Ask anyone in Britain how much sun they saw in February and you will usually get a wry smile. That matters for more than mood. The body makes most of its vitamin D from direct sunlight on the skin, and the NHS explains that between October and early March the sun in the UK is not strong enough for that to happen. A vitamin D blood test is one of the simplest ways to see where your levels actually sit, rather than guessing.
This guide covers what the Vitamin D test measures, why the time of year matters, how it differs from the less common calcitriol test, and when checking it tends to make sense.
What vitamin D does
Vitamin D helps regulate the amount of calcium and phosphate in the body. Both are needed to keep bones, teeth and muscles healthy. The NHS notes that a lack of vitamin D can lead to rickets in children and to bone pain from a condition called osteomalacia in adults. The laboratory text from LML, our testing partner, adds that low levels have been linked with fatigue, bone and muscle pain and low mood.
Large reviews describe low vitamin D status as common worldwide, particularly at higher latitudes, in people who spend little time outdoors and in people with darker skin (Amrein et al., 2020). The UK fits several of those descriptions for part of the year.
Why the vitamin D blood test is so relevant in the UK
According to the NHS, from about late March or early April to the end of September most people can make all the vitamin D they need from sunlight. Outside those months, it has to come from food and supplements. Food sources are limited: oily fish, red meat, egg yolks and some fortified foods. Cows' milk in the UK is generally not fortified.
LML states that up to a quarter of the UK population is low in vitamin D at the end of winter. That is why the topic keeps returning every spring, and why the official advice is for everyone to consider a daily supplement of 10 micrograms during autumn and winter. The NHS adds that some groups should consider a supplement all year round, including people who are rarely outdoors, people who cover most of their skin when outside and people with dark skin, such as those of African, African-Caribbean or south Asian background.
The Scientific Advisory Committee on Nutrition (SACN) reviewed the evidence behind these recommendations in its report on vitamin D and health, which is the basis of the current UK advice.
What the test actually measures
Vitamin D (25-OH)
The standard Vitamin D test in the BPR catalogue measures 25-hydroxyvitamin D, written as 25-OH vitamin D. This is the circulating form that reflects both what the skin has produced and what has come from diet and supplements. Reviews of testing practice describe it as the accepted marker of vitamin D status (Pilz et al., 2019).
1,25-dihydroxyvitamin D (calcitriol)
The catalogue also lists 1,25-dihydroxyvitamin D (calcitriol). LML describes it as the most active form of vitamin D, produced mainly in the kidneys from 25-hydroxyvitamin D. Because the body tightly controls its level, it is not the usual test for checking whether someone is getting enough vitamin D (Pilz et al., 2019). It tends to be requested in specific situations, for example when a doctor is looking at calcium balance or kidney function. If you are unsure which one you need, the 25-OH test is the usual starting point, and a professional can tell you whether calcitriol adds anything in your case.
Single test or a combination?
Depending on what you want to understand, the BPR catalogue offers a few options:
- Vitamin D: 25-OH vitamin D on its own.
- Calcium + Vitamin D: adds calcium, useful when bone health is the main question.
- Vitamin Profile: vitamin D together with vitamin B12 and folate, three nutrients that are often looked at together when someone feels tired or follows a restricted diet.
None of these is "better" in the abstract. The right choice depends on the question you are trying to answer, and that is worth a short conversation with a professional.
