Hypercalcemia in Dogs: 9 Causes of High Calcium

Hypercalcemia in dogs (i.e. high calcium in dogs) can be caused by a variety of conditions, some more concerning than others. To help explain what high calcium could mean for your dog, integrative veterinarian Dr. Julie Buzby discusses how the body regulates calcium, and describes nine causes of hypercalcemia. Then she explains the diagnosis, treatment, and prognosis for hypercalcemia in dogs.

When my client brought her beloved senior dog in to see me, I didn’t think I’d be calling her back the next day to talk about hypercalcemia (i.e. high calcium levels). But that’s exactly what ended up happening.

Being an astute dog mom, my client was quick to make an appointment for her dog when she suspected something was off with him. Walter, who had always been ready to take a walk or play at a moment’s notice, was now preferring to snooze the days away. And when he did get up, he seemed a bit weak sometimes. Plus, there had been a few nights recently where he turned his nose up at dinner—something he would normally never dream of doing.

Nothing out of the ordinary stood out to me about Walter’s physical exam, but I recommended running some lab work to get a better picture of his overall health and to look for indicators of kidney disease, liver disease in dogs, diabetes in dogs, or other common senior dog ailments.

But when the lab results came back, much to my surprise, the only abnormalities were a high blood calcium (i.e. hypercalcemia) and a slightly low phosphorus. This prompted a discussion about calcium and the various causes of hypercalcemia in dogs.

What does calcium do in the body?

As you probably learned in school (or from “Got Milk?” commercials), getting enough calcium helps you (and your dog) build strong bones. But it also has many other important jobs in the body, including playing a role in muscle contraction, heart rhythm, blood clotting, and nerve signal transmission.

With all of these important functions, it’s probably no surprise that calcium levels are tightly controlled by the body. In order to monitor the amount of calcium present at any given time, your dog has two tiny glands near the throat called the parathyroid glands.

These glands will release parathyroid hormone (PTH) if there is too little calcium in the blood. The result is increased absorption of calcium from the intestinal tract, decreased excretion of calcium by the kidneys, and reabsorption of calcium from the bones. All of these changes serve to increase blood calcium back to the normal range.

Additionally, PTH mediates the formation of calcitriol, the active form of Vitamin D. Calcitriol causes increased absorption of calcium and phosphorus from the intestines. And it decreases the excretion of calcium and phosphorus from the kidneys. When calcitriol levels are high, it will also decrease the synthesis of PTH.

PTH and calcitriol typically keep the calcium levels tightly controlled. However, there are situations where, for one reason or another, blood calcium levels can become too high. This is called hypercalcemia, and it can be dangerous for your dog.

What are the symptoms of hypercalcemia?

Many dogs with mildly elevated calcium levels will appear asymptomatic at first. This is why yearly physical examinations and blood work are so important, especially for middle-aged and senior dogs.

However, dogs will typically start showing clinical signs once calcium levels rise above 15 mg/dL. (For reference, normal calcium levels are typically considered to be around 9.4 to 11.1 mg/dL, but vary slightly depending on the laboratory).

High calcium levels in dogs can cause multiple different clinical signs. These symptoms can be categorized based on the body system that they affect—the gastrointestinal tract, the cardiovascular system, the neuromuscular system, and the urinary system.

Gastrointestinal system

Calcium has a large impact on how the muscles work. And the smooth muscle of the GI tract is no exception. Hypercalcemia typically slows down muscle contraction in the intestines, making it take longer for food to go from one end of the GI tract to the other. This can lead to constipation and inappetence.

Additionally, high calcium may cause increased stomach acid production, which can lead to gastric ulcers in dogs. And dogs with high blood calcium may also be lethargic dogs or have GI upset (i.e. vomiting or diarrhea).

Cardiovascular system

A dog’s heart is a muscle too, so it is also susceptible to the effects of high calcium. Elevated blood calcium can lead to changes in how the heart contracts. Severely hypercalcemic dogs may have a slower heart rate and sometimes an arrhythmia. Due to the slow heart rates and abnormal rhythms, these pups can appear lethargic and weak.

Neuromuscular system

In order for muscles to work, they must receive the nerve signals that tell them when to contract or relax. High calcium levels in the blood negatively affect the way the signaling molecules move across cell membranes, which alters signal transmission. Symptoms of hypercalcemia-related neuromuscular issues include muscle weakness, muscle wasting, exercise intolerance, muscle twitching, and lethargy.

Urinary system

Excess calcium that makes its way out of the body through the kidneys can damage the kidney tissue itself, leading to kidney failure in dogs. Plus, when high levels of calcium are present in urine, there is an increased risk of infection (i.e. UTI in dogs) and of bladder stone formation. Hypercalcemia can also interfere with the body’s ability to concentrate urine and lead to a condition called nephrogenic diabetes insipidus in dogs.

Dogs with diabetes insipidus or kidney failure may have increased thirst in dogs and increased urination. Plus, kidney failure can cause vomiting, diarrhea, lethargy, weight loss, or lack of appetite. And hypercalcemic dogs may strain to urinate or have bloody urine, especially if there is a bladder infection or stones present.

What are the causes of hypercalcemia in dogs?

In addition to these hypercalcemia-specific signs, affected dogs may also show symptoms of whatever condition caused the high calcium in the first place. Nine common causes of hypercalcemia in dogs include:

#1. Hypercalcemia of malignancy

In dogs, hypercalcemia of malignancy (i.e. high blood calcium due to cancer) is the most common cause of high calcium. Certain tumors can produce a substance called parathyroid hormone-related protein (PTHrP) which mimics the action of PTH in the body. Since PTH raises blood calcium, PTHrP has the same effect. And unlike PTH, its production isn’t turned off when blood calcium levels normalize, so PTHrP will continue to raise the blood calcium. Additionally, some cancers may produce other factors that ultimately lead to hypercalcemia.

Many different cancers can cause hypercalcemia, including:

• Lymphoma in dogs (this is the most common cancer associated with hypercalcemia of malignancy)
• Anal sac adenocarcinoma
• Mammary carcinoma
• Malignant melanoma
• Multiple myeloma
• Certain sarcomas

#2. Bone breakdown (i.e. osteolysis)

Additionally, bone tumors like osteosarcoma in dogs, or tumors that spread to the bone from other locations, can can cause hypercalcemia. And so can bone infections (i.e. osteomyelitis). All of these diseases lead to bone destruction, which causes the release of calcium into the bloodstream.

#3. Addison’s disease

Addison’s disease in dogs (i.e. hypoadrenocorticism) is another common cause of high calcium in dogs. In this condition, the dog’s adrenal glands don’t make enough of certain hormones—aldosterone and/or cortisol. While the exact connection between Addison’s disease and hypercalcemia isn’t well understood, it is possible that the lack of cortisol plays a role.

Although cortisol is better known for its role as a “stress hormone,” it does also have the ability to reduce calcium absorption from the intestines and increase calcium excretion from the kidneys. But since dogs with Addison’s disease have low cortisol, they may be prone to absorbing more calcium and excreting less calcium. This could potentially lead to hypercalcemia.

Thankfully, the high calcium levels that result from Addison’s disease are usually mild. And once the Addison’s disease is under control, the calcium levels typically normalize.

#4. Hyperparathyroidism

As we discussed earlier, the parathyroid glands release parathyroid hormone (PTH) whenever the body requires more calcium. But in certain situations, they can send out excess PTH, resulting in hypercalcemia. This excess PTH production is called hyperparathyroidism.

There are two types of hyperparathyroidism—primary and secondary. Primary hyperparathyroidism occurs when the glands themselves become diseased. For example, a PTH-secreting tumor may develop on one of the glands.

In secondary hyperparathyroidism, the glands begin to secrete excess PTH in response to something else going on in the body. A diet that is low in calcium, high in phosphorus, or doesn’t have the correct ratio of calcium to phosphorus can cause nutritional secondary hyperparathyroidism. Or, PTH levels may sometimes rise due to kidney disease (i.e. renal secondary hyperparathyroidism), which we will discuss in more detail next.

#5. Kidney disease

Your dog’s kidneys play an important role in calcium regulation. Both PTH and calcitriol work on the kidneys to decrease calcium excretion. Additionally, the kidneys normally remove PTH from the body and also convert Vitamin D into calcitriol. When the kidneys begin to fail, several things happen.

One important change is that the diseased kidneys don’t make as much calcitriol. This means there is less calcitriol available to limit PTH production. Without this inhibition, PTH levels may rise.

Plus, because the kidneys aren’t filtering the blood as efficiently as they should be, blood phosphorus levels begin to rise. This also triggers the secretion of PTH because, while we hadn’t mentioned it yet, PTH also acts to lower blood phosphorus.

The end result of these changes is higher PTH levels and high blood calcium levels.

However, not all dogs with renal failure will have hypercalcemia. Depending on the stage of kidney disease in dogs and various underlying factors, dogs with kidney disease may have low calcium, normal calcium, or high calcium.

#6. Vitamin D toxicity

Vitamin D is a fat-soluble vitamin and an essential nutrient for dogs. Its active form, calcitriol, works to increase calcium and phosphorus levels in the body. However, too much Vitamin D can lead to hypercalcemia. Dogs may develop vitamin D toxicity from:

• Exposure to toxic plants (e.g. Day Jessamine, Solanum)
• Cholecalciferol rodenticide poisoning
• Eczema cream ingestion (contains vitamin D)
• Over-supplementation with Vitamin D (e.g. multi-vitamins for dogs or ingesting Vitamin D supplements for humans)

#7. Granulomatous disease

Fungal infections such as blastomycosis or histoplasmosis, or other granulomatous diseases, can sometimes lead to hypercalcemia. In these conditions, a particular type of white blood cell called macrophages plays a key role in the immune response. Interestingly enough, sometimes these macrophages can secrete calcitriol, which can lead to high calcium levels in the blood.

Typically, the hypercalcemia associated with granulomatous disease in dogs will resolve after treating the underlying condition.

#8. Spurious (i.e. artificially elevated)

Sometimes, the hypercalcemia that is discovered on your dog’s blood test is false (i.e. spurious). Calcium levels in dogs can be artificially elevated due to any of the following blood sampling issues:

• Lipemia—too many fatty particles (i.e. lipoproteins) in a sample
• Hemoconcentration—a high amount of red blood cells in a sample
• Hemolysis—red blood cells have broken down or ruptured in a blood sample

To avoid some of these issues, your veterinarian may recommend fasting your dog for several hours prior to collecting a blood sample. He or she may also use a larger needle during sample collection. Plus, the vet may recommend a test called an ionized calcium level to determine if the hypercalcemia is true or spurious (more on that in the diagnostics section).

#9. Idiopathic hypercalcemia

If there is an elevated calcium level with no identifiable explanation, the dog may be said to have idiopathic hypercalcemia. This is the rarest cause of high calcium in dogs and tends to occur more often in cats.

How does the vet track down the cause of the hypercalcemia?

Since there are a variety of different causes of hypercalcemia, the diagnostic process may vary from dog to dog. Once the veterinarian detects an elevated blood calcium level, he or she will discuss the next best diagnostic steps for your dog’s situation.

Sometimes the initial bloodwork that revealed the high calcium will also hold clues to the cause. For example, elevated kidney values (e.g. BUN, creatinine, and SDMA) may point to kidney disease. Or low sodium and high potassium may be indicators of Addison’s disease.

In other situations, the vet may recommend additional tests such as:

• Ionized calcium test to confirm that the hypercalcemia is real (Ionized calcium is the active form of calcium. It gives a better picture of calcium status than the total blood calcium level, which is what a general chemistry panel measures.)
• X-rays or ultrasound to look for evidence of fungal infections, bone infections, bone tumors or other tumors, or bladder stones
• PTH, PTHrP, or vitamin D levels to rule in/out hyperparathyroidism, hypercalcemia of malignancy, or Vitamin D toxicity
• Fungal serology to diagnose a fungal infection

Additionally, the vet may want to repeat the physical exam to check for anal gland masses, enlarged lymph nodes in dogs (which could point to lymphoma or inflammation), mammary masses, or other clues.

What is the treatment for hypercalcemia in dogs?

Your veterinarian will create a treatment plan for your dog based on the severity of the elevation in blood calcium, the dog’s clinical signs, and the underlying cause. Discussing the treatment for each condition is beyond the scope of this article, so we will just focus on the treatment of hypercalcemia itself.

Sometimes mildly elevated calcium levels will resolve themselves once the vet addresses the underlying condition. However, hypercalcemic dogs who are extremely ill (usually with a calcium level greater than 18 mg/dL) will require supportive care.

This could mean that your furry friend will need to spend some time in the hospital for fluid therapy and medications to help with heart arrhythmias, muscle twitches, vomiting, etc. The vet may also recommend steroids, diuretics, bisphosphonates, calcitonin, or other medications to help control blood calcium levels.

What is the prognosis for high calcium in dogs?

The good news is that when caught early, dogs with Addison’s disease and vitamin D toxicity can have a good prognosis and great quality of life long term. However, dogs with renal failure have a guarded prognosis, and dogs with hypercalcemia of malignancy tend to have a poor prognosis.

It is also good to keep in mind that any uncontrolled hypercalcemia, regardless of cause, can lead to kidney failure, urinary tract infections or stones, heart arrhythmias, or other issues that can impact a dog’s quality of life and life expectancy.

Back to Walter

As it turned out, Walter’s hypercalcemia was being caused by a small anal sac adenocarcinoma. Thankfully, there was no evidence that the cancer had spread to his lymph nodes, and the surgery to remove the tumor was a success. After the surgery, Walter’s hypercalcemia resolved, and so did his symptoms. Although she knew he was living on borrowed time, Walter’s mom was thrilled that he was back to his old self again.

Work with your veterinarian

As you have learned, many different conditions cause an elevated blood calcium. Hypercalcemia could mean that your dog has cancer or severe kidney failure. Or it might indicate something as simple as a vitamin D overdose or a problem with the blood sample.

While the waiting and the unknown can be scary, I want to remind you that your veterinarian is here to help you. He or she can guide you through the process of determining the cause of your dog’s hypercalcemia and developing a treatment plan. Don’t hesitate to reach out to him or her with any questions or concerns along the way.

What was the cause of your dog’s hypercalcemia?

Please comment below.