Glossary

25-hydroxyvitamin D (25(OH)D)

25(OH)D is the accepted blood marker of vitamin D status; it reflects the total of vitamin D made in the skin and obtained from food and supplements.

Written by Metehan Yılmaz
In this guide

25-hydroxyvitamin D, 25(OH)D, is the form measured in blood after vitamin D has been converted in the liver. According to the international consensus by Giustina and colleagues, total serum 25(OH)D (the sum of 25(OH)D₃ and 25(OH)D₂) is the accepted biomarker of vitamin D status; however, the measurement method, standardization and desired levels remain important subjects of debate. [1]

Half-life

According to the same consensus, the half-life of serum 25(OH)D is 2–3 weeks, while that of the more water-soluble active form 1,25(OH)₂D is about 5–8 hours. [1] The form measured in routine assessment is 25(OH)D; it is not the same as the active form.

Thresholds

The Delphi consensus completed with a meeting in Istanbul in October 2025 and published in 2026 in Türkiye defines serum 25(OH)D below 20 ng/mL as deficiency and below 12 ng/mL as severe deficiency; it sets the target range as 20–50 ng/mL. The same document does not recommend routine screening in the general population; it supports targeted testing in high-risk adults. [2] The introduction of the document states that authorities treat below 20 ng/mL as the threshold for the general population (the approach reflected in the IOM and Endocrine Society guidelines) and below 30 ng/mL for older adults and high-risk groups. [2] For this reason, the threshold by which a result is interpreted is stated.

Units and laboratory differences

Results are given by laboratories in ng/mL (mass) or nmol/L (molar); the Giustina consensus writes that the unit should be clearly stated in the report and that mol/L should be preferred as the SI unit. According to the same consensus, measurement can be done with antibody-based methods or with liquid chromatography–mass spectrometry (LC-MS/MS); LC-MS/MS gives more consistent and accurate results. [1]

For the threshold table, unit conversion and test interpretation, the guide What should vitamin D be? can be read.

Sources

  1. Giustina A et al. — Consensus Statement on Vitamin D Status Assessment and Supplementation: Whys, Whens, and Hows (Endocr Rev) (2024)
  2. Gogas Yavuz D et al. — Best Practice Recommendations for the Assessment, Prevention and Treatment of Vitamin D Deficiency in Türkiye: A 2026 Update in a Setting with Limited Mandatory Food Fortification (2026)

Updated

This content is for information only and is not medical advice. It does not recommend doses, products or vendors. Talk to your clinician before making any health decision.