What Is a Vitamin D 25-Hydroxy Test?

The 25-hydroxyvitamin D (25-OH D) test measures the storage form of vitamin D — the best indicator of vitamin D status.

Why 25-OH D?

• Has a long half-life (2–3 weeks)
• Reflects vitamin D from sun, food, and supplements
• Not affected by short-term fluctuations

Reference Ranges

• Severe deficiency: below 12 ng/mL
• Deficiency: 12–20
• Insufficiency: 20–30
• Sufficient: 30–50
• Toxic: above 100

How to Interpret

• Many clinicians target 40–60 ng/mL
• If deficient, supplement and recheck in 8–12 weeks
• Don't supplement without testing — excess is harmful