Hypomagnesemia (Low Blood Magnesium)

What hypomagnesemia means

Magnesium is mostly stored in bone and inside cells. Less than 1% circulates in the blood. A blood test measures only that small fraction, so a normal level does not always mean body stores are full, and a low level usually means a real deficit.

Magnesium controls electrical stability in nerves, muscles and the heart, and it is needed for the hormone that regulates calcium (parathyroid hormone) and for the kidneys to hold on to potassium. That is why low magnesium so often travels with hypokalemia and hypocalcemia.

For diet, supplements and everyday deficiency, see magnesium deficiency. This page focuses on low blood levels found on testing and their medical management.

Magnesium levels: normal, low and severe

Symptoms

Symptoms usually appear only when levels are clearly low, and they overlap with those of low calcium and potassium:

• Muscle cramps, twitching and tremor
• Tingling around the mouth or in the fingers
• Weakness and fatigue
• Positive Chvostek's sign (facial twitch when the cheek is tapped) and Trousseau's sign (hand spasm under a blood pressure cuff)
• Tetany: sustained spasms of the hands and feet
• Irritability, confusion, abnormal eye movements (nystagmus)
• Seizures
• Palpitations and irregular heartbeat

Heart rhythm and ECG changes

Low magnesium can prolong the QT interval and widen the QRS complex on an ECG and trigger atrial fibrillation, ventricular ectopic beats and a dangerous ventricular rhythm called torsades de pointes. Risk is higher with low potassium, digoxin use, existing heart disease and other QT-prolonging drugs. Intravenous magnesium is the first treatment for torsades.

Causes of hypomagnesemia

Doctors sort causes by where the magnesium is being lost.

Loss through the gut

• Chronic diarrhea, laxative misuse, high-output stomas or fistulas
• Malabsorption: celiac disease, Crohn's disease, short bowel syndrome, bariatric surgery
• Long-term proton pump inhibitors (omeprazole, pantoprazole and others), which reduce magnesium absorption. The FDA warns this is most likely after a year or more of use, and levels recover when the PPI is stopped
• Acute pancreatitis
• Poor intake: malnutrition, alcohol use disorder

Loss through the kidneys

• Diuretics: loop diuretics (furosemide) and thiazides
• Alcohol, which directly increases urinary magnesium loss
• Uncontrolled diabetes: glucose in the urine pulls magnesium with it
• Medicines: cisplatin and other platinum chemotherapy, cetuximab and panitumumab, aminoglycoside antibiotics, amphotericin B, foscarnet, and the transplant drugs tacrolimus and cyclosporine
• Inherited tubule disorders: Gitelman syndrome, Bartter syndrome and rarer familial forms
• Recovery phase of acute kidney injury and after a kidney transplant

Shifts into cells or bone

• Refeeding syndrome when nutrition restarts after starvation
• Insulin treatment of diabetic ketoacidosis
• "Hungry bone" syndrome after parathyroid or thyroid surgery

How it is diagnosed

• Serum magnesium, usually checked with potassium, calcium, phosphate and kidney function
• ECG if levels are very low or you have symptoms
• Urine magnesium tests when the cause is unclear. A fractional excretion above about 2% (or more than about 24 mg in 24 hours) despite a low blood level points to kidney loss; a very low urine level points to gut loss or poor intake
• Medication review, with particular attention to PPIs, diuretics and chemotherapy

Treatment

Severe or symptomatic hypomagnesemia

Given in the hospital: intravenous magnesium sulfate, with heart monitoring when there are rhythm problems or seizures. Because the kidneys excrete a large part of an IV dose, repeat doses over several days are often needed to refill body stores. Doses are reduced in kidney failure, where magnesium can build up to dangerous levels.

Mild or chronic hypomagnesemia

• Oral magnesium salts, such as magnesium chloride, lactate, citrate, glycinate or oxide, taken in divided doses through the day. Diarrhea is the main side effect and limits how much can be absorbed. Use the dose your doctor recommends.
• Treat the cause: stop or switch a PPI if possible, review diuretics, control blood sugar, reduce alcohol.
• Amiloride, a potassium-sparing diuretic, can reduce ongoing kidney losses in some people.
• Magnesium-rich foods such as leafy greens, nuts, seeds, legumes and whole grains help maintain levels but cannot fix a significant deficit alone.

Correct magnesium first

Low potassium and low calcium often fail to correct until magnesium is replaced, because magnesium is needed for the kidney to retain potassium and for parathyroid hormone to work. Doctors therefore check and replace magnesium in anyone with stubborn hypokalemia or hypocalcemia.

Who should have magnesium checked

• People on long-term PPIs or diuretics, especially together, or with digoxin
• People receiving cisplatin, cetuximab or calcineurin inhibitors
• Anyone with chronic diarrhea, malabsorption or alcohol use disorder
• People with unexplained low potassium or calcium, arrhythmias or seizures

Frequently asked questions

What is a dangerously low magnesium level?

Levels below about 1.2 mg/dL (0.5 mmol/L) are considered severe and carry the highest risk of seizures and heart rhythm problems, especially with symptoms, ECG changes or low potassium. These usually need intravenous magnesium in the hospital. Lower levels are more dangerous in people with heart disease.

What is the most common cause of hypomagnesemia?

In hospitals, common causes are diuretics, poor intake and diarrhea. In the community, long-term proton pump inhibitor use, alcohol, diuretics and poorly controlled diabetes are frequent culprits. Often more than one factor is involved.

Can omeprazole cause low magnesium?

Yes. Omeprazole and other proton pump inhibitors can reduce magnesium absorption from the gut, usually after more than a year of use. Levels typically recover within weeks of stopping. If you need a PPI long term, your doctor may check magnesium periodically.

How long does it take to correct low magnesium?

Blood levels can rise within hours of an IV infusion, but refilling body stores takes several days. With oral supplements, mild deficiency usually improves over 1 to 4 weeks. If the cause continues, such as a diuretic or PPI, ongoing supplements may be needed.

Why won't my potassium go up?

Low magnesium makes the kidneys waste potassium, so potassium supplements may not work until magnesium is corrected. If you have repeated low potassium despite treatment, ask your doctor to check your magnesium level.