Cancer is the most common cause of high calcium in dogs, accounting for 45% to 65% of cases. But it’s far from the only one. Several other conditions, from overactive parathyroid glands to accidental poisoning, can push calcium levels above the normal range of 9.3 to 11.5 mg/dL. Understanding the possible causes helps you have a more informed conversation with your vet about what’s going on and what comes next.
How Cancer Raises Calcium Levels
When a tumor is responsible for high calcium, the condition is called hypercalcemia of malignancy. The two most common culprits are lymphoma (usually the T-cell type) and anal gland cancer (adenocarcinoma of the apocrine glands of the anal sac). Other cancers linked to elevated calcium include thyroid carcinoma, multiple myeloma, bone tumors, mammary gland tumors, melanoma, and lung cancers, though these are less frequent.
Most of these tumors raise calcium through a specific trick: they produce a protein called PTHrP that mimics parathyroid hormone, the body’s natural calcium regulator. This protein tells the bones to release calcium into the bloodstream and tells the kidneys to hold onto it rather than filtering it out. In anal gland cancer, PTHrP is the primary driver, with 50% to 70% of affected dogs developing high calcium that correlates directly with the protein’s blood levels. In lymphoma, PTHrP works alongside other inflammatory signals, making the relationship less straightforward.
High calcium is sometimes the first clue that a hidden tumor exists. Your vet may discover it on routine bloodwork before any other signs of cancer appear.
Primary Hyperparathyroidism
Dogs have four tiny parathyroid glands in their neck that regulate calcium. In primary hyperparathyroidism, one or more of these glands develops a benign growth (adenoma) or, rarely, a cancerous one, causing it to pump out too much parathyroid hormone. That hormone pulls calcium from bones and reduces how much the kidneys excrete, steadily raising blood calcium levels.
A study of 210 dogs with this condition found certain breeds were overrepresented. Keeshonds made up 20% of cases, followed by mixed breeds (14%), Labrador Retrievers (9%), German Shepherd Dogs (6%), and Golden Retrievers (6%). Shih Tzus, Poodles, Springer Spaniels, Australian Shepherds, and Cocker Spaniels also appeared more often than expected. If your dog belongs to one of these breeds and has persistent calcium above 12 mg/dL without another obvious cause, this diagnosis is worth investigating.
Vitamin D Poisoning
Vitamin D toxicity is one of the more urgent causes of high calcium because it can escalate quickly. The most common source is cholecalciferol rodenticide, a type of rat or mouse poison that works by flooding the body with vitamin D. Dogs that eat these baits absorb massive amounts of the vitamin, which supercharges calcium absorption from the gut and pulls it from bones simultaneously. Even small amounts of these products can be dangerous.
Other sources include certain psoriasis creams containing vitamin D analogs, dietary supplements left within reach, and, in rare cases, commercially manufactured dog foods with formulation errors. If your dog recently got into something they shouldn’t have and bloodwork shows spiking calcium, vitamin D poisoning should be considered immediately. The calcium levels in these cases need close monitoring because they can remain elevated for weeks as the body slowly clears the stored vitamin D.
Addison’s Disease
Addison’s disease, or hypoadrenocorticism, occurs when the adrenal glands stop producing enough hormones. It’s primarily caused by the immune system attacking the adrenal cortex. About 34.5% of dogs with Addison’s disease develop high calcium at the time of diagnosis.
The mechanism isn’t a single simple pathway. When adrenal hormones drop, dogs become severely dehydrated, which concentrates calcium in the blood. At the same time, the kidneys lose their ability to filter efficiently, so calcium that would normally be excreted stays in circulation. Metabolic acidosis, a shift in the blood’s acid-base balance, also frees calcium from its binding proteins, making more of it active. The good news is that once Addison’s disease is treated and hydration is restored, calcium levels typically come back down.
Kidney Disease
Chronic kidney disease (CKD) creates a complicated feedback loop involving calcium, phosphorus, and vitamin D. As kidney function declines, phosphorus builds up because the kidneys can’t clear it. The damaged kidneys also produce less calcitriol, the active form of vitamin D, which the body needs to properly regulate calcium. In response, the parathyroid glands ramp up hormone production to compensate, a condition called renal secondary hyperparathyroidism. This process begins early in kidney disease, sometimes before calcium or phosphorus levels on bloodwork even look abnormal.
The calcium picture in kidney disease is mixed. Some dogs develop low calcium, while others develop high calcium, depending on how advanced the disease is and whether the parathyroid glands have been overcompensating for a long time. Ironically, vitamin D supplements sometimes used to manage kidney disease can themselves push calcium too high. In one study, over half the dogs treated with a vitamin D compound for kidney-related parathyroid problems developed mild hypercalcemia during treatment.
Fungal and Granulomatous Infections
Certain infections, particularly fungal ones, trigger a type of inflammation called granulomatous disease. Inside these inflamed areas, immune cells called macrophages begin producing their own active vitamin D, bypassing the kidneys entirely. This extra vitamin D increases calcium absorption from the gut and raises blood levels.
In dogs, this has been documented with blastomycosis and a handful of rarer fungal infections. A recent case report also identified it in a dog with pythiosis, a water mold infection. The important detail is that the fungus itself isn’t producing the calcium spike. The body’s own immune response to the infection is the problem. Once the infection is controlled and the granulomatous inflammation resolves, calcium levels typically normalize.
Symptoms of High Calcium in Dogs
The classic signs are increased thirst and increased urination. High calcium interferes with the kidneys’ ability to concentrate urine. The kidneys essentially lose their responsiveness to the hormone that normally tells them to conserve water, so they produce large volumes of dilute urine regardless of how hydrated the dog is. The increased urination then drives excessive thirst as the body tries to keep up with fluid losses.
Beyond the drinking and urination changes, you might notice decreased appetite, lethargy, vomiting, or weight loss. Some dogs develop bladder or kidney stones because the excess calcium being filtered through the kidneys crystallizes in the urinary tract. If calcium remains high for a prolonged period, it can deposit in kidney tissue itself, causing lasting damage. Many dogs with mild elevations, however, show no obvious symptoms at all, which is why the finding often comes as a surprise on routine bloodwork.
How Vets Identify the Cause
Finding high total calcium is just the starting point. Your vet will likely want to measure ionized calcium, the biologically active form, to confirm the elevation is real. Total calcium above 12 mg/dL in a dog without high phosphorus predicts true ionized hypercalcemia with about 93% accuracy.
From there, the diagnostic workup typically involves drawing blood for parathyroid hormone (PTH) and PTHrP levels from the same sample. The pattern of these two hormones points in different directions. If PTH is normal or high while PTHrP is undetectable, the parathyroid glands themselves are the problem. If PTH is suppressed and PTHrP is elevated, a hidden tumor is likely producing it. Your vet may also check vitamin D metabolites, kidney values, and electrolytes to round out the picture.
Depending on results, imaging comes next. Neck ultrasound can reveal enlarged parathyroid glands. Chest X-rays and abdominal ultrasound help screen for tumors. A rectal exam checks for anal gland masses, one of the most common and easily overlooked cancer-related causes. For lymphoma, enlarged lymph nodes may be sampled with a needle for quick analysis.
In cats, idiopathic hypercalcemia (high calcium with no identifiable cause) accounts for about 12.6% of cases. In dogs, this diagnosis is significantly less common. Most dogs with persistent high calcium will eventually have a cause identified through thorough testing.