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

Primary hyperparathyroidism (PHPTH) is a rare endocrine disorder in cats, and bilateral parathyroid carcinoma is exceptionally uncommon. A 14-year-old neutered male Russian Blue cat presented with decreased appetite, reduced activity, vomiting, and persistent hypercalcemia. Hypercalcemia was documented by increased ionized calcium (iCa; 1.90 mmol/L) and total calcium (14.3 mg/dL), and an increased intact parathyroid hormone concentration (18.6 pg/mL) supported a diagnosis of PHPTH. Cervical ultrasonography, computed tomography, and fine-needle aspiration cytology identified bilateral parathyroid-associated lesions but could not determine malignancy or the functional contribution of each lesion. During preoperative stabilization with intravenous 0.9% sodium chloride and furosemide, the iCa concentration decreased from 1.90 to 1.47 mmol/L. On the fourth day of hospitalization, complete right thyroid lobectomy with excision of the associated right parathyroid mass and left parathyroidectomy with partial left thyroidectomy were performed. Histopathologic examination supported the diagnosis of synchronous bilateral low-grade, well-differentiated parathyroid carcinomas, with infiltrative growth into adjacent non-neoplastic parathyroid tissue in the left lesion and focal capsular penetration in the right lesion. Postoperative hypocalcemia was managed with intravenous calcium gluconate and oral calcitriol during hospitalization, followed by oral vitamin D supplementation after discharge. Serum iCa normalized by postoperative day 28 and remained within the reference interval thereafter. On postoperative day 161, serum iCa and iPTH concentrations were within their respective reference intervals, and follow-up CT showed no evidence of local recurrence or detectable metastasis. This case highlights that bilateral parathyroid carcinomas may exhibit subtle histopathologic evidence of malignancy that is not predicted by preoperative imaging or cytology, while lesion-specific functional status and anatomical relationships to the thyroid gland may independently influence surgical planning.