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Feline Primary Hyperparathyroidism: The Other Thyroid Condition You Should Know About

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Veterinarians frequently diagnose hyperthyroidism, or overactive thyroid gland, in their older feline patients. A less commonly diagnosed endocrine condition involves the parathyroid glands, which neighbor the thyroid gland and play a crucial role in your cat's health. Find out what these glands do, what happens when they malfunction, and what can be done if your cat is diagnosed with feline primary hyperparathyroidism.

Thyroid and Parathyroid Form and Function

Your cat's thyroid gland, which is one gland with two lobes, is located along the sides of your cat's trachea. A pair of smaller parathyroid glands is located at the upper portion of each lobe for a total of four parathyroid glands. Two of these glands are internal parathyroid glands, which are found within the thyroid gland, and the other two are external parathyroid glands, which lie just outside the lobes of the thyroid gland. The thyroid gland is responsible for producing T3 and T4 thyroid hormones to regulate your cat's metabolic function, which affects every organ and system in your cat's body. The parathyroid glands are responsible for producing parathyroid hormone, which is crucial for regulating your cat's blood calcium levels.

Blood Calcium Maintenance and Mayhem

Your cat's normal reference range for blood calcium falls within a narrow window of 8.2 to 10.8 mg/dL. When laboratory screenings reveal a result of 10.5 or greater, a cat is said to have hypercalcemia. Conversely, a result that is too low indicates hypocalcemia. In addition to maintaining strong bones, calcium is essential for blood clotting, muscle contraction, and enzymatic functions. If your cat's calcium level drops, the parathyroid glands respond by releasing parathyroid hormone to prompt the following corrective actions:

  • The kidneys produce more of an enzyme that increases the production of calcitriol, a hormone that enables the intestines to absorb larger amounts of calcium and phosphorus from food.
  • The kidneys also absorb more calcium when they filter waste.
  • The bones release calcium and phosphorus that they are storing into the bloodstream.

Once these actions take place, the parathyroid glands return to normal production levels of parathyroid hormone. However, when a cat has hyperparathyroidism, her parathyroid glands are on overdrive and produce too much parathyroid hormone.

Primary Hyperparathyroidism Source and Symptoms

Primary hyperparathyroidism is most commonly the result of a benign tumor on any one of the parathyroid glands. These benign tumors are called adenomas. On rare occasion, a malignant tumor, called a carcinoma, can be the culprit of primary hyperthyroidism. Enlargement of multiple parathyroid glands, known as parathyroid neoplasia, can also result in the abnormally high production of parathyroid hormone.

The average age for diagnosis of feline primary hyperthyroidism is 13 years. While the signs of hyperthyroidism include an increased appetite and an increase in activity levels, the opposite symptoms for hyperparathyroidism include the following:

  • Decrease in appetite
  • Vomiting
  • Lethargy

Cats that have developed calcium oxalate bladder stones as a result of high blood calcium levels may exhibit additional symptoms of straining to urinate, needing to urinate more frequently, inappropriate urination, and blood in the urine. Any of the symptoms of hyperparathyroidism are a red flag that something is amiss with your cat's health, and it needs to be investigated as soon as possible.

Hyperparathyroidism Tests and Treatment

During a physical examination in the animal clinic, your veterinarian will run his or her fingers along the sides of your cat's trachea to detect enlargement of the thyroid and parathyroid glands. The doctor will then order blood tests to evaluate your cat's blood calcium, serum ionized calcium, and parathyroid hormone levels. The normal reference range for a cat's ionized calcium levels is between 1.16 and 1.34 mg/dL, and the normal reference range for a cat's parathyroid hormone is between 0 and 40 mg/dL. If your veterinarian needs more information to make a definitive diagnosis, he or she may wish to perform an ultrasound to visualize your cat's thyroid and parathyroid glands. Once a diagnosis is made, treatment is imperative. The consequences of untreated hyperparathyroidism include damage to the kidneys, intestines, and brain.

The treatment for hyperparathyroidism is a parathyroidectomy, or surgical removal of the affected parathyroid gland. If your veterinarian recommends this procedure, pre-surgical tests and treatments will likely be required before admission into the animal hospital for the operation, particularly if your cat's calcium levels are excessively high. Such elevations can result in cardiac arrhythmias, hypertension and a low heart rate, all of which pose anesthetic complications. Once the surgery has been performed, your cat's blood parathyroid hormone level will drop temporarily, which puts your cat at risk for hypokalemia. Diligent post-surgical monitoring of your cat's calcium level will be critical for a few weeks while the remaining parathyroid glands adjust to produce the optimal amount of parathyroid hormone, and calcium and vitamin D supplementation may need to be administered during this time.

Although a parathyroidectomy is not without risks, your cat's life is in greater peril if hyperparathyroidism is left untreated. Addressing your cat's symptoms as soon as you observe them and working together with a vet, like those at Parkview Animal Hospital, before and after surgery will provide your cat with the best prognosis.