PTH1R and hypercalcemia disease: PTH-rP was first identified in 1987 as a tumor product that had the ability to activate PTH receptors and cause hypercalcemia by increasing bone resorption and renal tubular resorption of calcium, with a phosphaturic action also, by interacting with a common PTH/PTH-rP receptor, now called type 1 PTH receptor or PTH-R1 [11,12].