Magnesium
Mg· also: Magnesium, Serum Magnesium
Clinical Overview
Magnesium is the second most abundant intracellular cation and a cofactor in over 300 enzymatic reactions, including energy production (ATP synthesis), protein synthesis, DNA replication, and membrane transport. Approximately 60% is stored in bone, 38% in cells, and only ~2% in blood. Because most magnesium is intracellular, serum magnesium can be normal even with significant total body depletion.
Why This Test Matters
Hypomagnesemia is remarkably common in hospitalized patients (up to 65%) and is frequently overlooked. It is closely linked to hypokalemia — magnesium deficiency impairs renal potassium retention, making hypokalemia refractory to potassium replacement until magnesium is corrected. Hypomagnesemia also causes hypocalcemia (impairs PTH secretion). Magnesium is used therapeutically for pre-eclampsia, torsades de pointes, severe asthma, and eclampsia.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 1.7 – 2.2 | mg/dL | — |
Also reported in: mmol/L.
Critical (Panic) Values
Critical Low: < 1 mg/dL. Critical High: > 4.9 mg/dL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High Mg Causes
- Renal failure (most common — reduced excretion)
- Excessive magnesium supplementation or antacid use
- Intravenous magnesium therapy (pre-eclampsia treatment)
- Hypothyroidism and adrenal insufficiency
Low Mg Causes
- Chronic diarrhea and malabsorption (most common GI cause)
- Proton pump inhibitors (PPIs) — can cause renal magnesium wasting with long-term use
- Diuretics (loop and thiazide — increase renal magnesium excretion)
- Alcohol excess (increased renal losses and poor intake)
- Type 2 diabetes with poor control (osmotic diuresis)
- Hypoparathyroidism and vitamin D deficiency
- Refeeding syndrome
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation. Serum magnesium should ideally be drawn in the morning fasting. Note that serum magnesium reflects only the 2% circulating fraction — it can be normal even with significant total body depletion.
Factors That Can Affect Results
- Hemolysis (magnesium is intracellular — hemolysis falsely elevates serum magnesium)
- Prolonged tourniquet use
- Antacids and laxatives containing magnesium
- Renal function (kidney disease markedly affects serum magnesium)
Related Topics
Frequently Asked Questions
Why can't hypokalemia be corrected without fixing magnesium first?
Magnesium is essential for the activity of the Na-K-ATPase pump in the kidney's collecting tubule, which reabsorbs potassium. When magnesium is deficient, this pump works inefficiently, causing the kidneys to continuously waste potassium regardless of how much potassium is replaced. Hypokalemia that does not respond to potassium replacement should always prompt measurement and correction of magnesium — often 4–8 g of IV magnesium sulfate is needed.
Can long-term PPIs cause magnesium deficiency?
Yes. Long-term use of proton pump inhibitors (more than 12 months) is a recognized cause of hypomagnesemia, likely because PPIs impair intestinal magnesium absorption through an unclear mechanism. Symptoms include muscle cramps, tremors, and seizures. The FDA issued a warning about this association. Serum magnesium should be checked periodically in patients on long-term PPI therapy, especially if they also take diuretics.