A vitamin D blood test measures 25-hydroxyvitamin D, written 25(OH)D. This is the form your liver makes from vitamin D. It reflects what you get from sunlight, food and supplements. Most people do not need the test. In Ireland, HSE laboratory advice issued in 2024 says vitamin D testing should not be used as a general screen. It reserves testing for specific conditions, such as bone disease, malabsorption, kidney or liver disease and certain medicines. UK bone-health guidance and the US Endocrine Society also advise against routine testing.
If you do have the test, the number only makes sense alongside two things: the units, and which threshold your doctor is using. Irish and UK bodies work in nmol/L. US bodies usually quote ng/mL. And expert bodies do not agree on where "low" starts. This guide explains both, who guidelines say should and should not be tested, and what a result can and cannot tell you.
If you are here because of mood, memory or brain fog, what the vitamin D trials show for cognition and mood is covered in a separate article.
Your liver converts vitamin D into 25(OH)D. Most vitamin D blood tests measure 25(OH)D. The US National Library of Medicine says that is the most accurate way to see if you have enough. The US Institute of Medicine (now the National Academy of Medicine) called it a useful integrated marker of vitamin D exposure.
There is a second test, for 1,25-dihydroxyvitamin D, sometimes called "active vitamin D" or calcitriol. It is not the right test for checking whether you have enough. The Institute of Medicine describes it as a more complex hormone measure, regulated by calcium and parathyroid hormone and affected by kidney function. The National Library of Medicine says it may be used to monitor kidney problems or investigate abnormal calcium.
Some reports list vitamin D2 and D3 separately. The National Library of Medicine says total vitamin D is the number to check.
To convert ng/mL to nmol/L, multiply by about 2.5. To go the other way, divide by 2.5. The US Preventive Services Task Force (USPSTF), for example, gives 20 ng/mL as 49.9 nmol/L.
So a result of "20" means very different things depending on the unit. Check which one your report uses before comparing it with anything you read.
Different bodies set thresholds for different jobs. Some protect bone health across a whole population. Others were meant to guide individual care. Most are based mainly on bone outcomes. The list below shows what each body we checked actually says.
The FSAI uses the Institute of Medicine's definitions in its Irish recommendations. SACN notes that its 25 nmol/L level is not a clinical threshold for diagnosing rickets. It is a population target for protecting musculoskeletal health.
This is the change most likely to confuse you if you have read older advice. The Endocrine Society's 2011 guideline defined "sufficiency" as at least 75 nmol/L (30 ng/mL). It called 50 to 75 nmol/L (20 to 30 ng/mL) "insufficiency". In 2024, the Society said it no longer endorses either definition. Its guideline panel found that trial evidence does not support any threshold that predicts meaningful benefit from supplementation. The panel rated its overall certainty in the evidence as very low.
The 2024 guideline also suggests against routine 25(OH)D testing in the general population. It found no trial evidence to support screening people with obesity or a darker complexion either. It suggests vitamin D for some groups without testing first: ages 1 to 18, adults over 75, pregnancy, and high-risk prediabetes. These recommendations do not apply to people with established reasons for testing or treatment.
The HSE's 2024 laboratory advice note lists the accepted reasons for testing. In general, a test request should give one of these:
The Royal Osteoporosis Society recommends testing in three situations. One is musculoskeletal symptoms that could be due to deficiency. Another is suspected bone disease that vitamin D treatment may improve. The third is bone disease before specific treatment, where deficiency may need correcting first.
The National Library of Medicine says a provider may order the test for symptoms or conditions linked to deficiency. Examples include bone pain, muscle weakness or low bone density. It also lists high-risk situations such as malabsorption, weight-loss surgery and certain medicines.
If you have multiple sclerosis, what the vitamin D research does and does not show in MS is covered separately.
In the US, the USPSTF concluded in 2021 that the evidence is insufficient to weigh the benefits and harms of screening adults without symptoms. That is an "I statement". It is neither a recommendation for screening nor against it.
Our guide to blood tests for brain fog discusses vitamin D in that context. Whether a test is appropriate for you is a decision for your GP.
Very high levels are uncommon and usually come from supplements, according to the National Library of Medicine. It lists nausea, vomiting, poor appetite, constipation, weakness and weight loss as possible symptoms. The USPSTF says levels linked with toxicity are typically above 150 ng/mL (about 375 nmol/L). If your result is high, talk to your GP before changing anything.
It is an ordinary blood test from a vein in your arm. The National Library of Medicine says no special preparation is needed. Tell whoever takes the sample about all medicines, vitamins and supplements you take, because they can affect results. Do not stop any medicine unless your doctor tells you to.
A GP or hospital doctor requests the test, stating the clinical reason. The HSE advice note says laboratories should, as far as practical, reject requests that do not meet the criteria. They should then send a report saying the test was not done. If you pay for a private test, the same limits on interpretation apply.
In the UK, the Royal Osteoporosis Society list above is the main bone-health guidance we found. In the US, the USPSTF position means screening of people without symptoms is not recommended either way. Insurance cover for testing varies, so check before you pay.
There is no single agreed answer. In Ireland, the FSAI uses below 30 nmol/L as at risk of deficiency and 50 nmol/L or more as sufficient for practically everyone. UK bone guidance uses below 25 nmol/L as deficient. The Endocrine Society no longer endorses its earlier sufficiency and insufficiency cut-offs.
Multiply by about 2.5. So 20 ng/mL is about 50 nmol/L, and 30 ng/mL is about 75 nmol/L.
The HSE says that for most people without a specific reason, following Department of Health advice does not require testing. If you have a condition on the lists above, ask your GP.
No, not for checking whether you have enough. That test measures 1,25-dihydroxyvitamin D. It is mainly used for kidney problems or abnormal calcium.
The National Library of Medicine says no special preparation is needed. If other blood tests are being taken at the same time, follow the instructions for those.
This article is general information, not medical advice. It cannot tell you whether you need a test or what your result means for you. Talk to your GP or pharmacist about your own situation.
Reviewed by: Proco Team
Last updated: November 2026