Immunoperoxidase study of uncommon parathyroid tumors. Report of two cases of nonfunctioning parathyroid carcinoma and one intrathyroid parathyroid tumor-producing amyloid.

Nonfunctioning carcinomas of the parathyroid gland are rare and difficult to diagnose. They are often confused with thyroid tumors or with metastasis from other sites. We report two cases of nonfunctioning parathyroid carcinomas; one was originally diagnosed as follicular carcinoma of the thyroid gland. The immunohistochemical demonstration of parathormone in the tumor cells established the parathyroid origin of these neoplasms. An intrathyroid parathyroid tumor, associated with large amounts of interstitial amyloid, mimicking medullary carcinoma of the thyroid, in a patient with primary hyperparathyroidism is also reported. Positive immunoreaction in the tumor cells for parathormone, negative staining for calcitonin, and the return of patients' serum calcium levels to normal after tumor resection, confirmed the parathyroid nature of this neoplasm. Immunohistochemistry studies proved to be extremely helpful in establishing the diagnoses of these unusual parathyroid tumors.