Short answer: The vitamin D test measures 25-hydroxy vitamin D (25-OH D), the form of vitamin D that circulates in the blood for weeks and reflects how much the body has from sunlight, food and supplements. Results are given in ng/mL (common in the US) or nmol/L (common in Europe).
At a glance
| Measures | 25-hydroxy vitamin D, the form that circulates for weeks |
|---|---|
| Units | ng/mL (US) or nmol/L (Europe); 1 ng/mL = 2.5 nmol/L |
| LOINC | 1989-3 |
| Moves with | Season, sunlight, skin, supplements, weight |
| Reference ranges | Differ between laboratories and guidelines |
| Keep | Value, unit, date and the dose of any supplement |
What the test measures
The skin makes vitamin D in sunlight, and it also comes from food and supplements. The liver turns it into 25-hydroxy vitamin D, which is what the test measures; the kidneys then make the active hormone (1,25-dihydroxy vitamin D), which is a different test and is not used to check vitamin D status.
Some reports give 25-OH vitamin D3 only, others D2 + D3 together. The method is printed on the report.
When it is ordered
Common reasons, which your doctor adapts to you:
- when a doctor checks bone health or calcium levels;
- in people at risk of low vitamin D (little sun, darker skin, some conditions or medicines);
- to follow supplementation.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| 25-hydroxyvitamin D3 | 1989-3 | ng/mL | nmol/L (×2.496) |
| Calcium (often on the same report) | 17861-6 | mg/dL | mmol/L (×0.2495) |
nmol/L = ng/mL × 2.496 (about 2.5): 20 ng/mL ≈ 50 nmol/L, 30 ng/mL ≈ 75 nmol/L. Mixing up the units is the most common source of confusion with this test.
Reading your report
Thresholds for 'deficient', 'insufficient' and 'sufficient' differ between guidelines and countries, so the range on your report and your doctor's view matter more than numbers found online. Levels are usually lower at the end of winter and higher at the end of summer; when you compare results over the years, note the season. Very high levels are possible with large supplement doses; do not raise doses without medical advice.
Keeping the results in your record
Scan or photograph the report in Anpheros Daily: each value is read with its unit and the laboratory's reference range, you check it, and it is stored with its LOINC code as an HL7 FHIR Observation, a format used by many hospitals and national health systems. Results from different laboratories line up on one chart over time, and the lab unit converter helps when two reports use different units. Our guide on how to read lab results explains reference ranges and flags in general; the lab results guide for a new doctor shows how to share them.
What this page does not do
It explains what the test is and how the result is written. It does not interpret your result, diagnose or recommend treatment: the meaning of a value depends on your history, your other results and how the sample was taken, and that is your doctor's job.
Related pages
Sources
Frequently asked questions
How do I convert vitamin D from ng/mL to nmol/L?
Multiply by 2.496. For example, 30 ng/mL is about 75 nmol/L.
Why is my vitamin D lower in winter?
Because the skin makes most of it in summer sunlight. Seasonal changes are normal; compare results from the same season.
Is the 25-OH test the right vitamin D test?
For checking vitamin D status, yes. The 1,25-dihydroxy test measures the active hormone and is used for specific medical questions.
Do I need to fast for a vitamin D test?
Usually not, unless it is taken with other tests that need fasting.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
How it works in the app →
Anpheros builds a medical record that the patient controls and can share with any clinic, on an HL7 FHIR R4 platform, based in the EU. These guides help you understand and organise your health; they do not replace your doctor.
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