What your vitamin D result actually means
Reviewed by the My London Pharmacy clinical team, Zeenah Araim, Pharmacist Independent Prescriber
·17 August 2026
·6 min read
Quick answer
UK vitamin D results are reported in nmol/L. Below 25 nmol/L is deficient, 25 to 50 nmol/L is insufficient for most people, and above 50 nmol/L is generally adequate. What matters is not just the number but your symptoms, the season you tested in, and your individual risk factors, which is why the same result can mean different things for different people.
Reading the number: nmol/L, not ng/mL
The first thing to check on a UK result is the unit. Vitamin D (25-hydroxyvitamin D, usually written 25-OH-D) is reported in nanomoles per litre (nmol/L) in the UK and much of Europe, but in nanograms per millilitre (ng/mL) in the US, the two are easy to mix up, and the difference matters, since 30 ng/mL is roughly 75 nmol/L. If you are reading a result from an American source or app, make sure you are comparing like with like.
With that settled, UK guidance from the NHS and Public Health England groups results into three broad bands, described below. Bear in mind these thresholds are population guidance, not a diagnosis on their own, your clinician interprets the number alongside your symptoms and circumstances.
The three bands: deficient, insufficient, adequate
Below 25 nmol/L, deficient. This is a level low enough to affect bone health and, in adults, is linked with bone pain, muscle weakness and fatigue. Treatment usually means a structured course of higher-dose vitamin D (sometimes called loading doses) followed by an ongoing maintenance dose.
25–50 nmol/L, insufficient for some people. This range may be adequate for some people but not for others, depending on the time of year, bone health risk and other factors, a supplement is often reasonable here even without dramatic symptoms.
Above 50 nmol/L, generally adequate for most of the population, all year round, according to current guidance. There is no strong evidence that pushing levels much higher than this brings extra benefit, and very high levels (usually only from excessive supplementation) can cause problems of their own.
Why context matters as much as the number
The same reading can mean different things depending on when and why it was taken. A level of 45 nmol/L measured in March, at the end of a UK winter with minimal sunlight, is a very different situation from the same number measured in September after a summer outdoors, the first suggests your baseline may dip further before next spring; the second suggests something is limiting how well you make or absorb vitamin D even with good sun exposure.
Symptoms matter too. Fatigue, bone or muscle aches, and low mood alongside a borderline result carry more weight than the same number found incidentally with no symptoms at all. And certain groups are known to run lower more consistently, people with darker skin, those who cover most of their skin for cultural or religious reasons, older adults, and anyone who spends most of their time indoors, which your clinician will factor in rather than reading the number in isolation.
What affects your level
Vitamin D is unusual among vitamins because your body makes most of it from sunlight on skin, rather than relying mainly on diet. That is why levels in the UK follow a clear seasonal pattern, rising through spring and summer, falling through autumn and winter, and why Public Health England recommends everyone consider a daily 10 microgram (400 IU) supplement during autumn and winter, regardless of a test result.
Diet contributes a smaller share: oily fish, egg yolks and fortified foods are the main sources, which is one reason vegans and people with limited dietary variety often run lower. Certain digestive conditions that affect fat absorption, some medicines, obesity, and reduced kidney function can also lower levels independent of sunlight or diet.
What to do with your result
If your result is deficient, the usual approach is a short course of higher-dose treatment to correct the shortfall, followed by an ongoing lower daily maintenance dose, self-treating a genuinely deficient result with an over-the-counter tablet alone is usually too slow to correct it properly. If you are insufficient or borderline, a standard daily supplement is generally enough, alongside sensible sun exposure when the season allows it.
It is worth retesting after a course of treatment, typically after three months, to confirm the level has actually corrected rather than assuming it has, and because vitamin D interacts with calcium metabolism, anyone with kidney disease, high calcium levels, or certain other conditions should have dosing guided by a clinician rather than choosing a high-dose product independently.
Symptoms worth knowing about, deficiency or not
Because vitamin D deficiency is common and often has no dramatic symptoms, many people only find out through a test done for another reason. When symptoms do appear, they tend to be non-specific: persistent tiredness, low mood, particularly in winter, sometimes described as seasonal affective disorder, and generalised bone or muscle aching, occasionally significant enough to be mistaken for a musculoskeletal problem. Children with severe, prolonged deficiency can develop rickets, affecting bone development; the equivalent in adults is called osteomalacia, causing softened, painful bones.
None of these symptoms are unique to vitamin D deficiency, which is exactly why testing rather than assuming is worthwhile. Feeling fine is not proof your level is adequate, and having some of these symptoms is not proof it isn't; the blood test is what actually tells you, which is why it is often checked alongside other markers linked to fatigue and general wellbeing rather than assumed from how you feel.
Who is most at risk of low levels
Some groups run consistently lower and are worth knowing if you fall into one of them. People with darker skin need considerably more sun exposure to produce the same amount of vitamin D, because melanin reduces the skin's ability to synthesise it. Anyone who covers most of their skin outdoors for religious or cultural reasons, older adults (whose skin becomes less efficient at making vitamin D with age), and people who are housebound or spend most of their time indoors are all at higher risk, regardless of season.
Obesity is another recognised risk factor, vitamin D is fat-soluble and can become sequestered in body fat, effectively lowering the amount circulating and available for use, which sometimes means higher supplement doses are needed to reach the same blood level. Certain digestive conditions that reduce fat absorption, such as Crohn's disease or coeliac disease, and having had weight-loss surgery, can also significantly reduce how much vitamin D your body actually absorbs from supplements or diet, which is worth flagging to whoever is advising on your dose.
Getting tested
Vitamin D testing is straightforward, a simple blood test, available on the NHS in some circumstances or privately when you want a quicker result or a routine check outside of symptoms. It is often tested alongside other markers linked with fatigue and general wellbeing; our guide to blood tests that explain fatigue covers how vitamin D fits into that wider picture.
Our clinical team offers vitamin D testing with your result reviewed and explained by a clinician, so you know not just the number but what it means for you and what, if anything, to do next.
Frequently asked questions
What vitamin D level counts as deficient in the UK?
Below 25 nmol/L is generally classed as deficient. 25 to 50 nmol/L is insufficient for some people, and above 50 nmol/L is considered adequate for most of the population.
Why do UK and US vitamin D results look so different?
The UK reports results in nmol/L, while the US typically uses ng/mL. 30 ng/mL is roughly equivalent to 75 nmol/L, so always check which unit you're reading.
Do I need a vitamin D supplement even if my result is normal?
Public Health England recommends everyone consider a daily 10 microgram supplement during autumn and winter, regardless of a test result, because UK sunlight is insufficient for most of the year.
How soon should I retest after starting vitamin D treatment?
Retesting after around three months is typical, to confirm a deficient level has actually corrected rather than assuming the treatment has worked.
Can vitamin D levels be too high?
Yes, though this is rare and almost always linked to excessive supplementation rather than sunlight or diet. Anyone on high-dose treatment should have it monitored by a clinician.