There is a blood test for calcitriol. This is a tightly controlled pathway, hence it doesn't change much.
D3 shows how much "raw material" is in the blood.
From Dr. Cannell of the Vitamin D Council:
Calcitriol (1,25(OH)2D3 or 1,25D3)
Calcitriol (1,25-dihydroxyvitamin D) is made from calcidiol in both the kidneys and in other tissues and is the most potent steroid hormone derived from cholecalciferol. Calcitriol has powerful anti-cancer properties. It is sometimes referred to as the active form of vitamin D. Calcitriol levels should never be used to determine if you are deficient in vitamin D.
Calcitriol Contraindicated in Vitamin D Deficiency
Ergocalciferol has been used safely by physicians for years for a variety of indications. Unfortunately, when doctors don't prescribe ergocalciferol, they sometimes prescribe calcitriol or newer analogs of calcitriol, costing thousands of times more than cholecalciferol. Calcitriol, and its analogs, are contraindicated in vitamin D deficiency because they may cause hypercalcemia and they fail to address the real problem: low stores of 25(OH)D. Cholecalciferol repletes the vitamin D system by filling up your vitamin D tank with 25(OH)D, the vitamin D fuel. Vieth R. The pharmacology of vitamin D, including fortification strategies. In:Feldman D, Glorieux F, eds. Vitamin D, Chapter 61, in press, 2nd ed. Academic Press, San Diego.
Giving calcitriol, or its analogs, for vitamin D deficiency is like shooting ether into your engine to keep your car running. In addition, they pose a significant risk of hypercalcemia (high blood calcium). If you have a simple vitamin D deficiency and your doctor insists on prescribing calcitriol or an expensive analog of vitamin D (other than cholecalciferol or ergocalciferol), find another doctor.
Let the body regulate it, unless you like your soft tissues getting calcified.