High calcium levels in dogs, or hypercalcemia, is caused by conditions such as cancer, kidney failure, vitamin D poisoning, aluminum toxicity and diseases that cause bone deterioration, according to PetMD. Other causes include over-functioning of the parathyroid gland or an under-functioning adrenal gland.
According to veterinarian Dr. Mike Paul for Pet Health Network, unless the dog is given supplements containing calcium, diet is unlikely to be the cause of excess calcium in dogs.
PetMD explains that some common symptoms of hypercalcemia include depression, confusion, lack of appetite, constipation, vomiting, increased thirst and urination and enlarged lymph nodes in the neck. Hypercalcemia is diagnosed with a blood count, blood chemistry analysis and a urinalysis. Additional testing might be necessary to find the underlying cause of high calcium levels.