Single marker guide · Vitamin D (25-hydroxyvitamin D)
One in six UK adults is vitamin D deficient every winter and most have no idea, because the symptoms look like ordinary tiredness. A 25(OH)D test tells you your level in a single number. Whether that number explains how you feel depends on the markers tested beside it.
Venous draw at 103 UK clinics or at home. UKAS-accredited laboratory. Report reviewed by a medical professional.
The short answer
A vitamin D test measures 25-hydroxyvitamin D, the storage form circulating in your blood, in nanomoles per litre. Below 25 nmol/L is deficiency. Between 25 and 50 is insufficient. Above 50 is what the NHS calls sufficient, and most clinicians working in preventive medicine aim for 75 to 125, because that is where bone, immune and muscle outcomes look best in the research.
The problem is that low vitamin D shares its symptom list with low iron, an underactive thyroid, low B12 and poor sleep: tiredness, low mood, aches, getting every cold going. If you test vitamin D alone, find it low, supplement for three months and still feel flat, you have spent money learning one thing and ruled out nothing. That is why we test it as part of a panel, never on its own.
Reference ranges
| 25(OH)D result | Category | What it means |
|---|---|---|
| Below 25 nmol/L | Deficient | Bone and muscle effects likely over time. NHS threshold for treatment. Usually needs a higher corrective dose, then maintenance. |
| 25 to 50 nmol/L | Insufficient | Common in the UK from October to April. Enough to avoid bone disease, not enough for optimal immune and muscle function in most studies. |
| 50 to 75 nmol/L | Sufficient | Meets the NHS definition of adequate. Many clinicians consider the lower half of this band still worth improving. |
| 75 to 125 nmol/L | Optimal | The range most preventive clinicians target. Associated with the best outcomes for bone density, immunity and muscle strength. |
| Above 250 nmol/L | Excess | Almost always from high-dose supplements. Can raise blood calcium. Reduce dose and retest. |
UK laboratories report in nmol/L. If you have a result in ng/mL from elsewhere, multiply by 2.5 to convert. TrueVitals reports show your result against both the standard and the optimal range, with the reasoning written out.
Symptoms
The most common complaint. Low D affects muscle energy production directly, and the fatigue is heavy rather than sleepy. It is also the most common symptom of low iron, low B12 and thyroid problems, which is exactly why one marker is never enough. Always tired but tests normal?
Vitamin D regulates immune cells in the lining of your airways. Deficiency is associated with more frequent respiratory infections, and correcting it reduces them in people who start low. Feeling run down guide.
Vitamin D receptors sit in the parts of the brain that regulate mood. Low levels are consistently associated with low mood and seasonal dips. It is not the whole story of depression, but it is a cheap thing to rule in or out.
Vitamin D controls calcium absorption. Persistently low levels soften bone and weaken muscle, which shows up as vague aches in the lower back, hips and thighs, and difficulty climbing stairs.
Hair follicles need vitamin D to cycle properly. Deficiency is linked to increased shedding, though ferritin is the more common driver and needs checking alongside. Hair loss blood test guide.
Vitamin D affects muscle repair and strength. Athletes and gym-goers with low levels recover slower and are more injury-prone, and indoor training makes deficiency more likely, not less. Blood tests for gym-goers.
Why the UK is different
Vitamin D is made in skin exposed to UVB light. At UK latitudes the sun sits too low from roughly October to March for any UVB to reach the ground, so for half the year your only sources are food and supplements, and food alone rarely gets anyone above 50 nmol/L. This is why Public Health England advises everyone in the UK to consider a 10 microgram (400 IU) supplement through autumn and winter, and why deficiency is so common by February.
Some people are low all year: anyone with darker skin, anyone who covers up or works indoors, people over 65, anyone carrying extra body fat (vitamin D is stored in fat and held there), and people on certain medications. The northern half of the country sits at a higher latitude than most of Canada's population, which is part of why Manchester, Leeds and Glasgow see such high winter deficiency rates.
Read it in context
A vitamin D result on its own tells you whether to take a supplement. Read alongside the right markers, it tells you whether vitamin D was ever the problem.
Low iron is the other great cause of tiredness, hair shedding and breathlessness, and it is far more common than low D in women under 50. If both are low, treating only vitamin D leaves you tired. Iron blood test guide.
An underactive thyroid produces the same fatigue, low mood and weight gain. TSH alone misses conversion problems. Thyroid blood test guide.
Low B12 causes fatigue, brain fog and pins and needles, and is common in vegans, vegetarians and anyone on acid-reducing medication. B12 blood test guide.
When vitamin D has been low for a long time, parathyroid hormone rises to hold calcium steady. A raised PTH with low D tells you the deficiency has been going on for months and matters. Both are in the Ultimate panel.
Vitamin D cannot be activated without magnesium. Supplementing D with low magnesium is one of the most common reasons levels refuse to rise on retest.
Low vitamin D and raised inflammatory markers travel together. Seeing both tells you more about immune function than either alone.
Which test
Advanced
74 biomarkers · £269
Vitamin D with full iron studies, full thyroid including Free T3 and antibodies, B12, folate, magnesium, liver, kidney, lipids including ApoB, and hormones. The right choice if tiredness is the question.
View AdvancedMost chosen
Ultimate
114 biomarkers · £349
Everything in Advanced plus parathyroid hormone, complete hormones, fasting insulin and HOMA-IR, Lp(a), immunoglobulins, tumour markers and urinalysis. The full picture in one draw.
View UltimateSignature
230 biomarkers · £799
Processed by Randox. The deepest private panel in the UK, with extended cardiovascular, autoimmune, digestive and neurological screening.
View SignatureClinic draw £19 at 103 UK clinics. At-home phlebotomist £39 across 200+ areas. Fast from 10pm the night before. Pay monthly available.
Honest comparison
If you already know you are deficient and simply want to confirm your supplement is working, a standalone finger-prick vitamin D test from Medichecks or Thriva costs around £39 and does that job well. There is no need to pay for a panel to check one number you already understand.
A panel is the better choice when you are testing because of how you feel, when you have never had a full picture, or when you want to stop guessing which of five overlapping causes is behind your symptoms. The Advanced panel works out at under £4 per marker, and the report explains every result in plain English rather than leaving you with a spreadsheet. See how every UK provider compares.
After the result
Speak to your GP. Deficiency at this level is usually treated with a short higher-dose course before a daily maintenance dose. Your TrueVitals report flags this clearly and tells you what to say.
A daily vitamin D3 supplement through autumn and winter is standard advice, taken with food and, if your magnesium is low, alongside that. Your report gives a target and a retest date.
Vitamin D moves slowly. Three months of consistent supplementation is the right interval to see whether your dose is enough. Your second report compares against your first automatically.
This page is general information, not medical advice. If you have symptoms, your report will tell you which findings to discuss with your GP and why.
FAQs
It measures 25-hydroxyvitamin D (25(OH)D), the storage form of vitamin D in your blood, reported in nmol/L in the UK. This is the correct marker for assessing your vitamin D status. The active form, 1,25-dihydroxyvitamin D, is not useful for this and is only tested in specific kidney or calcium conditions.
The NHS defines below 25 nmol/L as deficient, 25 to 50 as insufficient and above 50 as sufficient. Most clinicians working in preventive medicine aim for 75 to 125 nmol/L, which is where bone, immune and muscle outcomes look best in the research. TrueVitals reports show both ranges.
Not for vitamin D itself. TrueVitals panels also include glucose, insulin and lipids, so we ask you to fast from 10pm the night before your appointment and drink water as normal.
Sometimes. GPs generally test vitamin D only when there are symptoms suggesting bone disease or a specific risk factor, and many areas restrict routine testing. If your GP declines, a private test is the straightforward route. What to ask your GP for.
A standalone finger-prick vitamin D test is around £39 from providers like Medichecks. A TrueVitals Advanced panel is £269 and includes vitamin D with 73 other markers covering iron, thyroid, B12, hormones, liver, kidney and cardiovascular risk, with a venous draw and a fully written report.
Your personalised report and dashboard are delivered within 48 hours of the laboratory receiving your sample for Advanced and Ultimate panels. Signature panels take longer because of their depth.
Public Health England advises 10 micrograms (400 IU) daily for everyone in autumn and winter. People who test low are often advised higher maintenance doses by their clinician, and deficiency below 25 nmol/L is usually treated with a short higher-dose course first. Retest after three months. Do not take very high doses long term without testing, since excess vitamin D raises blood calcium.
Yes. Vitamin D is included in Advanced (74 markers), Ultimate (114) and Signature (230). Ultimate also includes parathyroid hormone, which shows whether a low vitamin D level has been affecting calcium regulation.
74 to 230 biomarkers. Venous draw at 103 UK clinics or at home. A report that tells you what your results mean and what to do next.
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