Vitamin D3, 1,25-dihydroxyvitamin D (Calcitriol) Unit Conversion

Calcitriol, also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D₃, is the hormonally active form of vitamin D with three hydroxyl groups (abbreviated 1,25-(OH)₂D₃ or simply 1,25(OH)₂D). It increases the level of calcium (Ca2+) in the blood by

• increasing the uptake of calcium from the gut into the blood,
• decreasing the transfer of calcium from blood to the urine by the kidney, and
• increasing the release of calcium into the blood from bone.

Calcitriol is produced in the cells of the proximal tubule of the nephron in the kidneys by the action of 25-hydroxyvitamin D₃ 1-alpha-hydroxylase, a mitochondrial oxygenase and an enzyme which catalyzes the hydroxylation of 25-hydroxycholecalciferol (calcifediol). The activity of the enzyme is stimulated by PTH. The reaction is an important control point in Ca²⁺ homeostasis. The production of calcitriol is also increased by prolactin, a hormone which stimulates lactogenesis (the formation of breast milk), a process which requires large amounts of calcium. It is decreased by high levels of serum phosphate and by an increase in the production of the hormone FGF-23 by osteocyte cells in bone.

Aid in the diagnosis of primary hyperparathyroidism, hypoparathyroidism, pseudohypoparathyroidism, renal osteodystrophy, and vitamin D-resistant rickets.