Your vitamin D result is reported as 25-hydroxyvitamin D. What the number means, what the NHS considers deficient, and what optimal looks like.
Vitamin D is measured in the blood as 25-hydroxyvitamin D, also written as 25-OH-D or calcidiol. This is the storage form of vitamin D, the form that reflects how much your body has available. Understanding your result means knowing what the number represents, what the UK thresholds are, and the difference between not deficient and optimal.
When you have a vitamin D blood test, the lab measures the concentration of 25-OH-D in your serum. This reflects both the vitamin D you have synthesised from sun exposure and the vitamin D you have obtained from diet or supplements. It is the most reliable indicator of your overall vitamin D status.
In the UK, results are reported in nmol/L. In the US and some private labs, results may be reported in ng/mL. To convert, multiply ng/mL by 2.496.
| Level, nmol/L | Status | Guidance |
|---|---|---|
| Below 25 | Severe deficiency | Risk of rickets or osteomalacia. A loading dose is typically required. |
| 25 to 49 | Deficiency | NHS threshold for deficiency. Supplementation recommended. |
| 50 to 74 | Insufficient | Below the level many experts consider optimal. Supplementation often advisable. |
| 75 to 200 | Sufficient | Generally adequate. Most experts aim for 75 to 150 nmol/L. |
| Above 250 | Potentially toxic | Risk of high blood calcium. Possible with high dose supplementation. |
The NHS defines sufficiency as above 50 nmol/L, but many researchers and clinicians consider 75 to 100 nmol/L to be the optimal range for bone health, immune function and overall wellbeing. The evidence for benefits above 100 nmol/L is less clear.
Vitamin D levels in the UK follow a predictable seasonal pattern. They typically peak in late summer, August to September, after months of sun exposure, and reach their lowest point in late winter, February to March. The time of year your test was taken significantly affects interpretation.
A result of 55 nmol/L in September, after a summer of sun, represents a different situation from the same result in February. If you are testing to assess your baseline, late winter is the most informative time.
The NHS recommends 10 micrograms, 400 IU, of vitamin D3 daily for adults during autumn and winter. For those with confirmed deficiency, a GP may recommend higher doses: typically 800 to 3,000 IU daily, or a loading regimen for severe deficiency.
Vitamin D3 is more effective than D2 at raising blood levels. Taking it with a meal containing fat improves absorption. If you are supplementing at higher doses, retest after three to four months to confirm your levels have responded.
One number, read in season. biomarkr tracks your vitamin D across every test so you can tell the trend from the seasonal swing. Free for your first year.
Join early access →Thresholds and guidance in this article are drawn from NHS sources and NICE Clinical Knowledge Summaries. Your own lab report ranges always take precedence.
Educational purposes only · not medical advice · always speak to your GP or a qualified clinician about your results