Calcium is a very important electrolyte in the body. It not only gives structure to our teeth and bones, but it also plays a vital role as a messenger to cells, controlling muscle contraction, nerve conduction, and blood coagulation. Too much of a good thing can be bad, though.
Hypercalcemia is the state in which there is an increased amount of calcium circulating through the body. It’s not a disease in and of itself; it’s a symptom that something else is wrong. It can occur in dogs or cats (or any mammal, including us!), but thankfully, it is relatively rare.
Before we talk about what causes hypercalcemia, I thought it would be useful to first address how calcium is regulated in the body. It’s quite a complicated (but brilliant) system. Calcium is regulated in three ways:
Vitamin D increases the formation of a calcium binding protein in the intestines. This protein functions to absorb calcium into the bloodstream. Too much vitamin D will cause hypercalcemia.
Parathyroid hormone (PTH). PTH is secreted by the parathyroid gland. It increases calcium and phosphorus mobilization from skeletal bones.
Calcitonin has a mild blood calcium lowering effect.
These three regulators work continuously to ensure that blood calcium levels are normal.
The most common cause of hypercalcemia in dogs and cats is a condition known as hypercalcemia of malignancy. Some tumors, especially lymphosarcomas and adenocarcinomas, produce substances that affect bone resorption of calcium, resulting in abnormally high calcium levels.
Other reasons for hypercalcemia include:
Hypoadrenocorticism (Addison’s disease) in dogs.
Cholecaliferol-based rodenticides. Some rat poisons increase the intestinal absorption of calcium.
Conditions causing skeletal lesions, such as bone infections and hypertrophic osteodystrophy.
Excess calcium supplementation.
In cats, a condition called idiopathic hypercalcemia is becoming more common. An underlying cause for increased calcium is unknown. Often, these cats have no symptoms.
Clinical signs of hypercalcemia may or may not be present. In mild cases, clinical signs are often not observed. When clinical signs are noted, patients usually suffer from decreased appetite, vomiting, increased thirst, increased urination, constipation, and depression. Bladder stones and muscle twitching can also occur. In cases where hypercalcemia is an acute and rapidly occurring issue, death can occur.
High calcium levels will show up on some routine blood work, though it should be double-checked. There are a few factors that can cause spurious results, so before a big work up for high calcium is pursued, high calcium levels should be confirmed.
Treatment of high calcium depends on the clinical signs, the underlying cause, and the onset (whether hypercalcemia is sudden onset or chronic). Generally, removal or treatment of the underlying cause will resolve the hypercalcemia, though this isn’t always possible. Supportive treatments include fluid therapy, diet change, medications that target calcium release from the bones, and the use of diuretics and steroids.
Luckily, hypercalcemia is rare. But if it has been found in your pet, be prepared for a thorough search for the underlying cause before treatment can be initiated.