Vitamin D is in fact a hormone that the body produces in the skin under the sun. It is needed not only for bones and calcium but also for muscles, immunity and metabolism.
Its deficiency is one of the most widespread in the world, especially at northern latitudes and in winter. The test measures the 25(OH)D form — it reflects the vitamin's stores.
Reference ranges
Interpretation of 25(OH)D:
| Deficiency | < 20 ng/mL (< 50 nmol/L) |
| Insufficiency | 20 – 30 ng/mL |
| Normal | 30 – 50 ng/mL and above |
What an elevated level means
Too high a vitamin D comes almost exclusively from supplement overdose (the sun does not cause overdose).
Excess is dangerous: it raises blood calcium (hypercalcemia), which threatens nausea, weakness, kidney stones and impaired kidney function.
So high doses are taken only as prescribed and with test monitoring, not "just in case".
What a low level means
Vitamin D deficiency often runs unnoticed, but it can show up as fatigue, muscle and bone pain, frequent colds and low mood.
Why it is dangerous: fragile bones (rickets in children, osteoporosis in adults), muscle weakness, and possible links to immunity and metabolism.
Who is at risk: people at northern latitudes, those rarely in the sun, the elderly, people with dark skin, and those with excess weight.
How the test is done
Venous blood for 25(OH)D. It needs no special preparation.
Vitamin D is fat-soluble, so a supplement is best taken with a meal containing fat. The dose and the need to take it are determined by the test level — not "blindly".
What affects the value
- Sun — the main source; in winter and at northern latitudes it is insufficient.
- Latitude and season — in autumn and winter skin synthesis is minimal.
- Skin color — dark skin synthesizes vitamin D more slowly.
- Age — over the years the skin produces it less well.
- Excess weight — vitamin D gets "trapped" in fat tissue.
Calculate using this marker
Calculators where Vitamin D (25(OH)D) is used directly:
Frequently asked questions
What vitamin D level is normal?
A 25(OH)D of 30 ng/mL (75 nmol/L) and above is usually considered normal; 20–30 is insufficiency, below 20 is deficiency. The optimal target level is debated, but most recommendations advise keeping it in the 30–50 ng/mL range.
Should everyone take vitamin D?
Many people at northern latitudes in winter — yes, but the dose is best chosen by test rather than by guesswork. Vitamin D is fat-soluble and in large doses can accumulate and cause harm (raise calcium). It is wiser to test 25(OH)D and decide by the result.
Can vitamin D be topped up with sun?
In summer and at southern latitudes — largely yes, but in winter and in the north skin synthesis almost stops. Plus, with age and dark skin it is slower. Then a supplement (guided by the test level) and foods — oily fish, eggs — help.