Calcitriol

activating the vitamin D receptor within cells, regulating gene expression to increase blood calcium levels—primarily by enhancing intestinal calcium absorption.
As a medication, it is used to treat conditions such as osteoporosis, osteomalacia, hypoparathyroidism, and calcium disorders related to kidney disease. It can be administered orally or intravenously. 
First identified in 1971 and approved in the United States in 1978, calcitriol is now widely available as a generic drug and is included on the WHO List of Essential Medicines.