Vitamin D (25-OH)

Test description

Measurement of 25-hydroxyvitamin D [25(OH)D] is the gold standard for assessing the body's vitamin D supply. This form reflects the total pool derived from skin synthesis, diet, and supplementation, and its half-life (2–3 weeks) makes it the most reliable indicator of long-term status. Vitamin D regulates calcium and phosphorus absorption, conditions bone mineralisation, supports muscle function, and modulates immune response. Observational studies link its deficiency to more frequent respiratory infections, low mood and seasonal depression, musculoskeletal pain, weakened muscle strength, and accelerated bone loss. In Scotland and throughout the UK, the angle of the sun's rays for most of the year prevents effective skin synthesis, so deficiencies affect a significant portion of the population – including young and active individuals. Regular monitoring of 25(OH)D allows for precise adjustment of supplementation dosage instead of guesswork, and after 8–12 weeks, assessment of treatment effectiveness.

Biomarkers measured