medicines (especially proton-pump inhibitors and diuretics), alcohol intake and gut symptoms, plus checks of potassium and calcium. People with kidney disease should not take magnesium supplements or magnesium-containing laxatives without medical advice.

References

de Baaij JHF, Hoenderop JGJ, Bindels RJM. Magnesium in man: implications for health and disease. Physiol Rev. 2015;95(1):1–46. doi:10.1152/physrev.00012.2014
Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060. doi:10.1161/CIRCULATIONAHA.109.192704

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 9 May 2024