The parathyroid glands are small (1/4 inch diameter), flat glands that play a very important role in maintaining the blood calcium concentration in dogs and cats. Chemical sensors within the parathyroid glands monitor blood calcium levels and if the calcium levels decrease, the glands secrete parathyroid hormone (PTH). PTH acts on the kidneys, intestines, and bones to increase the amount of calcium in the bloodstream. There are usually four parathyroid glands in most mammals, two on either side of the throat, closely associated with the thyroid glands (hence the name, parathyroid). Tumors of the parathyroid glands are uncommon; however they can produce serious problems in dogs and cats if the tumors secrete excessive, unregulated amounts of PTH. Excessive PTH causes elevated levels of blood calcium which can have toxic effects on the kidneys, the intestines, and the brain.
Parathyroid tumors have been reported in many different breeds of dogs and cats. There is no known dietary or environmental cause; in most cases the occurrence seems to be random circumstance. However, certain breeds of dogs appear to be more at risk. A genetic predisposition for parathyroid tumors has been found in Keeshonds. Keeshonds with parathyroid tumors should not be bred.
Since parathyroid tumors are small and are located deep in the neck, there are usually no external signs to prompt an owner that a dog might have a parathyroid tumor. Instead, it is the clinical signs caused by excessive calcium levels in the bloodstream (hypercalcemia) that prompts an owner to seek veterinary attention. Hypercalcemia can cause dysfunction in many organs, especially the kidneys. Elevated calcium levels in the blood interfere with the ability of the kidneys to concentrate urine so affected pets urinate frequently and drink more water to compensate for the fluid loss from frequent urination. As more calcium passes out through the kidneys into the urine, calcium-based urinary stones (urolithiasis) can appear in the urinary bladder, ureters, or kidneys. Hypercalcemia also affects intestinal function; some pets may vomit, become constipated, or lose their appetite. Affected pets show signs of weakness as hypercalcemia interferes with normal muscle function. If hypercalcemia persists long enough, calcium-phosphate complexes will develop in the tissues of the kidney and other organs, causing permanent damage.
The most common way in which parathyroid tumors are diagnosed is through routine blood testing done as part of the yearly health evaluation of an older dog or cat. The early signs of hyperparathyroidism are very subtle and may be attributed to old age by a pet owner. However, if hypercalcemia is detected in a blood evaluation, then further testing is warranted to determine the cause.
If a parathyroid tumor is suspected in your pet, whether because of hypercalcemia detected in a blood screen or because of clinical signs related to hypercalcemia, confirmation of the disease requires measurement of the amount of PTH in the blood. This test may also be combined with another test measuring the level of PTH-related peptide (PTH-rP) in the blood. Both PTH and PTHrP cause elevation of calcium in the bloodstream. However, PTH is uniquely produced by the parathyroid glands, while PTHrP is released from certain other types of cancers, such as lymphoma, multiple myeloma, and anal sac adenocarcinoma. If PTHrP is elevated in the presence of hypercalcemia, then a diagnosis of a non-parathyroid tumor is supported. However, if PTHrP is non-detectable and PTH is in the normal or elevated range, then a parathyroid tumor is strongly suspected. It is important to note that a normal blood concentration of PTH in the presence of hypercalcemia is still “abnormal” and suggests poorly regulated hormone secretion by a parathyroid tumor. PTH secretion by normal parathyroid glands is suppressed by hypercalcemia.
Ultrasonography of the parathyroid glands is frequently performed in dogs to confirm the location and number of parathyroid tumors. Tumors normally appear as spherical enlargement of the parathyroid gland and maybe 2–4 times the size of normal parathyroid glands. Ultrasonography may be more difficult to perform in cats due to the small size of some tumors and may be further complicated in older cats because of the prevalence of nodular thyroid lesions. More advanced imaging using contrast-enhanced CT is being used to diagnose and characterize parathyroid disease in dogs and when looking for ectopic parathyroid tissue.
There are two treatment options for parathyroid tumors:
- surgical excision
- ultrasound-guided (ethanol) ablation
Both procedures require general anesthesia, so a thorough evaluation of the pet must be done to determine if the high calcium levels have damaged the kidneys, heart, or nervous system. If this is the case, treatment with medication and intravenous fluids will be needed prior to treatment. If blood calcium levels are significantly elevated, there may be an increased risk of cardiac arrhythmias and blood pressure issues during anesthesia.
Surgical excision is done through a surgical incision in the ventral neck just behind the throat. The ultrasound images are used to direct the surgeon to the location of the parathyroid tumor(s). Usually, a parathyroid tumor can be excised directly from the thyroid gland, sparing the thyroid gland and leaving the remaining parathyroid glands in place. The excised parathyroid tumor can be submitted for biopsy.
Ultrasound-guided ethanol ablation(chemical ablation) is a minimally invasive alternative to surgical incision and may be used in high-risk patients. The procedure requires general anesthesia to ensure that there is no movement during the ablation. Using ultrasound imaging, a needle is guided into the parathyroid tumor, and ethanol is injected into the parathyroid tumor. The ethanol is toxic to the parathyroid tumor and causes rapid necrosis, destroying the tumor. Care must be taken so that the ethanol does not leak out of the parathyroid capsule and damage the surrounding nerves. There is a possibility that the ethanol will not destroy the entire tumor, and the procedure may need to be repeated. This procedure does not allow for a biopsy of the parathyroid tumor.
After removal of a functional parathyroid tumor, the hypercalcemia will resolve rapidly. The remaining parathyroid glands will have been suppressed by the hypercalcemia, and it usually takes a few days (or longer) for the remaining normal parathyroid glands to regain function and start producing PTH. It is possible that calcium levels will become lower than normal during this period, and regular monitoring of blood calcium is important post-treatment. Dogs that have a high calcium level prior to surgery are at greater risk of the calcium falling well below the normal range. Warning signs of this problem can include rubbing the face, dilating pupils, muscle twitching, loss of appetite, weakness, and seizures. To prevent hypocalcemia, prophylactic treatment with a medication, calcitriol, is often started on the day of treatment. The medication is gradually weaned as the remaining parathyroid glands begin to function normally and regain control of blood calcium levels.
The overall prognosis should be good following treatment of a parathyroid tumor as this disease can usually be cured, but if left untreated, it can cause irreversible damage to the internal organs.
Premier Sponsors
