A Case of a Rare Parathyroid Hormone (PTH)-Producing Neuroendocrine Tumor

BACKGROUND Neuroendocrine neoplasms are commonly seen in association with hormone production, and clinical signs that arise from these hormonal effects often manifest as the first presentation of malignancy. The excess production of parathyroid hormone (PTH) in particular, however, is primarily sporadic (80-85%) in clinical settings. In the context of malignancy, hyperparathyroidism manifestations arise most frequently from non-neuroendocrine pulmonary tumors through a ligand mimicker, parathyroid hormone-related peptide (PHrP). Excess PTH or PTHrP production has been very rarely described in association with gastrointestinal tumors and almost never described as a primary paraneoplastic syndrome from a neuroendocrine tumor (NET) alone. CASE REPORT We present a patient with a prior surgically resected carcinoid tumor who later presented with an elevated parathyroid hormone level, hypercalcemia, and clinical manifestations of primary hyperparathyroidism. She was found to have a low-grade, recurrent neuroendocrine tumor on resection of a parathyroid mass suspected to be a productive adenoma. Despite no longer having parathyroid glands given the extent of resection, her PTH level remained elevated and was rising. Further investigation via repeat sestamibi nuclear scan excluded the possibility of exogenous parathyroid tissue, and subsequent dotatate positron emission tomography/computed tomography (PET/CT) revealed the source of the PTH production: multiple sites of metastatic neuroendocrine tumors producing native PTH. CONCLUSIONS This case highlights the rare possibility of NETs to secrete PTH and the importance of considering early staging with dotatate PET/CT to evaluate the extent of disease. Additionally, our case reveals the importance of considering NET as an alternative etiology for refractory hypercalcemia.