Active Vitamin D, also known as 1,25-dihydroxyvitamin D or calcitriol, is the hormonally active form of vitamin D produced in the kidneys from the storage form 25-hydroxyvitamin D (25-OHD). While the standard vitamin D blood test measures 25-OHD (the circulating storage form), active vitamin D is the form that actually binds to vitamin D receptors in the intestine, bones, kidneys, and immune cells to exert its biological effects.

The conversion of 25-OHD to active vitamin D is tightly regulated by parathyroid hormone (PTH) and calcium levels. Active vitamin D's most important role is stimulating calcium absorption from the gut and regulating calcium and phosphate homeostasis — essential for bone mineralisation, muscle function, and neuromuscular signalling. Active vitamin D also plays important roles in immune regulation, cell differentiation, and reducing inflammation.