Parathyroid hormone-related protein (or PTHrP) is a protein occasionally secreted by cancer cells (breast cancer, certain types of lung cancer including squamous cell carcinoma). However, it also has normal functions.

PTHrP acts as an endocrine, autocrine/ paracrine, and intracrine hormone. 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. PTHrP shares the same N-terminal end as parathyroid hormone and therefore it can bind to the same receptor (the Type I PTH receptor). PTHrP aids in normal mammary gland development and lactation as well as placental transfer of calcium.