How to Check Your Vitamin D Levels: The Test and What the Numbers Mean

You check your vitamin D level with a blood test for 25-hydroxyvitamin D, which a GP arranges when there are bone or muscle symptoms or suspected or known bone disease; NICE's clinical knowledge summary says "Do not routinely test for vitamin D deficiency in asymptomatic people" and advises people at higher risk to take the maintenance dose rather than be tested.¹ UK results come in nanomoles per litre: below 25 nmol/L the risk of poor musculoskeletal health is considered to increase, 25 to 50 nmol/L may be insufficient for some people, and above 50 nmol/L is sufficient for most people.^{2, 3} Home finger-prick kits measure the same marker in the same units. This is the testing page of the vitamin D guide, within the vitamins and minerals guide; the symptoms are on signs of vitamin D deficiency.

Can I get a vitamin D test on the NHS?

Yes, when a GP judges it indicated. NICE's summary for adults says to measure serum 25(OH)D if a person has "Musculoskeletal symptoms (such as bone pain, myalgia, and generalized weakness) that may be attributable to vitamin D deficiency", suspected bone disease such as osteomalacia or osteoporosis, or known bone disease where deficiency must be corrected before treatment.¹ The NHS colecalciferol page puts it plainly: "To find out if you have low vitamin D levels, your doctor will give you a blood test."⁴ If a GP suspects rickets or osteomalacia, the NHS says "they'll check your vitamin D and calcium levels".⁵ The test goes through the GP; a pharmacist can advise on the daily 10 microgram supplement.

Why do doctors not test everyone?

Because the result would not change the advice. NICE: "There is insufficient evidence to recommend screening the general population for vitamin D deficiency, as treating asymptomatic people with identified vitamin D deficiency has not been shown to improve health outcomes."¹ The Endocrine Society's 2024 guideline "suggests against routine 25(OH)D testing in all populations considered".⁶ UK advice goes straight to the supplement: people at higher risk "should be advised on the need for maintenance-dose vitamin D supplementation",¹ and the NHS says everyone should consider 10 micrograms (400 IU) a day "during the autumn and winter", the groups it names all year, and that "10 micrograms a day will be enough for most people".⁷

What does the test measure?

Serum 25-hydroxyvitamin D, the form the liver makes from vitamin D. NICE's summary: 25(OH)D "is the best clinical indicator of vitamin D status. It reflects both skin synthesis and dietary sources of vitamin D, has a fairly long half-life of 2–3 weeks, and is not subject to tight homeostatic control"; the hormone the kidneys then make from it, 1,25-dihydroxyvitamin D, is "a poor indicator of vitamin D status".¹ A laboratory can measure 25(OH)D2 and 25(OH)D3 together, as total 25(OH)D, or report them separately, which matters if you take vitamin D2 (below).

What do the numbers mean, and what is a normal vitamin D level?

UK laboratories report nmol/L. The national survey states that "In the UK, risk of poor musculoskeletal health is considered to increase at 25(OH)D concentrations below 25nmol per litre", the threshold SACN set in 2016; NICE's children summary gives the three bands in the wording below.^{2, 3}

The US Institute of Medicine tied its intake values to a level of at least 50 nmol/L (20 ng/mL) and found that "Higher values were not consistently associated with greater benefit" and that "for some outcomes U-shaped associations were observed, with risks at both low and high levels".⁸ A target of 100 nmol/L, often repeated online, has no basis in UK guidance. Very high results are covered on can you take too much vitamin D.

When in the year should you test?

For adults aged 19 to 64 in the National Diet and Nutrition Survey, Table 6.15 (2016 to 2023, by month of blood sample) gives 6.1% below 25 nmol/L in July to September, 13.7% in October to December, 31.4% in January to March and 17.1% in April to June, the mean falling from 62.4 to 39.9 nmol/L; across the year, Table 6.9 (2019 to 2023) puts 18.2% of adults aged 19 to 64 below 25 nmol/L, about one in six.² A late-winter test shows your lowest level of the year and a September test your highest, so a result is read with its date. Why UK sunlight makes no vitamin D in skin from October to March is on getting vitamin D from the sun in the UK.

Are home finger-prick tests reliable?

A home kit is a finger-prick sample posted to a laboratory, which measures 25(OH)D and reports it in nmol/L. The marker is the one the NHS uses, so the question is the laboratory's method, not the format. In a 2004 study, serum from ten adults sent to six laboratories came back with mean 25(OH)D that "ranged from 17.1–35.6 ng/ml", and the authors concluded that "whether an individual is found to have low or normal vitamin D status is a function of the laboratory used".⁹ Methods have moved on: the national survey standardises its results to the international reference method, and a laboratory that does the same and reports total 25(OH)D in nmol/L gives a result you can read against the table.² A below-25 result with symptoms from the NHS list is a reason to see a GP.

Does the test count vitamin D2?

Usually, but ask. In a 2012 comparison of automated assays, "All immunoassays measured total 25OH-D (including D(3) and D(2)), with the exception of" one, which measured 25(OH)D3 only.¹⁰ A 2009 antibody test specific to 25(OH)D3, with 0.7% cross-reactivity to 25(OH)D2, showed no significant change in people taking vitamin D2 for three months: it was not designed to see the D2 metabolite.¹¹ If you take vitamin D2, a D3-only test would under-read your status, so ask whether the laboratory reports total 25(OH)D or the two forms separately. The forms are explained on what vitamin D2 (ergocalciferol) is.

How long after starting a supplement does the level change, and how often should you re-test?

Weeks, not days. 25(OH)D has a half-life of two to three weeks; a tracer study in 36 healthy men measured about 14 to 15 days.^{1, 12} In a 22-week Irish winter trial, 238 adults took 0, 5, 10 or 15 micrograms of vitamin D3 a day from October to March; by March, 25(OH)D was 1.96 nmol/L higher for each microgram of daily intake.¹³ After a confirmed deficiency, NICE says "the first follow-up 25(OH)D level should not be arranged before 8–12 weeks after the beginning of treatment", and the NHS says "You'll usually take a blood test 1 month after starting your treatment and then again after 3 to 6 months to check your levels".^{4, 14} Otherwise "Routine monitoring of serum 25-hydroxyvitamin D (25[OH]D) levels is not necessary"; re-testing at three to six months is considered only in the groups NICE lists.¹⁴ For a severely low level a GP may prescribe, in the NHS's words, "a loading dose course of vitamin D" that "is only available on prescription".⁴ When in the day to take a supplement is on the best time to take vitamin D.

Frequently asked questions

How do I tell if my vitamin D is low without a test?

You cannot. The NHS signs, bone pain, muscle weakness, cramps and fractures after minor accidents, are shared with other conditions, and NICE's guidance speaks of "asymptomatic people with identified vitamin D deficiency", so a low level often causes nothing you would notice.^{1, 5}

Can you provide a chart of vitamin D levels?

The table above is the UK chart: below 25 nmol/L (10 ng/mL) an increased risk of deficiency, 25 to 50 nmol/L (10 to 20 ng/mL) possibly insufficient for some people, above 50 nmol/L (20 ng/mL) sufficient for most people. A result in ng/mL is multiplied by 2.5.^{2, 3}

How much does a vitamin D test cost?

Where a GP judges it indicated, the NHS test is free at the point of use. Home kits and private tests are priced by each provider; whichever you use, ask for total 25(OH)D in nmol/L.

Has UK vitamin D guidance changed recently?

Yes. In December 2025 NICE reviewed its 2014 public health guideline PH56 and decided: "We will stand down this guideline (PH56) because it is no longer applicable to the wider healthcare system context and it is inconsistent with current NHS advice on the use of vitamin D" (page last updated 9 December 2025).¹⁵

Sources

1. NICE Clinical Knowledge Summaries. Vitamin D deficiency in adults: when should I suspect or test? Revised August 2026. cks.nice.org.uk.
2. National Diet and Nutrition Survey 2019 to 2023: report, chapter 6, and data tables 6.9 and 6.15. 11 June 2025. gov.uk.
3. NICE Clinical Knowledge Summaries. Vitamin D deficiency in children: summary. cks.nice.org.uk.
4. NHS. About colecalciferol. Reviewed 10 January 2023. nhs.uk.
5. NHS. Rickets and osteomalacia. Reviewed 9 July 2025. nhs.uk.
6. Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 2024. PubMed 38828931.
7. NHS. Vitamin D. Reviewed 3 August 2020. nhs.uk.
8. Ross AC, et al. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine. Journal of Clinical Endocrinology and Metabolism, 2011. PubMed 21118827.
9. Binkley N, et al. Assay variation confounds the diagnosis of hypovitaminosis D. Journal of Clinical Endocrinology and Metabolism, 2004. PubMed 15240586.
10. Farrell CJ, et al. State-of-the-art vitamin D assays: automated immunoassays compared with LC-MS/MS. Clinical Chemistry, 2012. PubMed 22230812.
11. Li B, et al. Selective monitoring of vitamin D2 and D3 supplementation with a highly specific 25-hydroxyvitamin D3 immunoassay. Clinica Chimica Acta, 2009. PubMed 19336231.
12. Jones KS, et al. 25(OH)D2 half-life is shorter than 25(OH)D3 half-life and is influenced by DBP concentration and genotype. Journal of Clinical Endocrinology and Metabolism, 2014. PubMed 24885631.
13. Cashman KD, et al. Estimation of the dietary requirement for vitamin D in healthy adults. American Journal of Clinical Nutrition, 2008. PubMed 19064513.
14. NICE Clinical Knowledge Summaries. Vitamin D deficiency in adults: management and follow-up. Revised August 2026. cks.nice.org.uk.
15. NICE. December 2025 exceptional surveillance of vitamin D: supplement use in specific population groups (PH56). Updated 9 December 2025. nice.org.uk.

This article is general information, not medical advice; a GP decides whether to test and what to prescribe.