Both dogs and cats have a total of four parathyroid glands: two on each side of the neck. The parathyroid glands are attached to the surface or are imbedded within the thyroid glands. One parathyroid gland is located on the top pole (end) of the thyroid gland, and the other is located on the bottom pole. Normally these glands are about 2 to 3 mm in diameter. The glands produce parathyroid hormone, which causes the calcium level in the blood to increase. Because many electrical systems of the body’s organs such as the kidneys, bowels, muscle and brain are totally dependent on calcium, a change in the normal level of this important electrolyte in the blood can be very harmful to the pet. In addition, high calcium levels can cause stones to form in the urine.
Parathyroid tumors are uncommon in dogs and much less common in cats. These tumors usually are benign, meaning that they usually do not invade into nearby tissues (metastasize). They usually are very small tumors that produce excessive amounts of parathyroid hormone. Multiple parathyroid tumors are found in about 10% of affected patients. Breeds that are most predisposed to develop parathyroid tumors include Keeshonds, Labrador retrievers, and German shepherds.
Signs and diagnosis
Signs of hyperparathyroidism develop as a result of abnormally high calcium levels in the blood. Early signs of this condition include increased thirst and urination. Subsequently, lethargy, weakness and poor appetite may be noted. Signs such as straining to urinate, passage of blood-tinged urine or the inability to pass urine may be caused by calcium-based urinary stones. Cats may have diarrhea and vomiting as additional signs.
Because the relatively small parathyroid tumors are located deeply within the neck in dogs, they usually cannot be palpated. In cats, however, these same tumors can be palpated along the side of the neck. Ultrasound of the neck frequently can identify these small tumors. In addition, ultrasound is used to check the bladder and kidneys for stones. Chest X-rays are used to identify visible spread of the cancer; however, microscopic spread of the tumor to other organs cannot be detected with this test.
A complete blood count, chemistry profile and urine testing are completed in the initial stages of the evaluation. Blood tests showing an elevation of the calcium level may be due to a parathyroid tumor; however, there are many other diseases that may elevate the calcium in the blood. To confirm a diagnosis of a functioning parathyroid tumor, a parathyroid hormone level is measured from a blood sample. The blood work will also be used to check for damage to internal organs that may have occurred from the high blood calcium levels.
Initial evaluation of the patient will 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 may be needed prior to removal of the parathyroid tumor. If your companion’s blood calcium level is greater than 14 mg/dl prior to surgery, there is a great risk that the calcium level will fall well below the normal range and cause seizures and other signs. We therefore recommend prophylactic treatment with a medication, calcitriol, starting two days prior to surgery.
Surgery is performed by making an incision on the underside of the neck. The parathyroid tumor is removed with a relatively simple procedure.
The overall prognosis for your pet should be favorable following treatment of a parathyroid tumor, as this disease can be cured. Yet left untreated, it can cause irreversible damage to internal organs.
For more information on this subject, speak to the veterinarian who is treating your pet.