Renal, Fluids, Electrolytes & Acid-Base · Topic 16 of 22

Magnesium Disorders

Renal · electrolytes

Magnesium Disorders

Low magnesium keeps potassium and calcium low — replete it first. Torsades gets 2 g IV at any level; high magnesium is reversed with IV calcium.

Key numbers

Normal magnesium 1.8–2.6 mg/dLMg < 1.8 mg/dL

Seizures: magnesium sulfate 2–4 g IV2–4 g IV over 5–10 min

Torsades: magnesium sulfate 2 g IV push2 g IV over 1–2 min

Hypermagnesemia: give IV calcium gluconate10% Ca gluconate 10–20 mL

01 · The numberNormal magnesium 1.8–2.6 mg/dL

Normal serum magnesium is 1.8–2.6 mg/dL; hypomagnesemia is below 1.8 mg/dL (0.70 mmol/L), severe usually below 1.25 mg/dL, and hypermagnesemia is above 2.6 mg/dL (1.05 mmol/L).

02 · The ruleReplete magnesium to correct refractory hypokalemia

Hypokalemia and hypocalcemia are hard to correct until magnesium is repleted: suspect magnesium deficiency in refractory hypokalemia or unexplained hypocalcemia, even when serum magnesium is normal, and replace it.

03 · GiveHypermagnesemia: give IV calcium gluconate

Treat severe hypermagnesemia with circulatory and respiratory support plus 10% calcium gluconate 10–20 mL IV, which temporarily reverses magnesium effects including respiratory depression.

Causes & risk factors

Why it happens

Hypermagnesemia: kidney failure plus magnesium intakeSymptomatic hypermagnesemia is uncommon and occurs mostly in kidney failure after magnesium-containing antacids or laxatives, or with IV magnesium sulfate infusions; it is rare with normal kidney function.

Causes: alcohol, GI loss, diuretics, PPIs, nephrotoxinsHypomagnesemia follows alcohol use disorder and undernutrition, chronic diarrhea or malabsorption, loop and thiazide diuretics, PPI use over 1 year, DKA, hypercalcemia, and nephrotoxins such as amphotericin B, cisplatin, aminoglycosides and cyclosporine.

Presentation

What you'll see

Lost deep tendon reflexes signal magnesium toxicityRising magnesium first abolishes deep tendon reflexes (as levels approach 12 mg/dL), then causes hypotension, respiratory depression and narcosis; cardiac arrest may occur above about 15 mg/dL.

Hypomagnesemia: tremor, tetany, hyperreflexia, seizuresHypomagnesemia causes weakness, tremor, fasciculations, tetany (Chvostek and Trousseau signs, carpopedal spasm), hyperreflexia, personality change, seizures and arrhythmias; tetany tracks the accompanying hypocalcemia or hypokalemia.

Diagnosis & monitoring

Work-up

Magnesium sulfate infusion: check reflexes and breathingDuring IV magnesium sulfate infusion (eg, preeclampsia, 4 g load then 2 g/h, reduced in renal insufficiency), check reflexes and breathing; for toxicity with lost reflexes or hypoventilation give calcium gluconate 1 g IV.

Kidney impairment: replete cautiously, recheck oftenMonitor serum magnesium frequently during replacement, especially with renal insufficiency or repeated parenteral doses, because impaired excretion turns repletion into hypermagnesemia.

Do right now

In this order

First moves

Torsades: magnesium sulfate 2 g IV push2 g IV over 1–2 min

→

Seizures: magnesium sulfate 2–4 g IV2–4 g IV over 5–10 min

→

Hypermagnesemia: give IV calcium gluconate10% Ca gluconate 10–20 mL

Why each step

Torsades: magnesium sulfate 2 g IV pushFor torsades give magnesium sulfate 2 g IV over 1–2 minutes regardless of the serum magnesium, and correct hypokalemia; repeat once in 5–10 minutes if needed, then infuse 3–20 mg/min if renal function allows.

Seizures: magnesium sulfate 2–4 g IVSevere symptomatic hypomagnesemia (seizures or other severe symptoms, magnesium usually < 1.25 mg/dL): give magnesium sulfate 2–4 g IV over 5–10 minutes, repeating up to 10 g over 6 hours if seizures persist.

Hypermagnesemia: give IV calcium gluconateTreat severe hypermagnesemia with circulatory and respiratory support plus 10% calcium gluconate 10–20 mL IV, which temporarily reverses magnesium effects including respiratory depression.

Management

Orders

Remove magnesium: furosemide or hemodialysisIf kidney function is adequate, IV furosemide with maintained volume increases magnesium excretion; hemodialysis works in severe hypermagnesemia because about 70% of serum magnesium is not protein-bound.

Mild or asymptomatic: oral magnesium saltsTreat when symptomatic or persistently below 1.25 mg/dL; oral magnesium salts (eg, magnesium gluconate 500–1000 mg three times daily for 3–4 days) suit patients without severe symptoms, but diarrhea limits the dose.

Moderate deficiency: slow IV magnesium, 1 g/hWith magnesium ≤ 1.25 mg/dL but milder symptoms, infuse magnesium sulfate about 1 g/hour for up to 10 hours; about half of given magnesium is lost in urine, so full repletion takes days.

Low or high magnesium?

Commonly confused

Serum level	< 1.8 mg/dL
Usual setting	Alcohol use, diarrhea, diuretics, long-term PPI, nephrotoxins
Reflexes and neuro	Tremor, tetany, hyperreflexia, seizures
Cardiac danger	Arrhythmias, torsades
Treatment	Magnesium sulfate IV or oral magnesium

More detail

Reference

Show 6 reference facts

IV magnesium sulfate given alone can worsen hypocalcemia because sulfate binds ionized calcium; treat concurrent hypocalcemia and hypokalemia specifically.

At magnesium 6–12 mg/dL the ECG shows PR prolongation, QRS widening and increased T-wave amplitude.

Only about 1% of body magnesium is extracellular and half sits in bone, so serum magnesium can be normal despite depleted intracellular and bone stores.

Severe hypomagnesemia often comes with hypocalcemia and hypocalciuria, and with hypokalemia from renal potassium wasting plus metabolic alkalosis.

In alcohol use disorder deficits of 12–24 mg/kg are possible; with normal kidneys give about twice the estimated deficit because roughly 50% is excreted.

Cisplatin renal magnesium wasting can be severe and persist after the drug stops; liposomal amphotericin B causes less hypomagnesemia than conventional amphotericin B.

Pearl

Hypokalemia that will not correct is a magnesium problem until proven otherwise.

Sources · Merck Manual Professional — Hypomagnesemia (Jun 2025) · Merck Manual Professional — Hypermagnesemia (updated Dec 2025) · Merck Manual Professional — Overview of Disorders of Magnesium Concentration · Merck Manual Professional — Torsades de Pointes Ventricular Tachycardia · Merck Manual Professional — Preeclampsia and Eclampsia (magnesium sulfate dosing and toxicity) · Tzivoni D et al. Treatment of torsade de pointes with magnesium sulfate. Circulation 1988;77:392–397 (2 g IV bolus effective; serum magnesium normal in all patients tested) — Europe PMC abstract · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).