Hypocalcemia and parathyroid function in metastatic prostate cancer

To report the occurrence of hypocalcemia in a patient with metastatic prostate cancer, discuss its pathogenesis, and review the related medical literature.

An 82-year-old man with a known history of prostate cancer was found to have a serum calcium level of 5.4 mg/dL during an admission to the hospital for small bowel obstruction. A thorough review of his medical history revealed a temporal relationship between the diagnosis of malignant disease and progressive hypocalcemia. A complete evaluation was performed, including laboratory and imaging studies, to ascertain the cause of the hypocalcemia.

The patient had no history of hypocalcemia before the diagnosis of, and initiation of antiandrogen therapy for, advanced prostate cancer. Serum magnesium and phosphorus levels were within normal limits. The serum calcium level responded to therapy in the hospital but remained between 5.8 and 7.1 mg/dL. The parathyroid hormone level was normal, and the 25-hydroxyvitamin D value was low. A 24-hour urine collection showed substantially reduced calcium excretion, and a whole-body bone scan revealed widespread metastatic deposits. These findings were compatible with hypocalcemia related to prostate cancer and bone metastatic lesions.

This case serves as a reminder that hypocalcemia can be a manifestation of prostate cancer metastatic to bone. In contrast to the occurrence of secondary hyperparathyroidism in this setting, however, this patient had normal levels of parathyroid hormone. Review of similar previous reports and the causes and implications of a possible functional hypoparathyroid state are discussed.