Hypercalcemia is an abnormally high level of calcium in the blood (greater than 11 mg/dl). Calcium is the most abundant mineral found in the body, approximately 99% is found in bone and the remaining 1% in extra cellular fluid (fluid found outside of the cells and between the cells in body tissues). It is essential for many important functions including providing strength to bones and teeth, cardiac function, proper nerve impulses and muscle contractions, blood clotting, cell growth and division and hormone secretion. In combination with phosphorus, it forms calcium phosphate, the dense, hard material of bones and teeth. It is stored in the skeleton and released as it is required.
Cats any age can develop hypercalcemia. In one review of 427 cats with idiopathic hypercalcemia, the average age being around 9.8 years of age, long-haired cats were over-represented and both genders were equally represented.
What causes hypercalcemia in cats?
The most common cause of hypercalcemia is idiopathic (IHC), meaning no underlying cause can be found. Most cases of hypercalcemia are due to increased gastrointestinal uptake from the food or excessive mobilisation of stored calcium from the bones. Some diseases which can lead to hypercalcemia (in order of incidence) include:
Acute or chronic renal failure.
Cancer (lymphoma, squamous cell carcinoma, multiple myeloma).
Primary hyperparathyroidism (a condition where the parathyroid gland produces too much parathyroid hormone).
Less common causes include:
Ingestion of certain houseplants.
Vitamin D toxicity, generally caused by over supplementation or rodenticide poisoning.
Cats are more resistant to the clinical consequences of hypercalcemia than dogs and many may remain asymptomatic. Often hypercalcemia is discovered only during routine blood tests.
All organs of the body can be affected by hypercalcemia, however, most symptoms are related to neuromuscular, gastrointestinal, kidney or the heart. When symptoms do occur, they are typically the following:
Urinalysis. High urine calcium can be indicative of parathyroidism. Low urine calcium can be caused by hypocalciuric hypercalcemia.
Serum ionized calcium (iCA): Ionized calcium is calcium that is freely flowing in your blood and not attached to proteins. It is also called free calcium.
Obviously, the goal is to find out what is causing hypercalcemia and further investigative tests will be required to determine this. These may include:
Ultrasound of the parathyroid glands.
ACTH stimulation test for Addison’s disease: This test measures the ability of the adrenal glands to respond to a hormone known as adrenocorticotropic hormone (ACTH) which is made in the pituitary gland, travelling through the bloodstream to the adrenal glands where it stimulates the secretion of other hormones such as hydrocortisone from the cortex. The ACTH stimulation test measures levels of cortisol in the blood before and after an injection of synthetic ACTH.
Serum parathyroid hormone concentration: To check levels of parathyroid hormone (PTH) in the blood.
Parathyroid hormone-related peptide (PTHrP) is a protein secreted by some cancers.
X-Ray to look for calcium oxalate uroliths and cancers.
Fine needle aspirates from the lymph nodes to evaluate for lymphoma.
Blood test to check vitamin D levels.
How is hypercalcemia in cats treated?
Treatment depends on the severity and cause of hypercalcemia. Obviously treating the cause if one is found is the first course of action. Managing symptoms related to high calcium levels.
Loop diuretics such as Furosemide (Lasix®) to increase calcium excretion from the kidneys. Loop refers to the drug’s action on the loop of Henlé, a structure of the kidney involved in reabsorbing water.
Glucocorticoids such as prednisone to decrease bone resorption.
Sodium bicarbonate helps decrease serum calcium levels by increasing the alkaline level of the blood. This helps to shift the ionized calcium into protein-bound calcium, which is less harmful.
Cats with primary hyperparathyroidism may require surgery to remove the abnormal parathyroid gland.
There are other medications including; diphosphonates which inhibit bone resorption, calcitonin which inhibits bone resorption and mithramycin which inhibits osteoclastic bone resorption.
For mild cases of hypercalcemia, dietary changes may be recommended such as switching to a high fibre diet and/or a diet designed for cats suffering from chronic kidney disease.