Intact Parathyroid Hormone Suppression in Primary Hyperparathyroidism by Hypercalcemia of Malignancy

ABSTRACT Objective Hypercalcemia is a common metabolic disorder, with primary hyperparathyroidism and malignancy accounting for a large majority of the cases. In a few instances, hypercalcemia can have multiple, concomitant etiologies. We hypothesize that elevated intact parathyroid hormone (iPTH) due to primary hyperparathyroidism is suppressible in the setting of hypercalcemia of malignancy due to osteolytic lesions. Methods We present a case of a 44-year-old female with 2 causes of hypercalcemia: primary hyperparathyroidism secondary to parathyroid adenoma and hypercalcemia of malignancy secondary to osteolytic lesions from metastatic invasive ductal carcinoma of the breast. Results The patient had previously been diagnosed with a parathyroid adenoma with calcium of 11.4 mg/dL (reference range: 8.5 to 10.4 mg/dL) and elevated iPTH of 141.3 pg/mL (reference range: 8.5 to 75.0 pg/mL). She presented to the hospital approximately 6 weeks later with acute back pain. Lab values included elevated calcium at 17.1 mg/dL with iPTH found to be suppressed from her prior value to a level of 61.2 pg/mL. Spinal imaging noted extensive osteolytic lesions with biopsy confirming metastatic breast cancer. Conclusion This case illustrates that iPTH secreted by a parathyroid adenoma is capable of at least partial suppression in the setting of hypercalcemia from another source and it is important to evaluate for multiple etiologies of hypercalcemia.