Magnesium deficiency: symptoms, the magnesium blood test and what a low level means

Magnesium deficiency means your body is running short of magnesium, a mineral that more than 300 enzymes (the proteins that drive chemical reactions in cells) need to keep muscles, nerves, blood sugar and the heart rhythm working. ¹ On a blood test it shows up as hypomagnesemia (low blood magnesium): a serum magnesium below about 1.7 to 1.8 mg/dL, which is 0.70 to 0.75 mmol/L. ¹² The catch is that less than 1% of your magnesium sits in the blood, so a normal result does not prove your stores are full, while a confirmed low result is a real finding that nearly always has a cause. What to do next: if a result comes back low, bring it to your clinician and ask them to check potassium, calcium and kidney function alongside it and to look for the cause, which is often a medicine.

What is magnesium deficiency?

An adult body holds about 25 grams of magnesium. Between 50% and 60% of it is in bone and most of the rest is inside the cells of muscles and soft tissues; less than 1% circulates in the blood serum, and the body keeps that small amount under tight control. ¹ The kidneys do most of the controlling: when magnesium runs short they hold on to more of it and let less out in the urine. ¹ That balancing act is why the blood level can look fine while stores in bone and cells drain slowly in the background.

Two different problems hide under the one name. The first is low intake: about half of Americans eat less than the Estimated Average Requirement (EAR, the intake that meets the needs of half of healthy people) for magnesium, and some age groups eat substantially less. ⁵ Men 71 and older and teenagers are the groups most likely to fall short. ¹ Low intake alone rarely drops the blood level, because the kidneys compensate. The second problem is hypomagnesemia itself, a measurably low blood level, which is reported in about 2% of the general population and is far more common in hospital, especially in intensive care. ³ The NIH Office of Dietary Supplements puts it plainly: symptomatic deficiency usually comes from a health condition or a medicine and is uncommon in otherwise healthy people. ¹

What causes low magnesium?

Magnesium is lost in two main ways: through the gut (too little absorbed, or too much lost in diarrhea) and through the kidneys (too much let out in the urine). Low intake makes either worse. ² The common causes below are worth matching against your own medicines and history before any supplement enters the picture.

These causes often stack. A person with high blood pressure on a thiazide diuretic who also takes a PPI for reflux and drinks most evenings has three separate drains on the same small blood pool. Pregnancy and breastfeeding raise needs too, though the drop in late pregnancy is usually physiological. ²

How is magnesium deficiency diagnosed?

The test is a serum magnesium, sometimes listed on a report as Mg, Mag or Magnesium, Serum. ⁴ It is a routine blood draw from the arm and needs no special preparation, although a clinician will want to know about supplements, laxatives and antacids, several of which contain magnesium. ⁴¹ It is not part of the basic metabolic panel in most labs, so it is usually ordered on its own when symptoms, a low potassium or calcium, or a risk factor such as long-term diuretics or PPIs raise the question. ⁴ A clinician reads it against the lab's printed range, with the cutoffs below as the common reference points.

Labs print magnesium in mg/dL, mmol/L or sometimes mEq/L. To convert, divide mg/dL by 2.43 to get mmol/L; mEq/L is double the mmol/L number. Each lab sets its own range, so the lower limit on your report may read 1.6, 1.7 or 1.8 mg/dL, and that printed range is the one that applies to your result. The lab units guide explains why the same blood can produce different-looking numbers.

Serum magnesium vs RBC magnesium: which test shows stores?

Because the serum test sees so little of the body's magnesium, some clinics offer a red blood cell (RBC) magnesium, which measures the magnesium inside red cells. It sounds like a window into stores, but reviews class it as a research test, and it has no agreed reference range. ³ For now the serum test, read with symptoms, medicines and the companion electrolytes, is the standard. ³¹

Clinicians have other tools when the serum level and the picture disagree, such as a 24-hour urine magnesium (low urine magnesium suggests the body is conserving it) and, rarely, a magnesium retention test. ³ The Merck Manual advises suspecting magnesium deficiency even with a normal serum level when a low calcium has no other explanation or a low potassium will not correct. ² The RBC magnesium marker page covers that test in more detail.

Blood tests to check alongside a low magnesium

A magnesium result is rarely read alone. These are the markers that tell a clinician how far the effects reach and where the magnesium is going. Each links to its own page with full ranges; see how reference ranges work for why your lab's range wins.

How the results read together

Magnesium 1.4, 1.6, 1.7, 1.9 or 2.4: what the number means

These readings use the NIH Office of Dietary Supplements' usual range of 0.75 to 0.95 mmol/L (about 1.8 to 2.3 mg/dL) and the Merck Manual's cutoffs; your lab's printed range comes first. ¹²

• Magnesium of 1.2 to 1.4 mg/dL (0.49 to 0.58 mmol/L): clearly low, at or near the severe line of 1.25 mg/dL. Low calcium and potassium are common at this level, and the result deserves prompt review the same week, or the same day with symptoms. ²
• Magnesium of 1.5 or 1.6 mg/dL (0.62 to 0.66 mmol/L): mildly to moderately low. Often found on a diuretic or long-term PPI, or after a bout of diarrhea. Worth a cause and a check of potassium and calcium. ²
• Magnesium of 1.7 mg/dL (0.70 mmol/L): at the lower limit; flagged low by some labs and normal by others. Read with symptoms and medicines, and recheck if a cause is present. ¹²
• Magnesium of 1.9 mg/dL (0.78 mmol/L): inside the usual range. It does not prove stores are full, but on its own it is not a deficiency. ¹⁵
• Magnesium of 2.4 or 2.5 mg/dL (0.99 to 1.03 mmol/L): just above the usual range. A small excess like this is usually checked against kidney function and any magnesium-containing laxatives, antacids or supplements. ¹⁴

Conditions that look like magnesium deficiency

Tiredness, cramps and tingling have many causes. These are the ones clinicians think of first, with the test that separates them.

Low magnesium vs low potassium: how they differ

The two often arrive together, and that pairing confuses people. Potassium is the electrolyte checked on almost every routine panel; magnesium usually has to be ordered on its own. ⁴ Low magnesium makes the kidney's potassium channel (called ROMK) leak potassium into the urine, so a potassium that keeps falling back despite replacement is a classic clue that magnesium is low too. ³ Read the electrolytes guide for how potassium, sodium, chloride and bicarbonate fit together.

Magnesium deficiency and restless legs

Restless legs, the urge to move the legs at rest with an uncomfortable crawling feeling, is one of the most searched links to magnesium. A 2019 systematic review found one randomized trial, which showed no significant effect, plus case reports, and concluded that it is not clear whether magnesium helps restless legs or which patients might benefit. ¹⁰ If restless legs is the main symptom, it is worth asking a clinician which other causes, including low iron, should be checked first.

Magnesium deficiency treatment: what can help, and how strong the evidence is

Treatment starts with the cause. If a diuretic or a long-term PPI is responsible, whether to change or keep it is a decision for the prescriber, because those medicines treat real problems. When the deficiency itself needs correcting, clinicians use magnesium by mouth for most people, and intravenous (IV) magnesium in hospital for severe or symptomatic cases, such as a level below 1.25 mg/dL with seizures or an abnormal heart rhythm; they recheck often, especially when the kidneys are weak. ² Low potassium and calcium are corrected alongside. ²³ The table grades the evidence for the uses people search for.

It is unlikely that magnesium supplementation provides clinically meaningful cramp prophylaxis to older adults experiencing skeletal muscle cramps.Garrison and colleagues, Cochrane review, 2020

Forms, food and the cautions that come first

Supplements use many forms of magnesium. Forms that dissolve well in liquid are absorbed better, and the aspartate, citrate, lactate and chloride forms tend to be absorbed better than magnesium oxide and magnesium sulfate. ¹ The magnesium guide compares the forms in plain language, and the supplements guide covers how to judge any product against your labs. Vitamin D and magnesium are often discussed together because both feed the calcium and PTH pathway; the vitamin D guide covers that side.

If you and your clinician decide magnesium makes sense, magnesium is one of the compounds LifeFrom can put in your daily pouch, on its own or as part of the Minerals pack with trace minerals, boron and calcium citrate. People with kidney disease or a reduced eGFR need a clinician's view before any magnesium, because the kidneys clear it, and anyone on antibiotics or bisphosphonates should ask about spacing doses apart.

Monitoring and retesting

How often magnesium is rechecked depends on how low it was and why. A single reading says less than a series, especially near the lower limit; see why a trend beats a single blood test.

When to see a doctor

Questions to ask your clinician

• Is my magnesium low enough to need treatment, and what do you think is causing it?
• Could one of my medicines, such as a diuretic or an acid reducer, be the cause, and is there an alternative?
• Should my potassium, calcium, PTH or vitamin D be checked at the same time?
• Is my kidney function (eGFR) normal enough for a magnesium supplement?
• Would magnesium by mouth be enough, and which form is least likely to upset my stomach?
• When should we recheck the level, and what result would change the plan?
• My result is normal but I still have cramps or tingling; what else should we look for?

For the bigger picture on electrolytes, see the electrolytes guide; for choosing any supplement against your labs, see the supplements guide. If poor sleep is what brought you here, LifeFrom's sleep page covers the tests and habits with the strongest evidence. When calcium comes back low with an abnormal albumin, the corrected calcium calculator adjusts it, and the eGFR calculator turns a creatinine into a kidney function estimate.

This page is general education, not medical advice or a diagnosis. The clinician who knows your history, medicines and full results reads them and makes decisions with you.

Free in LifeFromAdd the lab results you already have, and LifeFrom reads each marker next to the others it moves with, in plain language, with the sources it used. Free, on the web or in the app.

Start on the web Download the app