By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
Magnesium is a cofactor for hundreds of enzymes and is essential for nerve, muscle and heart function. Because almost all of it sits inside cells, the blood level understates true status — but a low magnesium matters, not least because it can make a low potassium or calcium refuse to correct.
Magnesium is one of the body's quiet essentials: a cofactor for hundreds of enzyme reactions, central to energy metabolism, and required for normal nerve conduction, muscle contraction and heart rhythm. It is also the electrolyte most often forgotten, partly because measuring it well is genuinely difficult.
Magnesium is a cofactor for hundreds of enzymes, and sits almost entirely inside cells — so the blood level understates true body status.
Why the blood level understates the truth
Roughly 99% of the body's magnesium sits inside cells and bone, with only about 1% circulating in blood. A serum magnesium therefore samples a tiny, unrepresentative pool: a pet can be genuinely magnesium-depleted while the blood level still reads normal. As with calcium, a portion of circulating magnesium is protein-bound, so the ionised (free) fraction is the more physiologically meaningful measurement where it is available. The practical consequence is that a normal serum magnesium provides only weak reassurance, while a low one usually indicates real, and often substantial, depletion.
Reading a low result
Low magnesium (hypomagnesaemia) is the direction that matters, and it is common in hospitalised and critically ill patients. Causes include gastrointestinal losses (vomiting, diarrhoea, malabsorption), kidney losses, diabetes (especially during treatment of ketoacidosis), and certain medications (some diuretics and chemotherapeutics).
Its most clinically useful feature is the company it keeps: magnesium depletion makes low potassium and low calcium refractory — that is, they stubbornly fail to correct with supplementation until the magnesium is replaced. So a potassium or calcium that will not come up despite appropriate treatment is a classic prompt to check magnesium. Marked hypomagnesaemia can also contribute to muscle weakness, tremors and cardiac arrhythmias.
Reading a high result
High magnesium (hypermagnesaemia) is much less common and is seen chiefly with reduced kidney excretion — significant kidney disease — or over-supplementation, particularly with intravenous fluids or magnesium-containing preparations. Marked elevation depresses nerve and muscle function and affects the heart, so it is corrected deliberately.
In chronic disease and critical care
Magnesium is monitored mainly in hospitalised, critically ill and diabetic patients, and in kidney disease. In practice it is often checked precisely when other electrolytes are misbehaving, and corrected as part of a broader electrolyte strategy rather than in isolation. Following it alongside potassium and calcium is what makes it useful.
Cat vs dog
Magnesium behaves the same way in both species, with species-specific reference intervals, and the refractory-potassium relationship applies to both. Hypomagnesaemia is a recognised finding in critically ill dogs and cats alike, and in diabetic patients of either species during stabilisation.
About the reference range
This page shows no range strip. Pawline draws a band only where it holds a verified reference interval, and magnesium intervals are laboratory- and method-specific — with the added caveat that total and ionised magnesium are different measurements with different ranges. Read your result against the range and method printed on your own report.
For orientation only, one published example: Cornell University's veterinary laboratory reports total magnesium of roughly 1.5–2.1 mEq/L (≈0.75–1.05 mmol/L) in dogs and 1.7–2.2 mEq/L (≈0.85–1.10 mmol/L) in cats. Note this is that lab's figure for total magnesium; a widely-agreed ionised-magnesium reference interval does not exist in dogs and cats, which is one reason total magnesium under-reports true depletion. Your own report's range is what counts.
What to do next
Magnesium is most usefully checked when the picture calls for it: a potassium or calcium that will not correct, a critically ill or diabetic patient, or unexplained weakness or arrhythmia. A low result is corrected with veterinary guidance, and — importantly — correcting it is often what finally allows the other electrolytes to come back into line. Remember that a normal serum value does not exclude depletion, given how little of the body's magnesium is in the blood.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
