Parathyroid Tumors

By Christopher Pinard, DVM

Tumors, Pet Services

What are parathyroid glands? 

Parathyroid glands are small hormone-secreting glands that are located on or near the thyroid glands in the neck region. These glands are responsible for producing specialized chemicals (or hormones) that regulate the concentration of calcium in the blood. Calcium is very important for the contraction of muscles (including the heart) and maintaining healthy bones.

What is a parathyroid tumor?

A parathyroid tumor develops as a result of replication or growth of cells in the parathyroid gland. Parathyroid tumors tend to be benign (non-cancerous). Benign tumors are called adenomas. A malignant form of tumor, called parathyroid carcinoma, has been described, but is rare. Parathyroid tumors are uncommon in cats and dogs.

What causes this cancer?

The reason why a particular pet may develop this, or any tumor or cancer, is not always straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors. Although there are no known dietary or environmental causes for parathyroid tumors, there appears to be a genetic predisposition in Keeshonds. No breed or genetic predispositions have been described in cats.

What are the signs of a parathyroid tumor?

The signs of a parathyroid tumor are related to a condition known as hypercalcemia. Parathyroid tumors cause an overproduction of parathyroid hormone (PTH), a hormone that regulates the circulating level of blood calcium, causing it to rise. This condition is known as hypercalcemia. With hypercalcemia, your pet may become anorexic, start vomiting, drink and urinate more, and show signs of weakness, muscle trembling, and general lethargy.

How is this type of tumor diagnosed?

Since parathyroid tumors are small and located deep in the neck, there are usually no external signs to look for, and it is unlikely that your veterinarian may feel (palpate) a mass on physical examination. The most common way in which a tumor is diagnosed is through blood testing, finding hypercalcemia, and following up with specialized bloodwork and ultrasound. If a parathyroid tumor is suspected, whether because of hypercalcemia detected in a blood screen and/or the clinical signs of hypercalcemia, the definitive test to determine the presence of a parathyroid tumor is to measure the level of PTH in the bloodstream. The final step in the diagnosis of a parathyroid tumor is ultrasound imaging of the parathyroid glands, finding a spherical enlargement of a gland.

How does this type of tumor typically progress?

Both the benign and malignant forms of parathyroid tumor are locally invasive, meaning they grow locally with little evidence of spread to other organs. The metastatic (spread) rate is low, however metastasis is known to occur with parathyroid carcinomas. Regardless, if left untreated, your pet will continue to experience the clinical signs of hypercalcemia. Prolonged elevated calcium in the bloodstream can cause permanent kidney damage, as well as other health problems.

What are the treatments for this type of tumor?

Once other causes of elevated blood calcium have been ruled out and a diagnosis of a parathyroid tumor has been made, surgery is typically pursued. Ultrasound is useful to specifically locate the tumor prior to surgery. Usually the affected parathyroid gland is excised directly from the thyroid gland, sparing the thyroid gland and leaving the remaining parathyroid glands in place. After surgery, your veterinarian will carefully monitor the blood calcium level and supplement with calcium or vitamin D if the level drops too low, as low blood calcium (hypocalcemia) can lead to serious complications. Supplementation is then gradually weaned as the remaining parathyroid glands begin to function normally and regain control of blood calcium levels.

Another possible treatment is ultrasound-guided ablation, either ethanol ablation or heat ablation. Although ultrasound-guided ablation is less invasive, surgery seems to have the highest success and lowest rate of complications.

Is there anything else I should know?

The prognosis for long-term survival after surgery is good. Since parathyroid tumors are usually benign, excision is usually curative. Post-surgical hypocalcemia can cause serious, even fatal, complications, so your pet will need careful monitoring during the first few days after surgery until the remaining parathyroid glands return to normal function.

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