Magnesium: The Underdiagnosed Electrolyte Deficiency

The serum value samples only a small extracellular pool. Most magnesium is locked inside cells (especially muscle) and bone. The blood number reliably catches severe deficiency and acute changes, but borderline values miss a lot of chronic, milder deficiency.

Two practical takeaways:

• A "normal" serum magnesium does not rule out deficiency. If symptoms or refractory electrolyte abnormalities point that direction, an empirical trial of supplementation is reasonable.
• RBC magnesium and intracellular assays are more sensitive but rarely available outside research settings. Useful as a second-line check when needed.

In routine practice, the most reliable signal is functional. Refractory low potassium, refractory low calcium, or cramps that resolve when you replace magnesium.

• Normal: 1.7–2.2 mg/dL (0.70–0.91 mmol/L).
• Mild hypomagnesemia: 1.4–1.6 mg/dL. Often asymptomatic; common with diuretics, PPIs.
• Moderate hypomagnesemia: 1.0–1.3 mg/dL. Cramps, palpitations, fatigue, refractory hypokalemia.
• Severe hypomagnesemia: below 1.0 mg/dL. Arrhythmia risk; often hospital management.
• Hypermagnesemia: above 2.3 mg/dL. Almost always iatrogenic (IV magnesium, antacids in CKD).

Some clinicians argue that "optimal" magnesium runs in the upper half of the range (2.0 to 2.2). Evidence is mixed. There's no harm in keeping levels above 2.0 in patients with cardiovascular disease, diabetes, or chronic muscle complaints, and probably some upside.

High magnesium is uncommon and almost always iatrogenic. Healthy kidneys clear an oral load with no trouble, so high levels usually require both an external source and impaired excretion.

Common causes:

• Magnesium-containing antacids and laxatives in CKD. The leading outpatient cause. Milk of Magnesia and a creatinine of 2.5 don't mix.
• IV magnesium for eclampsia, asthma, or arrhythmia. Therapeutic in the right dose, dangerous in the wrong one.
• Acute or chronic kidney failure. Any magnesium load gets stuck.
• Tumor lysis, rhabdomyolysis. Cellular release.
• Adrenal insufficiency. Uncommon, but classic.

Symptoms appear above 4 to 5 mg/dL: nausea, flushing, weakness, decreased reflexes. Higher levels bring hypotension, respiratory depression, and cardiac arrest. Treatment is calcium gluconate, IV fluids, loop diuretics, and dialysis if it's severe.

Low magnesium is common and frequently missed. Major causes cluster into a few categories.

Drug-induced (the biggest bucket):

• Proton pump inhibitors (omeprazole, pantoprazole, esomeprazole). Long-term use blocks intestinal magnesium absorption. The FDA issued a warning in 2011. Anyone on a PPI longer than a year deserves a magnesium check, especially if they're also on a diuretic.
• Loop and thiazide diuretics. Direct renal magnesium wasting.
• Aminoglycosides, amphotericin B, cisplatin, cyclosporine, tacrolimus. All waste magnesium through the kidneys.
• Laxative abuse. Chronic GI losses.

GI losses and malabsorption:

• Chronic diarrhea, inflammatory bowel disease, celiac disease, post-bariatric surgery.
• Short bowel syndrome, pancreatic insufficiency.

Other causes:

• Alcohol use disorder. Poor intake, GI losses, and renal wasting all at once.
• Uncontrolled diabetes. Osmotic diuresis flushes magnesium out.
• Inadequate intake. Western diets average 60 to 70% of the RDA. Whole grains, leafy greens, nuts, seeds, legumes carry it. Processed food does not.
• Hungry bone syndrome, refeeding syndrome. Rapid cellular uptake.

The symptom list is long: cramps and muscle twitches, restless legs, fatigue, palpitations, anxiety, migraines, insomnia, constipation. Severe deficiency causes tetany, seizures, and ventricular arrhythmias including torsades de pointes.

Magnesium isn't on the basic metabolic panel and rarely gets trended. The patients who benefit most from periodic monitoring:

• Long-term PPI users. Annual at minimum, more often if also on diuretics.
• Anyone on chronic diuretics. Every 6 to 12 months.
• Heart failure or arrhythmia patients. Magnesium below 2.0 tracks with worse outcomes, and many cardiologists target above 2.0.
• Diabetics. Deficiency is more prevalent in this group and worsens insulin resistance.
• Chronic alcohol use. Periodic monitoring in recovery, active replacement in withdrawal.
• Anyone with chronic cramps, restless legs, or migraines. At least one check before chalking it up to idiopathic anything.

For oral repletion: glycinate, citrate, malate, and threonate are well absorbed. Magnesium oxide is poorly absorbed and mostly works as a laxative. Typical dose is 200 to 400 mg of elemental magnesium daily. Cut the dose if stools get loose.

• Magnesium below 1.0 mg/dL. Hospital evaluation, IV replacement, ECG monitoring. Risk of torsades de pointes.
• Refractory hypokalemia or hypocalcemia. Check magnesium first. These almost never correct without magnesium repletion.
• New cramps, palpitations, or restless legs in a patient on PPIs or diuretics. Check magnesium and replace if low or low-normal.
• Magnesium 1.4–1.6 mg/dL. Replace if symptomatic, on diuretics, in heart failure, or post-MI. Many cardiologists treat to above 2.0.
• Persistent atrial fibrillation or PVCs. Replenish magnesium and potassium; both lower the arrhythmia threshold.
• Magnesium above 2.5 in a CKD patient on antacids. Review for magnesium-containing OTC products.

• Potassium. Refractory hypokalemia is the loudest functional sign of magnesium deficiency.
• Calcium (and ionized calcium). Magnesium is required for parathyroid hormone secretion; severe hypomagnesemia causes functional hypoparathyroidism and hypocalcemia.
• Phosphate. Often disturbed alongside magnesium in malabsorption and refeeding states.
• Vitamin D. Magnesium is a cofactor in vitamin D activation; deficiencies cluster.
• BUN and creatinine. Kidney function determines clearance and dictates safe replacement strategy.
• ECG. For symptomatic or severely low magnesium; assess QT interval and look for U waves.