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 hormoneParathyroid hormone (PTH) is a hormone secreted by the parathyroid glands, which regulates calcium levels in the blood. PTH is secreted in response to low blood calcium levels (hypocalcaemia). An increase in calcium concentration above the normal value inhibits PTH production (negative feedback). PTH stimulates... levels, reduced calcium levels or indirectly via reduced phosphate levels. Calcitriol acts on four organs:
1) Bones: It promotes bone and inhibits bone resorptionThe re-uptake of substances from tissues, in particular bone, into the blood. It should not be confused with absorption, the initial uptake of substances into the body, for instance from food in the intestines, through the lungs and via the skin. Reabsorption, in contrast, occurs....
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.