We’ve talked before about hyperthyroidism in cats and hypothyroidism in dogs. Sitting very close to the thyroid gland are four smaller glands called parathyroid glands. Though they live in close proximity to the thyroid gland, their purpose is completely different. Just as the thyroid gland produces thyroid hormone, the parathyroid gland produces parathyroid hormone (abbreviated PTH). This hormone maintains calcium levels in the body—a very important job, indeed.
You may recall from our blog on hypercalcemia that calcium is necessary for proper cell function, and this is especially true for the cells in your pet’s neurologic and muscular systems. Muscle cells depend on calcium to contract, from the skeletal muscles that power your pet’s limbs to the smooth muscles that move food down the esophagus and through the intestines.
Calcium and phosphorus are in a very delicate balance in the body. They are maintained closely at a ratio of about 2:1—disturbances in this balance are detrimental to the kidneys and other organs. The parathyroid gland monitors this balance and, if needed, releases parathyroid hormone (PTH) to keep the ratio in check. If low calcium levels are detected, PTH is released to increase calcium levels by releasing it from bones. PTH also alerts the gastrointestinal system to absorb more calcium and tells the kidneys to conserve it as well. The same thing happens if high phosphorus levels are detected. In this way, calcium levels are increased to maintain the 2:1 ratio.
Just as the thyroid gland can be overactive (as is the case in cats with hyperthyroidism), the parathyroid gland can also be affected by hyperparathyroidism. There are several types of hyperparathryoidism, but in all cases, the parathyroid gland secretes too much parathyroid hormone, with dangerous effects on the calcium/phosphorus balance.
Primary hyperparathyroidism is pretty rare in cats and dogs, but is usually caused by a solitary benign parathyroid gland tumor. Excess PTH causes hypercalcemia in affected animals. Because clinical signs of hyperparathyroidism are often absent, it is routine blood work that picks up increased calcium levels and eventually leads to a diagnosis of hyperparathyroidism. When clinical signs are present, they are generally due to hypercalcemia and include weakness, increased water intake and urination, and bladder stones.
Pets affected by primary hyperthyroidism are typically older—between 6 and 17 years of age, and some breed predilections exist. Siamese cats are predisposed, as are German Shepherds, Keeshonds, Australian Shepherds, Dachshunds, Golden Retrievers, Labrador Retrievers, Poodles, Rhodesian Ridgebacks, and Siberian Huskies.
Treatment of primary hyperthyroidism requires surgical removal of the abnormal parathyroid tissue. If this isn’t possible, blood calcium levels should be closely monitored for the life of the affected pet.
Secondary hyperparathyroidism can occur because of nutritional deficiencies or kidney disease. In these cases, calcium levels are usually normal or low, but parathyroid levels are increased in order to maintain the calcium/phosphorus balance in relation to high phosphorus. The treatment of secondary hyperparathyroidism centers on correcting the underlying nutritional deficiency or managing underlying kidney disease.