Vitamin D is measured in the blood as 25-hydroxyvitamin D (25-OH-D), also written as 25(OH)D or calcidiol. This is the storage form of vitamin D — the form that reflects how much vitamin D your body has available. Understanding your result means knowing what the number represents, what the UK thresholds are, and what the difference is between "not deficient" and "optimal."
How vitamin D is measured
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 (nanomoles per litre). In the US and some private labs, results may be reported in ng/mL — to convert, multiply ng/mL by 2.496.
UK reference ranges and thresholds
| Level (nmol/L) | Status | NHS / NICE guidance |
|---|---|---|
| Below 25 nmol/L | Severe deficiency | Risk of rickets (children) or osteomalacia (adults); loading dose supplementation typically required |
| 25–49 nmol/L | Deficiency | NHS threshold for deficiency; supplementation recommended |
| 50–74 nmol/L | Insufficient | Below the level many experts consider optimal; supplementation often advisable |
| 75–200 nmol/L | Sufficient | Generally considered adequate; most experts aim for 75–150 nmol/L |
| Above 250 nmol/L | Potentially toxic | Risk of hypercalcaemia; unlikely from diet or sun, possible with high-dose supplementation |
Seasonal variation
Vitamin D levels in the UK follow a predictable seasonal pattern. They typically peak in late summer (August–September) after months of sun exposure, and reach their lowest point in late winter (February–March) after months of minimal UV radiation. This means the time of year your test was taken significantly affects interpretation.
A result of 55 nmol/L in September (after a summer of sun exposure) represents a different situation from the same result in February (after months of winter). If you are testing to assess your baseline, late winter is the most informative time.
What affects your vitamin D level
Several factors influence how your body produces and maintains vitamin D:
- Sun exposure — the primary source; requires UVB radiation, which is absent in the UK from October to March
- Skin tone — melanin reduces UV absorption; darker skin requires more sun exposure for equivalent synthesis
- Age — skin becomes less efficient at synthesising vitamin D with age
- Body weight — vitamin D is fat-soluble and can be sequestered in adipose tissue, reducing circulating levels
- Gut absorption — conditions such as Crohn's disease and coeliac disease impair vitamin D absorption from food and supplements
- Kidney and liver function — both organs are involved in converting vitamin D to its active form
Supplementation: what to take and how much
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 800–3,000 IU daily, or a loading regimen for severe deficiency.
Vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) at raising blood levels. Taking it with a fat-containing meal improves absorption. If you are supplementing at higher doses, retest after three to four months to confirm your levels have responded.
What to ask your GP
- What is my exact vitamin D level, and what does it mean for my health?
- What dose do you recommend, and for how long?
- Should I retest after supplementing, and when?
- Is there any reason I might not be absorbing vitamin D normally?
- Should I also be taking vitamin K2 alongside vitamin D?
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