Magnesium Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Magnesium?

Magnesium is the fourth most abundant mineral in the human body and the second most abundant intracellular cation (after potassium). It serves as an essential cofactor in over 300 enzymatic reactions, including those involved in energy production, protein synthesis, DNA replication, nerve transmission, and muscle contraction. Without adequate magnesium, fundamental cellular processes cannot function properly.

Approximately 60% of the body's magnesium is stored in bone, 39% resides within cells (particularly muscle and soft tissue), and only about 1% circulates in the blood. This distribution is clinically important because serum magnesium levels may appear normal even when total body stores are significantly depleted. A blood test measures only the circulating fraction, which the body tightly regulates through renal conservation, intestinal absorption, and bone mobilisation.

Despite its critical importance, magnesium is one of the most commonly deficient minerals in Western diets. The UK National Diet and Nutrition Survey consistently shows that a significant proportion of adults - particularly older people, those with chronic diseases, and individuals taking certain medications - fail to meet recommended daily intakes. Magnesium deficiency has been linked to cardiovascular disease, type 2 diabetes, osteoporosis, migraine, and mental health disorders.

Why Is Magnesium Tested?

A serum magnesium test may be requested for a variety of clinical reasons:

• Unexplained muscle cramps or spasms: Persistent muscle twitching (fasciculations), cramps, or tetany may indicate magnesium deficiency.
• Cardiac arrhythmias: Magnesium plays a key role in cardiac electrical stability. Low levels can contribute to atrial fibrillation, ventricular tachycardia, and torsades de pointes.
• Refractory hypokalaemia or hypocalcaemia: Magnesium depletion impairs potassium and calcium regulation. If potassium or calcium remain stubbornly low despite supplementation, magnesium must be checked and corrected first.
• Diabetes monitoring: Type 2 diabetes is associated with increased urinary magnesium losses. Low magnesium may worsen insulin resistance, creating a vicious cycle.
• Medication monitoring: Proton pump inhibitors (PPIs), loop and thiazide diuretics, aminoglycosides, and some chemotherapy agents can deplete magnesium.
• Chronic kidney disease: Impaired renal excretion can lead to hypermagnesaemia, while earlier-stage CKD and dialysis may cause deficiency.
• Alcohol use disorder: Chronic excessive alcohol intake is one of the most common causes of magnesium deficiency due to poor dietary intake, gastrointestinal losses, and increased renal excretion.
• Pre-eclampsia management: Intravenous magnesium sulphate is used to treat eclamptic seizures, and serum levels must be closely monitored during infusion.

Normal Ranges

Serum magnesium is measured in millimoles per litre (mmol/L) in the UK. The table below shows standard reference intervals:

Category	Normal Range (mmol/L)	Notes
Adults	0.70–1.00 mmol/L	Standard UK reference range
Optimal range	0.80–0.95 mmol/L	Some evidence suggests upper half of normal is associated with better outcomes
Hypomagnesaemia	< 0.70 mmol/L	Clinically low; symptoms likely
Severe hypomagnesaemia	< 0.50 mmol/L	Medical emergency; risk of arrhythmias and seizures
Hypermagnesaemia	> 1.00 mmol/L	Usually iatrogenic or renal failure
Pregnancy	0.60–0.90 mmol/L	Physiological dilution lowers levels slightly

It is worth emphasising that serum magnesium is an imperfect marker of total body magnesium status. A patient can have a "normal" serum level of 0.72 mmol/L while being significantly depleted at the cellular level. If clinical suspicion for deficiency is high despite a normal serum result, clinicians may consider a 24-hour urinary magnesium excretion test or red blood cell (RBC) magnesium level for a more accurate assessment.

What Do High Magnesium Levels Mean?

Hypermagnesaemia (serum magnesium above 1.00 mmol/L) is relatively uncommon in clinical practice because healthy kidneys efficiently excrete excess magnesium. When it occurs, causes include:

• Renal failure: The most common cause. As kidney function declines, the ability to excrete magnesium is impaired. Hypermagnesaemia is particularly common in advanced chronic kidney disease (stages 4–5) and in patients on dialysis.
• Excessive magnesium supplementation: Over-the-counter magnesium supplements, magnesium-containing antacids (e.g., milk of magnesia), and magnesium-containing laxatives can cause elevated levels, especially in elderly patients with reduced renal function.
• Iatrogenic causes: Intravenous magnesium sulphate for pre-eclampsia/eclampsia or tocolysis can cause hypermagnesaemia if infusion rates are too high or monitoring is inadequate.
• Adrenal insufficiency (Addison's disease): Reduced aldosterone leads to impaired renal magnesium excretion.
• Hypothyroidism: Severe hypothyroidism can mildly raise magnesium levels.
• Familial hypocalciuric hypercalcaemia: A rare genetic condition that can cause mild hypermagnesaemia alongside hypercalcaemia.
• Lithium therapy: Lithium can reduce renal magnesium clearance in some patients.
• Tumour lysis syndrome: Rapid cell breakdown releases intracellular magnesium into the bloodstream.

Symptoms of hypermagnesaemia become apparent above approximately 1.5 mmol/L and include nausea, flushing, hypotension, and muscle weakness. Severe hypermagnesaemia (above 2.5 mmol/L) can cause respiratory depression, cardiac arrest, and death.

What Do Low Magnesium Levels Mean?

Hypomagnesaemia (serum magnesium below 0.70 mmol/L) is far more common than excess and is frequently underdiagnosed. Causes include:

• Inadequate dietary intake: Diets low in wholegrains, nuts, seeds, legumes, and green leafy vegetables are often magnesium-deficient. Highly processed foods are particularly poor sources.
• Gastrointestinal losses: Chronic diarrhoea, vomiting, malabsorption (coeliac disease, Crohn's disease, short bowel syndrome), and bariatric surgery all impair magnesium absorption.
• Medication-induced depletion: PPIs (omeprazole, lansoprazole) are a major culprit - long-term use reduces intestinal magnesium absorption. Diuretics (furosemide, bendroflumethiazide) increase renal magnesium losses. Other offenders include aminoglycosides, cisplatin, ciclosporin, and amphotericin B.
• Alcohol excess: Chronic alcohol consumption depletes magnesium through multiple mechanisms: poor intake, gastrointestinal losses, and increased renal excretion.
• Diabetes mellitus: Glycosuria causes osmotic diuresis, increasing urinary magnesium loss. Up to 40% of patients with type 2 diabetes have low magnesium.
• Renal tubular disorders: Gitelman syndrome and Bartter syndrome cause inherited renal magnesium wasting.
• Hungry bone syndrome: After parathyroidectomy, bones avidly take up magnesium (and calcium), causing acute deficiency.
• Pregnancy and lactation: Increased demands can deplete maternal magnesium stores.

How to Improve Your Magnesium Levels

Dietary Optimisation

The UK Reference Nutrient Intake (RNI) for magnesium is 300 mg/day for men and 270 mg/day for women. Excellent dietary sources include:

• Nuts and seeds: Pumpkin seeds (150 mg per 30g serving), almonds (80 mg per 30g), cashews, and Brazil nuts.
• Dark leafy greens: Spinach, Swiss chard, and kale - the magnesium at the centre of chlorophyll gives these vegetables their green colour.
• Wholegrains: Brown rice, wholemeal bread, oats, and quinoa. Refining grains removes up to 80% of magnesium content.
• Legumes: Black beans, lentils, chickpeas, and edamame.
• Dark chocolate: A 30g serving of 70%+ dark chocolate provides approximately 50 mg of magnesium.
• Oily fish: Mackerel, salmon, and halibut provide both magnesium and omega-3 fatty acids.
• Avocados and bananas: Moderate magnesium sources that also provide potassium.

Supplementation

• Magnesium glycinate: Well-absorbed, gentle on the stomach, and may support sleep quality. A good first-choice supplement.
• Magnesium citrate: Well-absorbed but can have a mild laxative effect, which may be beneficial for those with constipation.
• Magnesium oxide: Contains more elemental magnesium per tablet but is less well absorbed. Often used in over-the-counter products.
• Magnesium taurate or threonate: Emerging evidence suggests potential benefits for cardiovascular and cognitive health, respectively.
• Dosage: The NHS advises that magnesium supplements of up to 400 mg/day are unlikely to cause harm. Higher therapeutic doses should be guided by your clinician.

Lifestyle and Medical Measures

• Review medications: If you take PPIs, diuretics, or other magnesium-depleting drugs, discuss alternatives or concurrent magnesium supplementation with your prescriber.
• Moderate alcohol intake: Reducing alcohol consumption can significantly improve magnesium status.
• Manage underlying conditions: Optimising blood glucose control in diabetes, treating coeliac disease, and managing chronic diarrhoea all help restore magnesium.
• Epsom salt baths: Transdermal magnesium absorption through Epsom salt (magnesium sulphate) baths is sometimes promoted, though evidence for clinically meaningful absorption is limited.

When Should You Get Tested?

Consider a magnesium blood test if:

• You experience persistent muscle cramps, twitching, or spasms.
• You have chronic fatigue, weakness, or poor exercise recovery.
• You suffer from frequent migraines or headaches.
• You have cardiac arrhythmias, palpitations, or a history of heart disease.
• You take PPIs, diuretics, or other magnesium-depleting medications.
• You have type 2 diabetes, chronic kidney disease, or a gastrointestinal disorder.
• You drink alcohol regularly or heavily.
• Your potassium or calcium levels are persistently low despite supplementation.
• You are pregnant or planning a pregnancy and want to optimise mineral status.
• You have osteoporosis or are concerned about bone health.

Which Lola Health Tests Include Magnesium?

Magnesium is an essential mineral that should be part of any thorough health assessment. Lola Health includes magnesium in the following panels:

• Peak Insights - our most comprehensive test, covering magnesium alongside a full mineral, vitamin, and metabolic profile.
• Vital Check — a broad wellness panel that includes key minerals and electrolytes.
• Core Health — an essential health check covering fundamental mineral and metabolic markers.

All Lola Health tests use venous blood draws for clinical-grade accuracy. Your results are reviewed by qualified healthcare professionals who provide personalised dietary and lifestyle recommendations.