6. Urinalysis

Although profound polydipsia is not commonly noted in cats with hypercalcemia, urine concentration may be lower than expected (specific gravity < 1.040). Isosthenuria may indicate intrinsic renal failure, but it has been noted in cats with severe hypercalcemia for many reasons.

Perform a urine sediment examination in all cats with hypercalcemia. Calcium oxalate urolithiasis is commonly noted in hypercalcemic cats, and morbidity may occur from urinary tract obstruction and infection.^5,10,11

A three-view thoracic radiographic examination (right and left lateral plus ventrodorsal or dorsoventral) is appropriate since both primary and metastatic neoplasia may cause hypercalcemia. Pay particular attention to the mediastinal region, and scrutinize all bony structures for lytic lesions.

8. Abdominal radiography

Although soft tissue changes are more easily identified with ultrasonography than with abdominal radiography, you can readily identify calculi in the kidneys, ureters, or the urinary bladder by using abdominal radiography. In addition, distinctive lytic bony lesions may be noted in the vertebrae of cats with multiple myeloma.

9. Abdominal ultrasonography

A scan to look for organomegaly, lymphadenopathy, bladder tumors, or other masses is a logical part of the hypercalcemic work-up. Any lesions should be aspirated or biopsied.

As calcium oxalate uroliths are commonly noted in hypercalcemic cats, pay close attention to the renal pelves and ureters.^10,11 If you have any concerns about ureteral obstruction, an abdominal radiographic examination may assist with calculi detection.

10. PTH concentration

Serum PTH concentrations will be low in cats with neoplasia, vitamin D-related toxicosis, or osteolytic disease. Although elevated PTH concentrations are the hallmark of primary hyperparathyroidism, PTH concentrations must be interpreted in conjunction with iCa concentrations.⁸ In cats with appropriate feedback mechanisms, elevated iCa concentrations suppress PTH release. Consequently, high normal PTH concentrations are inappropriate in patients with elevated iCa concentrations. Cats with renal failure have variable PTH concentrations, but iCa concentrations are generally normal. Patients with idiopathic hypercalcemia are expected to have low normal PTH concentrations.

11. PTH-related protein concentration

Many tissues synthesize a protein with marked similarities to endogenous PTH. This protein is called parathyroid hormone-related protein (PTH-rP), and it binds with PTH receptors in bone and kidney. In normal animals, PTH-rP concentrations are essentially undetectable, but one of the mechanisms for paraneoplastic hypercalcemia is the release of PTH-rP by malignant tissues.¹²

In dogs, PTH-rP production has also been documented in association with inflammatory disorders, but that is not the case in cats.¹³ Although PTH-rP is not a highly sensitive marker for cancer in cats, it is regarded as specific. Consequently, an elevated concentration strongly suggests malignancy. Various types of carcinoma (e.g. squamous cell, bronchogenic, renal, thyroid) and lymphoma are most often associated with PTH-rP secretion in cats.^2,12

12. Vitamin D concentration

Vitamin D metabolism is complex, as dietary sources require hepatic and renal activation to become metabolically active. Elevated serum vitamin D concentrations indicate excessive dietary supplementation or accidental ingestion of a cholecalciferol rodenticide. However, the standard vitamin D assay only measures 25-hydroxycholecalciferol; some potent vitamin D products (e.g. calcipotriene, present in psoriasis cream; calcitriol, commonly used in cats with renal failure) are not detected. If vitamin D toxicosis is suspected but not substantiated by test results, carefully question owners. Also consider granulomatous diseases, as a high serum calcitriol concentration was documented in a cat with atypical mycobacterium infection and secondary hypercalcemia.¹⁴

Depending on the dose ingested, vitamin D toxicosis can cause profound hypercalcemia and hyperphosphatemia, with progressive and refractory acute renal failure.

Audrey K. Cook, BVM&S, MRCVS, DACVIM, DECVIM-CA
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
Texas A&M University
College Station, TX 77843