Calcitriol is the active form of vitamin D, normally made in the kidney. It is also known as 1,25-dihydroxycholecalciferol. It is a hormone which binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. Calcitriol increases blood calcium (Ca2+) mainly by increasing the uptake of calcium from the intestines.It can be given as a medication for the treatment of low blood calcium and hyperparathyroidism due to kidney disease, low blood calcium due to hypoparathyroidism, osteoporosis, osteomalacia, and familial hypophosphatemia, and can be taken by mouth or by injection into a vein. Excessive amounts or intake can result in weakness, headache, nausea, constipation, urinary tract infections, and abdominal pain. Serious side effects may include high blood calcium and anaphylaxis. Regular blood tests are recommended after the medication is started and when the dose is changed.Calcitriol was identified as the active form of vitamin D in 1971 and the drug was approved for medical use in the United States in 1978. It is available as a generic medication. In 2017, it was the 256th most commonly prescribed medication in the United States, with more than one million prescriptions.It is on the World Health Organizations List of Essential Medicines. Medical use
Calcitriol is prescribed for:
Treatment of hypocalcaemia – hypoparathyroidism, osteomalacia (adults), rickets (infants, children), renal osteodystrophy, chronic kidney disease
Treatment of osteoporosis
Prevention of corticosteroid-induced osteoporosisCalcitriol has been used in an ointment for the treatment of psoriasis, although the vitamin D analogue calcipotriol (calcipotriene) is more commonly used.
Calcitriol has also been given by mouth for the treatment of psoriasis and psoriatic arthritis. Research on the noncalcemic actions of calcitriol and other VDR-ligand analogs and their possible therapeutic applications has been reviewed. Adverse effects
The main adverse drug reaction associated with calcitriol therapy is hypercalcemia – early symptoms include: nausea, vomiting, constipation, anorexia, apathy, headache, thirst, pruritus, sweating, and/or polyuria. Compared to other vitamin D compounds in clinical use (cholecalciferol, ergocalciferol), calcitriol has a higher risk of inducing hypercalcemia. However, such episodes may be shorter and easier to treat due to its relatively short half-life.High calcitriol levels may also be seen in human disease states in patients not on supplementation. In someone with hypercalcaemia and high calcitriol levels, low intact parathyroid hormone levels are usually present. The major conditions with hypercalcaemia due to elevated calcitriol levels are lymphoma, tuberculosis and sarcoidosis where excess production occurs due to ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages. Other conditions producing similar findings including:

Fungal infections; Pneumocystis jiroveci, histoplasmosis, coccidioidomycosis, paracoccidioidomycosis, candidiasis
Other granulomatous conditions; PR3+ vasculitis, Crohns disease, acute granulomatous pneumonia, talc granuloma, silicone-induced granuloma, BCG-associated, granulomatous hepatitis, paraffin-associated granuloma
Genetic conditions; Williams syndrome, pseudoxanthoma elasticum, CYP24A1 mutation (adult / infantile), SLC34A1 mutation
Miscellaneous; mycobacterium avium, leprosy, lipoid pneumonia, cat scratch fever, berylliosisSome plants contain glycosides of 1,25-dihydroxycholecalciferol. Consumption of these glycosides by grazing animals leads to vitamin D toxicity, resulting in calcinosis, the deposition of excessive calcium in soft tissues. Three rangeland plants, Cestrum diurnum, Solanum malacoxylon, and Trisetum flavescens are known to contain these glycosides.