Parathyroid hormone-related protein (or PTHrP) is a protein occasionally secreted by cancer cells (breast cancer, certain types of lung cancer). PTHrP is related in function to the "normal" parathyroid hormone. When a tumor secretes PTHrP, this can lead to hypercalcemia. As this is sometimes the first sign of the malignancy, hypercalcemia caused by PTHrP is considered a paraneoplastic phenomenon.
