calcidiol

Calcidiol, also known as 25-hydroxyvitamin D [25(OH)D], is the principal circulating form of vitamin D in humans and the standard measure used to assess vitamin D status. It is produced in the liver, where cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) are hydroxylated by enzymes including 25-hydroxylase to form 25(OH)D. Calcidiol is transported in the blood largely bound to vitamin D–binding protein and has a relatively long circulating half-life of about two to three weeks. It reflects vitamin D obtained from sun exposure, diet, and supplements, and serves as a reservoir for the active hormone.

Calcidiol is not itself the biologically active vitamin D hormone. The active form is calcitriol (1,25-dihydroxyvitamin D), produced mainly in the kidney by 1-alpha-hydroxylase, and regulated by parathyroid hormone, calcium, and phosphate levels.

In clinical practice, serum total 25(OH)D is the preferred marker of vitamin D status. Assays include immunoassays and LC-MS/MS; results may vary between methods. Common interpretive cutoffs are: deficiency (<20 ng/mL or <50 nmol/L), insufficiency (20–29 ng/mL or 50–75 nmol/L), and sufficiency (≥30 ng/mL or ≥75 nmol/L). Factors such as obesity, liver disease, age, ethnicity, season, and sunscreen use can influence levels. Management typically involves vitamin D supplementation and monitoring to achieve sufficiency.