Magnesium
Magnesium is the fourth most abundant cation in the body and a cofactor for 300+ enzymes. Low magnesium is common, frequently missed, and explains many "refractory" potassium and calcium issues.
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What it measures
Magnesium is a cofactor for hundreds of enzymes — ATP biology, protein synthesis, nerve transmission, vascular tone. Only about 1% of body magnesium is in serum, so a normal serum level doesn't rule out tissue deficiency; conversely, when serum magnesium IS low, total-body deficiency is usually severe. Magnesium status quietly influences whether potassium and calcium can be corrected effectively — magnesium-deficient patients have refractory hypokalaemia and hypocalcaemia until magnesium is replaced.
What a high value can mean
- Renal failure — magnesium accumulates without normal renal excretion.
- Magnesium-containing antacids and laxatives in patients with reduced renal function.
- Iatrogenic — eclampsia/pre-eclampsia treatment with magnesium sulfate.
What a low value can mean
- Gastrointestinal loss — chronic diarrhoea, malabsorption, proton-pump inhibitor use, gastric bypass.
- Renal loss — diuretics (loop and thiazide), alcohol, aminoglycoside antibiotics, cisplatin.
- Reduced intake — alcoholism, refeeding syndrome.
- Hypoparathyroidism, diabetes.
- Look for low magnesium whenever low potassium or calcium is refractory to replacement.
When to discuss with a doctor
Symptoms of magnesium deficiency — muscle cramps, tremor, palpitations, refractory hypokalaemia — combined with a borderline-low serum level warrant a primary-care work-up. Patients on long-term PPI, diuretic or alcohol use benefit from periodic magnesium screening. Severe hypomagnesaemia with neurological symptoms is treated with IV replacement.