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, the definitive test done by your primary care veterinarian is the measure of PTH in the bloodstream. This is a very specific test that measures two types of parathyroid hormone, PTH and PTHrP (PTH related-peptide). 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 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 healthy parathyroid glands is suppressed by hypercalcemia.
The final step in the diagnosis of a parathyroid tumor is ultrasound imaging of the parathyroid glands. This is a very sensitive test and an experienced ultrasonographer can easily detect normal parathyroid glands associated with thyroid glands in the neck of a dog. Parathyroid tumors are usually single and appear as a spherical enlargement of the parathyroid gland. Typical tumors are 2–4 times the size of normal parathyroid glands.