A Rare Case of Primary Hyperparathyroidism and Hypercalcemia of Malignancy Seen in a Patient With Prostate Adenocarcinoma

Hypercalcemia of malignancy is commonly associated with several malignancies, but its existence in prostate cancer is an uncommon finding. The concurrent existence of a parathyroid adenoma and a history of hypercalcemia over several decades further adds to the enigma. Our case is of an 82-year-old man with a history of prostate cancer who presented to the endocrinology clinic with hypercalcemia. His PET-CT showed osteolytic metastasis to the T10 vertebrae which were presumed to be the cause of his high serum calcium. Further investigations revealed increased parathyroid hormone-related peptide (PTHrP). Denosumab therapy was started but his calcium remained elevated and hence, he underwent palliative radiation therapy. A follow-up PET-CT revealed significant disease regression and his serum calcium decreased from 11mg/dL to 10mg/dL. However, one month post radiation his serum calcium started showing an upward trend. Further investigations revealed an elevated parathyroid hormone (PTH) and an ultrasound of the thyroid revealed parathyroid adenoma. The patient subsequently underwent a parathyroidectomy with resolution of hypercalcemia.