Magnesium is an essential mineral involved in over 300 enzymatic reactions in the body — including muscle contraction, nerve function, blood glucose regulation and blood pressure maintenance. Despite its importance, magnesium deficiency (hypomagnesaemia) is among the most commonly overlooked electrolyte disturbances.
The serum magnesium test is routinely included in many electrolyte panels, yet many patients who receive a borderline result have no idea what magnesium does or why the number matters. Here's a plain-language guide to what the test shows and what to do if your level is low.
Normal ranges and interpretation
Normal serum magnesium ranges from approximately 0.7–1.0 mmol/L (1.7–2.4 mg/dL). However, serum magnesium is a poor reflection of total body magnesium — about 99% of the body's magnesium is intracellular or in bone, not in the blood. This means you can have a normal serum level while being significantly depleted overall.
If clinical suspicion is high despite a normal serum result, a magnesium retention test (measuring how much of an infused magnesium dose the body retains) gives a more accurate picture of cellular stores.
Why does magnesium become depleted?
- Poor dietary intake — magnesium-rich foods include nuts, seeds, dark chocolate, leafy greens and whole grains
- Gastrointestinal losses — persistent diarrhoea, vomiting, malabsorption (coeliac disease, Crohn's)
- Renal wasting — medications (diuretics, aminoglycosides, amphotericin) or kidney tubular disorders
- Alcohol excess — impairs absorption and increases urinary excretion
- Diabetes mellitus — hyperglycaemia drives urinary magnesium losses
- Proton-pump inhibitor (PPI) use — long-term use of drugs like omeprazole causes magnesium malabsorption
Symptoms of magnesium deficiency
- Muscle cramps, spasms and twitching
- Fatigue and weakness
- Sleep disturbance and restless legs
- Headaches and migraines — magnesium deficiency is a recognised migraine risk factor
- Irritability and anxiety
- In severe deficiency: cardiac arrhythmias, seizures, low potassium and calcium (magnesium is needed for their regulation)
Treatment
Mild deficiency is treated with oral magnesium supplements — glycinate, malate and citrate forms are better absorbed than oxide. Dietary changes help maintain levels. Severe deficiency, particularly if causing cardiac symptoms, requires intravenous magnesium in hospital.
If the deficiency is driven by PPI use, your doctor may consider switching to an H2 blocker, reducing the dose, or supplementing magnesium long-term.
Magnesium and potassium: a dependency
Persistent hypokalaemia (low potassium) that doesn't correct with potassium supplementation often reflects an underlying magnesium deficiency — magnesium is required for the kidney to retain potassium. Always check magnesium when treating low potassium. For a full picture of electrolyte balance, our guide to electrolyte tests explains the sodium, potassium and chloride panel — and our calcium blood test guide covers the closely related calcium-magnesium relationship.