1,25-dihydroxycholecalciferol

Active form of vitamin D

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1,25-Dihydroxycholecalciferol

Drug Information
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Identifiers IUPAC Name		Chemical Data C H N O Molecular Weight

1,25-Dihydroxycholecalciferol, also known as calcitriol, is the hormonally active form of vitamin D. It is a secosteroid hormone that plays a crucial role in the regulation of calcium and phosphate metabolism in the body. Calcitriol is produced in the kidneys from its precursor, 25-hydroxycholecalciferol, which is synthesized in the liver from cholecalciferol (vitamin D3).

Biological Function

Calcitriol functions by binding to the vitamin D receptor (VDR), which is present in various tissues throughout the body. This binding initiates a cascade of events that lead to the transcription of specific genes involved in calcium and phosphate homeostasis. The primary actions of calcitriol include:

• Intestinal Absorption of Calcium and Phosphate: Calcitriol increases the absorption of calcium and phosphate from the gastrointestinal tract, which is essential for maintaining adequate levels of these minerals in the blood.

• Bone Resorption: It stimulates the release of calcium from bone by promoting the differentiation and activity of osteoclasts, the cells responsible for bone resorption.

• Renal Reabsorption: Calcitriol enhances the reabsorption of calcium in the kidneys, reducing its excretion in urine.

Synthesis and Regulation

The synthesis of calcitriol is tightly regulated by several factors:

• Parathyroid Hormone (PTH): PTH is a major regulator of calcitriol production. When blood calcium levels are low, PTH is secreted by the parathyroid glands, stimulating the conversion of 25-hydroxycholecalciferol to calcitriol in the kidneys.

• Serum Calcium and Phosphate Levels: High levels of calcium and phosphate in the blood can inhibit the production of calcitriol.

• Fibroblast Growth Factor 23 (FGF23): This hormone, produced by osteocytes in bone, can decrease calcitriol synthesis.

Clinical Significance

Calcitriol is used therapeutically in the treatment of conditions such as:

• Hypocalcemia: Particularly in patients with chronic kidney disease or hypoparathyroidism.
• Osteoporosis: To help maintain bone density.
• Rickets and Osteomalacia: Conditions caused by vitamin D deficiency.

Pharmacokinetics

Calcitriol is administered orally or intravenously. It has a short half-life of approximately 5 to 8 hours and is highly protein-bound in the bloodstream. It is metabolized in the liver and kidneys and excreted primarily via the biliary route.

Also see

• Vitamin D
• Parathyroid hormone
• Calcium metabolism
• Osteoporosis
• Rickets