calcitriol

Calcitriol, scientifically known as 1,25-dihydroxyvitamin D3, is the hormonally active metabolite of vitamin D. It is produced primarily in the kidney by the enzyme 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D into calcitriol. The activity of 1-alpha-hydroxylase is increased by parathyroid hormone and decreased by high serum phosphate and by fibroblast growth factor 23.

Calcitriol exerts its effects by binding the vitamin D receptor, a nuclear transcription factor. The calcitriol–VDR complex regulates gene expression, leading to increased intestinal absorption of calcium and phosphate through upregulation of calcium-binding proteins and transport channels. It also participates in bone remodeling by influencing osteoblast and osteoclast activity. Beyond mineral metabolism, calcitriol can affect immune function and cell differentiation.

Pharmacologically, calcitriol is administered orally or by injection, with a plasma half-life of approximately four to six hours. It is metabolized in the liver and excreted mainly in bile. Its use is monitored by measuring serum calcium, phosphate, and PTH levels to avoid hypercalcemia and hyperphosphatemia.

Indications include the treatment of hypocalcemia due to hypoparathyroidism or chronic kidney disease–associated hypocalcemia, and the management of secondary hyperparathyroidism in CKD. In some settings, topical calcitriol is used for dermatologic conditions such as psoriasis. Adverse effects center on hypercalcemia and vascular calcification risk, particularly in CKD or in the presence of other calcium-raising therapies.