Late recurrence of a single gland primary hyperparathyroidism-Atypical parathyroid adenoma or misdiagnosed parathyroid carcinoma.

Key Clinical Message: This case report aims to raise awareness of differential diagnoses of hypercalcemia and primary hyperparathyroidism, including parathyroid carcinoma and atypical adenoma, and to highlight the diagnostic challenges. Abstract: Parathyroid carcinoma is a rare and often fatal cause of primary hyperparathyroidism and hypercalcemia. To date, there is still no clear-cut diagnostic pathway for parathyroid carcinoma established, which results in major diagnostic ambiguity and complexity. Clinical differentiation between benign parathyroid adenoma and carcinoma is challenging and ultimately the diagnosis remains histopathological. We present a case of a 58-year-old female patient with parathyroid tumor recurrence after parathyroidectomy because of primary hyperparathyroidism. The first tumor was histologically classified as an atypical parathyroid adenoma by a specialized endocrine pathologist. Eleven years after the primary tumor resection a new tumor recurred. Retrospectively, after the tumor recurrence, the primary diagnosis of the atypical adenoma was questioned, and the tumor was temporarily classified to rather be a parathyroid carcinoma. This case aims to raise awareness for the diagnostic challenge of parathyroid carcinomas as a rare cause of primary hyperparathyroidism and therewith to improve treatment and prognosis.