1,25(OH)2 Vitamin D (Calcitriol)

1,25-Dihydroxy Vitamin D (Calcitriol) is the hormonally active form of vitamin D, produced mainly in the kidneys from 25(OH)D under tight regulation by PTH, calcium, and phosphate. Calcitriol acts on the intestines, bones, and kidneys to increase calcium absorption and regulate bone metabolism. Measuring calcitriol is generally reserved for specific situations (e.g., kidney disease, calcium disorders, suspected granulomatous disease like sarcoidosis where it can be high) as it does not reflect overall vitamin D stores (use 25(OH)D for that).