Vitamin D Blood Test

• 25-hydroxyvitamin D (measured): the main circulating form of vitamin D, usually reported in nmol/L. Some laboratories report ng/mL; 1 ng/mL is roughly 2.5 nmol/L.

The result is normally given as a total, combining vitamin D3 (from skin and most supplements) and vitamin D2 (from some fortified foods and supplements). The Royal Osteoporosis Society notes that some immunoassay methods may not detect all vitamin D2, while mass spectrometry methods recognise both forms equally. If you take a D2 product, mention it when you book.

This test does not measure 1,25-dihydroxyvitamin D, calcium, phosphate, parathyroid hormone or alkaline phosphatase. A clinician investigating bone or calcium problems would usually look at these as well.

Most of our vitamin D is made in the skin when it is exposed to UVB light. In the UK, the NHS notes that sunlight is only strong enough for this from about late March or early April to the end of September, so through autumn and winter we rely on stores, food (oily fish, egg yolks, red meat and fortified foods) and supplements.

Whether it comes from skin, food or a tablet, vitamin D is first converted in the liver to 25-hydroxyvitamin D. The kidneys then convert a small amount of this into 1,25-dihydroxyvitamin D, the active hormone. The body controls the active form closely, through parathyroid hormone, calcium and phosphate, so it can look normal even when supplies are running low. That is why laboratories assess status by measuring 25-hydroxyvitamin D instead.

The best-established job of vitamin D is helping the gut absorb calcium and phosphate so bones can mineralise properly. Severe, prolonged deficiency causes rickets in children and osteomalacia (soft bones) in adults. Vitamin D receptors are found in many tissues, which has driven a great deal of research into other roles, but the UK Scientific Advisory Committee on Nutrition based its 2016 recommendations on musculoskeletal health because evidence for other outcomes was not strong enough.

UK guidance, including the Royal Osteoporosis Society's clinical guideline, uses these bands for bone health:

• Below 25 nmol/L: deficient.
• 25 to 50 nmol/L: may be inadequate in some people, depending on symptoms, bone health and other risk factors.
• Above 50 nmol/L: sufficient for almost the whole population.

Laboratories word their ranges differently, and some print higher "target" figures. Results from different assay methods can also differ by a few nmol/L, so a small change between two laboratories may not be a real change in you.

Your level also moves with the seasons. It is usually lowest in late winter and early spring and highest in late summer, so the month of your sample matters when you compare results.

A very high result is almost always caused by taking more supplement than needed. Long-term excess can raise blood calcium (hypercalcaemia), which can affect the kidneys, heart and bones. The NHS advises adults not to exceed 100 micrograms (4,000 IU) a day unless a doctor has advised otherwise.

If your result is low and you have bone pain, muscle weakness or a history of fractures, or if you are on prescribed vitamin D, discuss it with your GP. Do not change prescribed treatment because of an online test. Where a result needs medical attention, we will advise you to see your GP and, with your consent, can write to them.

The NHS already advises everyone over four to consider a daily 10 microgram supplement in autumn and winter, and people who rarely go outside, cover most of their skin, or have dark skin to consider one all year. Many people can follow that advice without a test. The Royal Osteoporosis Society also advises against routine testing of people who have no symptoms, even in higher-risk groups.

A test becomes more informative when the answer would change what you do. Examples include symptoms that a clinician thinks could relate to vitamin D, a medical condition or medicine that affects how vitamin D is absorbed or used (some anticonvulsants are one example), uncertainty about whether a long-standing supplement is enough or too much, or a request from your GP for a result.

May find it useful: people with limited sun exposure or darker skin who want to know their status; people taking supplements long term who want to check their level is sensible; people asked by a clinician to provide a result.

Probably less useful: people who feel well and already follow NHS supplement advice, because the result is unlikely to change what they do.

Better assessed elsewhere: anyone with new bone pain, a suspected fracture or a known high calcium result should see their GP. People under specialist care for kidney disease, parathyroid or calcium problems, or sarcoidosis should have vitamin D checked through their own team. If you are pregnant, follow your midwife's advice on vitamin D.

Established: adequate vitamin D supports bone and muscle health throughout life. This is why the NHS highlights year-round supplements for people who are frail, housebound or living in care homes.

Associations: in observational studies, lower 25-hydroxyvitamin D is linked with a wide range of conditions. People who are unwell often spend less time outdoors, so a low level can be a consequence of poor health as well as a possible contributor. Correlation alone does not show cause.

Trial evidence: the VITAL trial randomised almost 26,000 US adults (men aged 50 and over, women 55 and over) to 2,000 IU of vitamin D3 a day or placebo. Over a median of 5.3 years, supplementation did not reduce invasive cancer or major cardiovascular events. A lower cancer death rate appeared in one secondary analysis, but it was one of many outcomes examined, so the authors advised caution.

Unproven: there is no good evidence that pushing vitamin D well above the sufficient range slows ageing or extends life.

• Fasting is not usually needed. Eat and drink normally unless the laboratory instructs otherwise.
• Keep taking any prescribed vitamin D. Note the product, dose and when you last changed it, as this helps anyone reading your result.
• If you want to see the effect of a new dose, book at least three months after starting or changing it. The Royal Osteoporosis Society notes that levels may take up to six months to settle.
• Some laboratory methods use biotin-based chemistry. If you take high-dose biotin (common in hair, skin and nail supplements), tell us when booking so the laboratory can advise.

Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.

• Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
• Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
• Postal home finger-prick kit: Ask before ordering. We offer a home kit only where the laboratory confirms this test is suitable for a finger-prick sample.

Sample: Blood sample (serum for venous collection; capillary sample type for home kits confirmed by the laboratory)

Collection notes: No fasting needed. If you are checking the effect of a new or changed supplement dose, book at least three months after the change. A postal finger-prick kit is offered only where the laboratory confirms this test is suitable for capillary samples.

The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.

Beyond the blood test: optional support, priced separately

• Personalised written interpretation report: £50
• General consultation: £50
• Results consultation (results discussion and follow-up advice): £100
• Personalised programme: from £150, after we assess whether a programme suits you

Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.

If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.

Will my report show vitamin D2 and D3 separately?

Usually the result is reported as a single total. Separate D2 and D3 values depend on the laboratory's method, and we will confirm what your report includes before you book.

When in the year should I test?

It depends on your question. A sample in late winter or early spring shows your level near its seasonal low. If you are checking a supplement, timing relative to the dose change matters more than the month.

Should I stop my supplement before the test?

No. The test is meant to reflect your usual intake. Never stop or change prescribed vitamin D without speaking to the prescriber.

Can vitamin D be too high?

Yes. Excess from supplements can raise blood calcium. Sunlight alone does not cause vitamin D toxicity.

Can I use my result to choose a supplement dose?

Over-the-counter doses should stay within NHS guidance. If you are on a prescribed dose or your result is very low or high, your GP or pharmacist should advise.

Is interpretation included?

The price covers the laboratory analysis and report. If you would like help understanding the result, you can add a personalised written interpretation report (£50) or book a results consultation (£100).

1. SACN vitamin D and health report. Scientific Advisory Committee on Nutrition, Public Health England, 2016. https://www.gov.uk/government/publications/sacn-vitamin-d-and-health-report
2. Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management. Royal Osteoporosis Society, 2018 (reviewed 2020). https://theros.org.uk/media/ef2ideu2/ros-vitamin-d-and-bone-health-in-adults-february-2020.pdf
3. Vitamin D: vitamins and minerals. NHS, 2020. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
4. Vitamin D. Lab Tests Online UK. https://labtestsonline.org.uk/tests/vitamin-d
5. Manson JE, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine, 2019;380(1):33-44. https://pubmed.ncbi.nlm.nih.gov/30415629/