Vitamin D (25-OH)
Also reported as: 25-hydroxyvitamin D · Calcidiol · 25(OH)D
The circulating storage form of vitamin D, and the standard index of vitamin D status.
Below 20 ng/mL deficient, 20–30 insufficient, 30–100 sufficient.
Most observational associations plateau between 40 and 60 ng/mL, with no added benefit above that and possible harm well above 100.
Guideline thresholds from: Endocrine Society clinical practice guideline for the deficiency thresholds
To convert to nmol/L, multiply by 2.496.
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Where the thresholds sit
| Band | Range (ng/mL) | Note |
|---|---|---|
| Deficient | < 20 | — |
| Insufficient | 20–30 | — |
| Sufficient | 30–40 | — |
| Optimal | 40–60 | — |
| High | 60–100 | No added benefit demonstrated. |
| Excessive | > 100 | Risk of hypercalcaemia. |
Why it matters
Deficiency is genuinely common at UK latitudes, where skin synthesis effectively stops between October and March, and it has clear effects on bone and muscle. The wider claims — that correcting it prevents cancer or cardiovascular disease — have largely not survived randomised trials such as VITAL, so the honest position is: correct a deficiency, do not expect a miracle.
What raises it
- Sun exposure
- Supplementation
- Oily fish
What lowers it
- Northern latitude between October and March
- Darker skin pigmentation
- Obesity — vitamin D is sequestered in adipose tissue
- Malabsorption and coeliac disease
- Some anticonvulsants
How to move it
- 1In the UK, supplement through winter. Public Health England advises 10 µg (400 IU) daily for everyone from October to March; correcting an actual deficiency usually needs more.
- 21,000–2,000 IU/day suits most adults. Higher doses need monitoring.
- 3Take it with a meal containing fat — absorption is substantially better.
- 4Retest after 3 months. Levels take that long to plateau.
- 5Have magnesium adequacy in mind; it is a cofactor in vitamin D metabolism.
What moves this marker
Interventions on this site with evidence behind them for this specific marker. None of them replaces working out why yours is where it is.
What makes a single reading misleading
- ng/mL and nmol/L differ by a factor of about 2.5 — a great deal of online confusion comes from mixing them up.
- Levels swing seasonally, so compare like for like.
- Higher blood levels have not translated into the disease prevention that observational studies suggested.
Sources
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Educational only. Reference ranges are not diagnostic thresholds and this page cannot account for your medical history, medication or symptoms. Discuss any result that concerns you with a qualified clinician. See our medical disclaimer.