calcitriol
Calcitriol is vitamin D after it has been switched fully on, the actual working hormone. If raw vitamin D is the unsharpened tool, calcitriol is the version that finally does the job: opening the door for calcium to enter the body from food.
It is made when the kidney adds the last chemical group to 25-hydroxyvitamin D, a step driven mainly by PTH (and held back by FGF23 and by high phosphate). As a lipophilic, steroid-like hormone, calcitriol slips into cells and binds an intracellular vitamin D receptor, switching on genes. Its headline action is in the intestine, where it produces the proteins that pull calcium and phosphate across the gut lining into the blood.
Because the kidney is where calcitriol is finally activated, people with kidney failure cannot make enough of it and develop low calcium and weak bones, often treated with calcitriol or its analogs. Conversely, too much calcitriol (for example from certain granulomatous diseases like sarcoidosis) can cause dangerous hypercalcemia.
A dialysis patient is prescribed active calcitriol because their failed kidneys can no longer perform the final activation step on their own.
Kidney failure bypasses the body's own calcitriol production.
Blood tests for vitamin D status usually measure 25-hydroxyvitamin D, not calcitriol, because calcitriol levels are tightly regulated and can stay normal even when stores are low.