Magnesium in cats and dogs
Magnesium is the second most abundant positively charged ion inside cells after potassium, but less than 1% of the body’s magnesium is in the fluid outside cells. That is why a blood value is a poor guide to the body’s total stores. It is not on every panel; vets often add it when calcium or potassium behave oddly.
What it shows
According to the Merck Veterinary Manual, 50–60% of the body’s magnesium is in bone and 40–50% in soft tissues; both sources stress that the blood level does not reflect what is stored inside cells and bone. Magnesium is not held in place by a dedicated hormone: the gut absorbs it and the kidneys adjust how much they excrete.
Like calcium, magnesium in the blood is partly free (ionized) and partly bound to albumin and small molecules. Only the ionized part is active, and Merck notes that it cannot be calculated from total magnesium and albumin — it has to be measured directly. A routine panel reports total magnesium.
Magnesium is closely tied to its neighbours: eClinPath advises measuring it when low calcium has no clear explanation, and notes that low potassium may not correct until magnesium is corrected too.
Higher than reference — what it may be a sign of
- Kidneys excrete too little: kidney failure — eClinPath reports that kidney failure with little or no urine in dogs and cats consistently raises magnesium; urinary obstruction.
- Addison’s disease (lack of aldosterone; eClinPath: possible in theory).
- Too much intake: magnesium-containing fluids, supplements, antacids.
- Massive tissue damage, which releases magnesium from cells (eClinPath: possible in theory).
- Artefact: severe hemolysis or an old unseparated sample can raise the result, though not always.
Lower than reference
Low magnesium comes from losses through the gut (diarrhea, malabsorption) or the kidneys (heavy urination of any cause, including diabetes and intravenous fluids), from long periods of poor appetite, hyperthyroidism, and from a shift into cells, for example under insulin. Low albumin is listed as a possible contributor. In critically ill animals it is common: Merck cites low magnesium in 39–46% of dogs and cats in intensive care. Low magnesium can go together with low potassium and low calcium and may contribute to heart rhythm problems.
Look at together with
- Calcium — unexplained low calcium is a reason to check magnesium.
- Potassium — low potassium that does not respond as expected may come with low magnesium.
- Creatinine and urea — the kidneys decide how much magnesium stays in the body.
- Albumin — part of magnesium is carried on it, though eClinPath found little link between the two in practice.
Units and conversion
Magnesium appears on forms in mmol/L, mg/dL or mEq/L. To get mmol/L, multiply mg/dL by 0.4114 or mEq/L by 0.5: 2 mg/dL is about 0.82 mmol/L, and 2 mEq/L is 1 mmol/L. Ionized magnesium, if a lab offers it, is a separate test. In the app each row keeps the unit printed on the form, and the chart converts results to one unit.
Reference range
The reference range is on your lab’s form. Because the blood holds about 1% of the body’s magnesium, a value inside the range does not prove that body stores are full, and a vet reads it together with the clinical picture and the other electrolytes. Tubes with EDTA, oxalate or fluoride are not suitable for magnesium; lithium heparin is preferred for plasma.
Cats and dogs
The Merck Veterinary Manual notes that magnesium disorders are less common in cats than in cattle and sheep. Low magnesium is common in critically ill cats and usually causes no visible signs. High magnesium is rare; Merck links it to cats with kidney failure receiving intravenous fluids. Kidney failure with little or no urine raises magnesium in cats as in dogs.
In dogs in intensive care, those with low magnesium more often had low potassium and low sodium and stayed in hospital longer, according to Merck. eClinPath notes that high magnesium is not a feature of hyperparathyroidism in dogs, and that kidney failure with little or no urine consistently raises it. Otherwise the reading is the same as in cats.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Magnesium
- Overview of Disorders of Magnesium Metabolism in Animals. Merck Veterinary Manual
- Subclinical Hypomagnesemia in Critically Ill Animals. Merck Veterinary Manual
- Disorders of Magnesium Metabolism in Cats. Merck Veterinary Manual, cat owners version
- Whitbread TJ. Clinical Biochemistry. Merck Veterinary Manual
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.