Vitamin D blood test: what it measures and what can change it
25-OH vitamin D is the stored form of vitamin D. In the UK it falls in autumn and winter, when sunlight does not make it.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
25-OH vitamin D is the stored form of vitamin D. In the UK it falls in autumn and winter, when sunlight does not make it.
UK guidance is mostly about the supplement rather than the test: routine testing is not advised without bone or muscle symptoms.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
- SACN uses 25 nmol/L as the population threshold below which the risk of bone problems rises. SACN, Vitamin D and health (2016)
- NICE advises testing vitamin D only with symptoms of deficiency, a very high risk (for example very little sunlight) or a clinical reason such as osteomalacia or a fall, not as a routine check. NICE PH56 (2014, updated 2017), recommendation 7; Royal Osteoporosis Society, Vitamin D and Bone Health (2018)
What can shift the number
Now or in the last 3 months
- An infection or illness in the 6 weeks before. The vitamin D level falls during acute inflammation, so a result taken during an illness can read lower. SACN Vitamin D and Health (2016), S.10 and 6.8
Long-term conditions
- Coeliac disease, Crohn's or colitis, a stomach ulcer, or stomach or bowel surgery. Conditions that reduce the absorption of fat lower vitamin D. Lab Tests Online UK: Vitamin D
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
Read together with: Calcium (adjusted). NHS.uk: Rickets and osteomalacia; Royal Osteoporosis Society, Vitamin D and Bone Health (2018)
A GP usually treats a low level with a course of vitamin D and then a maintenance dose, and looks for a reason when it is very low or keeps coming back.
Questions to ask your GP
- Is a treatment course needed, or a daily supplement?
- Should calcium or bone health be checked too?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What are the symptoms of vitamin D deficiency?
Low vitamin D and calcium can cause a bone condition called osteomalacia in adults. The NHS lists symptoms such as pain in your back, shoulders, ribs, pelvis or legs, and tingling, cramping or twitching of your muscles.
NHS: Rickets and osteomalacia, Page last reviewed 9 July 2025When does a GP check your vitamin D?
If a GP thinks you may have osteomalacia, they'll check your vitamin D and calcium levels. This may involve blood tests and a urine sample. If another condition is causing your symptoms, treating that condition helps.
NHS: Rickets and osteomalacia, Page last reviewed 9 July 2025How can I get more vitamin D?
From about late March or early April to the end of September, most people can make all the vitamin D they need from sunlight. Between October and early March you do not make enough from sunlight. Foods with vitamin D include oily fish, red meat, egg yolks and fortified foods such as some spreads and breakfast cereals. Government advice is that everyone should consider a daily vitamin D supplement in autumn and winter; take one as your GP or pharmacist advises.
NHS: Vitamin D, Page last reviewed 3 August 2020Who is more likely to have low vitamin D?
The NHS names people who are not often outdoors, such as those who are frail, housebound or living in a care home. People who usually cover most of their skin outdoors, and people with dark skin, may also not make enough vitamin D from sunlight. The NHS advises these groups to take a supplement all year; ask your GP or pharmacist which suits you.
NHS: Vitamin D, Page last reviewed 3 August 2020Can you have too much vitamin D?
Yes, from supplements. Taking too many vitamin D supplements over a long time can cause too much calcium to build up in your body, which can weaken bones and damage the kidneys and heart. You cannot overdose on vitamin D from sunlight. Take supplements as your GP or pharmacist advises.
NHS: Vitamin D, Page last reviewed 3 August 2020Got your result? Start with this question in your AI.
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Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- NHS: Rickets and osteomalacia, Page last reviewed 9 July 2025
- NHS: Vitamin D, Page last reviewed 3 August 2020