Calcitriol is the active form of vitamin D3. It is synthesised in the kidney from vitamin D. It is independently activated by three factors: increased parathyroid hormone levels, reduced calcium levels or indirectly via reduced phosphate levels. Calcitriol acts on four organs:

1) Bones: It promotes bone and inhibits bone resorption.

2) Intestines: It increases calcium and phosphate absorption in the small intestine.

3) Kidneys: It increases reabsorption of calcium from the urine.

4) Parathyroid gland: It inhibits the synthesis and secretion of parathyroid hormone (PTH) from the parathyroid gland.