Blood tests
High calcium in dogs: why a tumour is looked for first
What a raised calcium signals in a dog, why it always has to be confirmed with an ionised calcium, and the household vitamin D sources owners forget about.
What this marker measures
Calcium does far more than build bone. It drives muscle contraction, nerve transmission and clotting. The body holds it within a narrow band using two hormones, parathyroid hormone and vitamin D.
The routine test measures total calcium, which is two fractions mixed together. Part of it circulates bound to albumin and is inactive. The other, ionised calcium, is the part that acts. This distinction is not theory: it changes what the result means.
What vets look into when the value sits outside the range
In dogs, a confirmed high calcium sends the vet looking for a tumour first, not last. That is not alarmism, it is real frequency. Lymphoma comes well ahead, followed by anal sac adenocarcinoma, a discreet tumour the vet looks for on rectal examination because it cannot be seen from the outside. Some tumours make a molecule that mimics parathyroid hormone and pulls calcium out of bone.
Chronic kidney disease comes next, and the relationship runs both ways: a diseased kidney unsettles calcium, and a high calcium damages the kidney.
Idiopathic high calcium exists mainly in cats, where it is often picked up by chance on a routine panel.
Primary hyperparathyroidism, a small nodule on a parathyroid gland, is seen in older dogs and responds well once identified.
The avoidable causes also need thinking about. Too much vitamin D poisons. The sources are household ones: cholecalciferol rodenticides, dermatology creams for psoriasis that the dog licks off its owner's skin, badly dosed supplements. This line of enquiry is settled by a question, not by a test.
In a growing puppy, a calcium slightly above the adult range is normal. The range has to be the one for its age.
What this value does not tell you on its own
The first reflex is to look at albumin. A dehydrated dog concentrates its blood and shows a higher total calcium without the active calcium having moved at all. The other way round, a low albumin can hide a genuinely high calcium. That is why a high total calcium is not treated, it is confirmed with an ionised calcium.
How urgent it is depends less on the figure than on how fast it came on and on the signs. A dog drinking enormous amounts, passing a lot of urine, eating less and looking dull is a dog whose calcium is already acting.
The logical next step, once ionised calcium is confirmed high, is to measure parathyroid hormone and the related molecule, then imaging: radiographs, abdominal ultrasound, examination of the lymph nodes and a rectal examination.
When to see a vet without waiting
Thirst and urine output markedly up over a few days. Marked dullness, vomiting, loss of appetite. Muscle weakness, tremors, seizures. Suspected ingestion of a rodenticide or of a skin cream. A lymph node you can feel under the jaw or behind the knee. These situations call for an immediate appointment.
Questions to ask your vet
- Has an ionised calcium been run to confirm this, and has albumin been taken into account?
- Is the search for a tumour under way, lymph nodes and rectal examination included?
- Could there have been exposure to vitamin D at home?
- Is kidney function affected, and should fluids be given while we wait for a diagnosis?
- Which test comes next if the ionised calcium comes back high?
Frequently asked questions
What is the difference between total calcium and ionised calcium?
Total calcium mixes together a fraction bound to albumin, which is inactive, and ionised calcium, which is the genuinely active fraction. A high total calcium is not treated, it is confirmed with an ionised calcium.
Can a psoriasis cream poison my dog?
Yes. The vitamin D derivatives used in human dermatology cause severe high calcium in dogs that lick treated skin on their owner. This is something to mention to the vet without being asked.
Can a puppy have a higher calcium than an adult?
Yes, bone growth goes with a physiologically higher calcium. The reference range used has to match the dog's age.
This article explains and points the way. It is not a diagnosis and is not a substitute for a veterinary consultation. It is published as a preliminary version and will be signed once a veterinary surgeon has approved it.
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Indicative UK ranges for taking a sample, a basic panel and an extended panel, what is included, why out of hours costs more, and how to ask for a written estimate.
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