25OHD3

25OHD3, or 25-hydroxyvitamin D3, is the major circulating metabolite of vitamin D and a standard biomarker for vitamin D status. It is produced in the liver by hydroxylation of vitamin D3 (cholecalciferol) and reflects vitamin D obtained from sun exposure, diet, and supplements. With a half-life of roughly two to three weeks, it provides a relatively stable measure of vitamin D status compared with the active hormone form.

Circulating 25OHD is largely bound to the vitamin D binding protein. Blood tests typically report total 25-hydroxyvitamin D, which may include both 25OHD2 and 25OHD3 depending on the assay used. The measurement is not about the active hormone but about the body's vitamin D reserve and sufficiency.

Interpretation of 25OHD levels varies by guideline. Common thresholds place deficiency at less than about 20 ng/mL (50 nmol/L), insufficiency around 20–29 ng/mL (50–74 nmol/L), and sufficiency at or above 30 ng/mL (75 nmol/L) in many populations. Some guidelines use alternative cutoffs or target ranges, especially for specific groups such as pregnant people, older adults, or those with certain medical conditions.

Analytical aspects include the use of LC-MS/MS as a gold-standard method and various immunoassays, which can differ in accuracy and cross-reactivity with 25OHD2 versus 25OHD3. Standardization efforts aim to harmonize results across laboratories.

Clinical significance centers on bone health, with deficiency linked to rickets in children and osteomalacia in adults. Adequate 25OHD3 levels support calcium and phosphorus balance and overall skeletal integrity; supplementation and monitoring are considered for individuals at risk of deficiency or excess, following local guidelines. 25OHD3 itself is not the active vitamin D hormone, which is 1,25-dihydroxyvitamin D.