Q & A: Hypercalcemia in an Older Lab - Is This Primary Hyperparathyroidism?

My patient is an 11-year-old spayed female Lab who has polyuria, polydipsia, and weight loss but is otherwise normal. Diagnostic testing has been relatively normal except that she has isosthenuria, hypercalcemia, and hypophosphatemia. These abnormalities are repeatable. 

Her serum parathyroid hormone (PTH) concentration is high (32.3, normal 3.0-17.0), ionized calcium is high (1.92, normal 1.25-1.45), and the parathyroid hormone-related polypeptide (PTHrp) is negative (0.0, normal 0.0-1.0). No gross evidence of perianal masses are seen including on rectal exam, and no lymph node enlargement can be palpated.

Will this be enough evidence to say that this dog definitely has primary hyperparathyroidism? Or should we do more to rule out other malignancy as the cause of hypercalcemia in this dog?

My Response:

Yes, it certainly does appear that your dog has primary hyperparathyroidism. I'd recommend chest radiographs and abdominal ultrasound to complete the workup, but they will likely be normal.

If you can do a parathyroid ultrasound, that would be recommended especially if surgical parathyroidectomy is planned.

Next Question:

Sorry to be a bother, but no chance that this dog couldn't have hypercalcemia of malignancy?

How about renal renal secondary hyperparathyroidism or nutritional secondary hyperparathyroidism?

My Response:

Hypercalcemia of malignancy is possible but extremely unlikely in this dog. While hypercalcemia of malignancy does not always produce high levels of PTH-rp, the PTH concentration should be low in that situation, not high as is the case in your dog.

Unless the dog is eating some bizarre diet, we can rule out nutritional disease in a dog of this age, and I assume the dog is not in renal failure at this time, which largely eliminates that consideration. So primary hyperparathyroid disease remains the most likely by far.

Follow-up and Final Diagnosis:

A single, parathyroid mass was identified and removed at time of exploratory surgery. Results of histopathology confirmed parathyroid adenoma.

The dog developed iatrogenic hypoparathyroidism and hypocalcemia during the immediate post-operative period. But now, 2 months after surgery, the serum calcium concentration is normal without any calcium or vitamin D supplementation.