Rare Case: Cat Survives Synchronous Bilateral Parathyroid Carcinomas

A 14-year-old neutered male Russian Blue cat has become one of the rarest documented cases in veterinary endocrinology: a feline patient diagnosed with primary hyperparathyroidism driven not by one, but by two simultaneous parathyroid carcinomas, one on each side of the neck. The case, reported in Frontiers in Veterinary Science, describes how subtle histopathologic clues—easily missed on preoperative imaging and cytology—ultimately revealed the malignant nature of both tumors, and how careful surgical planning tailored to each lesion’s anatomy led to an apparently successful outcome at five months of follow-up.

Primary hyperparathyroidism is already an uncommon diagnosis in cats. It is most often caused by a functional parathyroid adenoma, a benign tumor that secretes excess parathyroid hormone and drives blood calcium to abnormally high levels. Parathyroid carcinoma is exceedingly rare in the species, and bilateral parathyroid neoplasia is exceptional. Most reported feline cases involve solitary, unilateral lesions, which is what makes this cat’s presentation so unusual. The cat was brought to a veterinary hospital with a ten-day history of decreased appetite and reduced activity, along with vomiting once or twice daily. Diarrhea was absent, and whether the cat was drinking and urinating excessively could not be reliably determined from the owner’s observations.

Bloodwork told a striking story. Serum biochemistry revealed azotemia, with creatinine at 3.2 mg/dL against a reference interval of 0.7 to 2.1 mg/dL, along with hypercalcemia—ionized calcium measured 1.90 mmol/L (reference interval 1.11 to 1.46 mmol/L) and total calcium 14.3 mg/dL (reference interval 8.9 to 12.6 mg/dL)—and hypophosphatemia, with phosphorus at 2.2 mg/dL. Serum symmetric dimethylarginine, a kidney function marker, was also increased at 17 μg/dL. Because chronic kidney disease can itself raise calcium, the clinicians had to work through a differential diagnosis that included primary hyperparathyroidism, idiopathic hypercalcemia, neoplasia, vitamin D toxicosis, and renal disease. On reevaluation, the persistent hypercalcemia occurred alongside an elevated intact parathyroid hormone concentration of 18.6 pg/mL (reference interval 2.0 to 14.0 pg/mL), a combination that supported a diagnosis of primary hyperparathyroidism. Serum 25-hydroxyvitamin D was 83.6 ng/mL, classified as insufficient by the laboratory and not suggestive of vitamin D toxicosis.

Imaging then localized the problem—and complicated it. Thoracic and abdominal radiographs and abdominal ultrasonography were unremarkable, but cervical ultrasonography revealed an oval cystic mass protruding from the left thyroid lobe containing a well-defined hypoechoic nodule measuring 8.4 by 5.2 mm in the left parathyroid region. On the right side, ultrasound showed a well-circumscribed heterogeneous mass of 10.0 by 13.0 mm adjacent to the thyroid lobe, with solid and cystic components. Contrast-enhanced computed tomography refined the picture: the left mass measured 15.2 by 9.3 by 8.9 mm and protruded from the caudal thyroid margin, while the larger right mass measured 25.8 by 13.4 by 9.5 mm and sat adjacent to the right thyroid lobe. Crucially, no regional lymphadenopathy and no CT evidence of vascular or local invasion was identified—nothing on imaging to suggest cancer.

Fine-needle aspiration of both lesions yielded similar cytologic findings: round to polygonal cells with fine chromatin and moderate anisokaryosis, suggestive of a well-differentiated endocrine neoplasm of probable parathyroid origin. Like the imaging, the cytology could not distinguish benign from malignant disease. This limitation is well recognized—preoperative imaging and cytology may not reliably separate benign from malignant parathyroid lesions, and histopathologic distinction can be challenging even in equivocal cases examined under the microscope. The cat was hospitalized for preoperative stabilization, receiving two concurrent intravenous infusions of 0.9% sodium chloride at a combined rate of 4 mL/kg/h, one containing furosemide at 1 mg/kg/h, with additional furosemide doses on subsequent days. By surgery, serum ionized calcium had fallen from 1.90 to 1.47 mmol/L, while phosphorus rose from 2.4 to 4.0 mg/dL. Mild azotemia persisted during hospitalization, with creatinine ranging from 2.4 to 3.1 mg/dL, a transient increase considered potentially related to furosemide-associated volume contraction.

Surgery on the fourth day of hospitalization required two different approaches. The right-sided lesion was closely associated with and partially embedded within the thyroid parenchyma, making separation from the thyroid lobe impractical; the surgeons therefore performed a complete right thyroid lobectomy together with excision of the associated parathyroid mass. On the left, the parathyroid mass was completely excised with partial resection of adjacent thyroid tissue, preserving the uninvolved portion of the left thyroid lobe—an approach intended to minimize thyroid loss and reduce the risk of postoperative hypothyroidism. No intraoperative complications occurred. This divergence in technique illustrates a point the authors emphasize: the anatomical relationship of each lesion to the thyroid gland can independently determine the feasible extent of resection, a consideration separate from the question of malignancy.

Histopathology delivered the diagnosis that imaging and cytology could not. The left parathyroid mass was a well-differentiated endocrine epithelial neoplasm with no mitotic figures and no vascular or lymphatic invasion—but it displayed areas of modest infiltrative growth extending into adjacent non-neoplastic parathyroid gland tissue and the fibrous capsule, with neoplastic tissue extending to the histologic margins. That infiltrative growth supported a diagnosis of low-grade parathyroid carcinoma. The right mass, likewise well differentiated and mitotically inactive, showed focal extension into the fibrous capsule consistent with capsular penetration, which was considered supportive of its interpretation as a low-grade parathyroid carcinoma; it was completely excised, with neoplastic tissue extending to within 1 mm of the specimen margin. The authors note that distinguishing benign from malignant parathyroid neoplasms is particularly difficult in well-differentiated tumors lacking overt cytologic atypia or mitotic activity, making careful assessment for invasive or infiltrative growth essential in equivocal cases. Neither malignancy-associated feature had been predicted by the preoperative workup.

The postoperative course was eventful. Within 24 hours, ionized calcium fell to 1.27 mmol/L and then to 1.02 mmol/L—postoperative hypocalcemia consistent with the recognized consequence of extensive or bilateral parathyroid excision, which may cause transient or permanent hypoparathyroidism, particularly when residual functional tissue is limited or suppressed. The cat received 10% calcium gluconate intravenously twice on postoperative day 1, and oral calcitriol was initiated and continued through day 4. After discharge, calcitriol was discontinued because the owner could not administer it, and oral vitamin D supplementation was continued using alternative preparations adjusted by serial calcium measurements. Ionized calcium normalized by day 28 at 1.15 mmol/L and remained within the reference interval on day 42 at 1.26 mmol/L. Vitamin D supplementation was discontinued on day 70 once calcium concentrations stabilized. Kidney parameters also improved: creatinine decreased to 2.4 mg/dL on postoperative days 1 and 2 and remained at 1.6 to 2.2 mg/dL through day 161, measured SDMA concentrations stayed within the reference interval, and total thyroxine remained within its reference interval at every measured time point.

At the final follow-up on postoperative day 161, serum ionized calcium was 1.43 mmol/L and intact parathyroid hormone was 4.7 pg/mL, both within their reference intervals, although serum 25-hydroxyvitamin D had fallen to 69.8 ng/mL, again in the insufficient range, prompting resumption of vitamin D supplementation. Follow-up contrast-enhanced CT revealed no evidence of local recurrence or detectable metastasis. The authors are careful about the limits of these findings: serum iPTH was not measured at every postoperative visit, and follow-up beyond approximately five months was unavailable, so long-term disease control could not be evaluated. They also acknowledge that preoperative PTH measurement in fine-needle aspiration washout fluid—which can identify parathyroid tissue and provide lesion-specific information on PTH secretion—was not performed, precluding determination of each lesion’s relative functional contribution, though persistent preoperative hypercalcemia and postoperative hypocalcemia supported excess PTH secretion from at least one resected lesion. An immunohistochemical panel including parafibromin, galectin-3, and Ki-67 was likewise not performed, since species-specific validation of such markers in feline parathyroid neoplasia is lacking; the diagnoses rested on routine H&E findings.

The significance of the case, the authors argue, lies less in the bilateral occurrence itself—bilateral cystic parathyroid carcinoma has previously been reported in a cat—than in what it demonstrates about diagnosis and surgical planning. Synchronous bilateral parathyroid carcinomas can be well differentiated and histologically subtle, with different features supporting malignancy in each lesion despite similar preoperative appearances. In bilateral disease, they conclude, histopathologic assessment, lesion-specific functional evaluation such as PTH-FNA washout, and the anatomical relationship of each lesion to the thyroid gland should be treated as distinct but complementary factors. For now, the Russian Blue remains clinically stable and normocalcemic at approximately five months, with continued surveillance through periodic parathyroid hormone and calcium measurements recommended.

Subject of Research: Synchronous bilateral parathyroid carcinomas causing primary hyperparathyroidism in a cat

Article Title: Primary hyperparathyroidism in a cat with synchronous bilateral parathyroid carcinomas: a case report

Article References: Choi, U., Kwon, G., Lim, J.-H., Oh, S.-I., & Ock, S. (2026). Primary hyperparathyroidism in a cat with synchronous bilateral parathyroid carcinomas: a case report. Frontiers in Veterinary Science, 13, Article 1930590. https://doi.org/10.3389/fvets.2026.1930590

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1930590

Keywords: primary hyperparathyroidism, parathyroid carcinoma, feline, hypercalcemia, parathyroidectomy, histopathology, thyroidectomy, veterinary oncology, parathyroid hormone, case report, hypocalcemia, computed tomography