25-OH Vitamin D
Vitamin D storage form reflecting individual supply status
Also known as
Definition
25-Hydroxy vitamin D (calcidiol) is the primary storage form of vitamin D in the blood and the most reliable marker of individual vitamin D status. It is produced in the liver and converted to the active form calcitriol in the kidneys as needed. Optimal levels are essential for bone health, immune function, and hormonal balance.
| Parameter | Value |
|---|---|
| Unit | ng/ml (nmol/l) |
| Reference Range | 30–100 ng/ml (per DGE/laboratory standard) |
| Optimal Range (lab2go) | > 60 ng/ml (lab2go Optimum Zone for immune function and well-being) |
US labs report in ng/ml (= µg/l); internationally nmol/l is also used. Conversion: 1 ng/ml × 2.496 = nmol/l. Reference ranges vary by lab — always check the unit on your report.
↓ What a low value means
Levels below 30 ng/ml indicate insufficiency; below 20 ng/ml is deficiency. Consequences include impaired calcium absorption (osteoporosis/osteomalacia risk), weakened immune defense with increased susceptibility to infection, elevated risk of autoimmune disease, depressive mood, and muscle weakness. Vegans, people with darker skin tones, older adults, and those with little sun exposure are particularly at risk.
↑ What a high value means
Sustained levels above 100–150 nmol/l (40–60 ng/ml) from supplementation can lead to hypercalcemia: nausea, vomiting, weakness, and kidney calcification (EFSA safety limit: 100 µg/day = 4,000 IU). Toxicity almost exclusively occurs from uncontrolled supplementation — not from sun exposure. Levels of 60–80 ng/ml without symptoms are clinically safe.
✓ How to optimize this marker
Sun exposure: 15–30 min of midday sun on arms and legs (May–September, temperate climates). Supplementation: 2,000–4,000 IU vitamin D3 daily, combined with vitamin K2 (MK-7, 100–200 µg) to guide calcium to bones. Fat-soluble — take with a meal containing fat. Lab recheck after 3 months. Food sources (minor contribution): fatty fish, egg yolks, fortified products. Target: 60–80 ng/ml.
When to test
Once yearly, ideally at the end of winter (Feb–Mar) to capture the seasonal low point. Recheck 3 months after starting or adjusting supplementation. No fasting required. Always measure 25-OH-D, not 1,25-OH-D (active calcitriol does not reflect supply status).
Frequently asked questions
How much vitamin D should I take daily? +
The DGE recommends 800 IU/day when sun synthesis is absent. Achieving levels above 60 ng/ml often requires 2,000–4,000 IU/day — individually dependent on starting level and body weight. Doses above 4,000 IU/day should be lab-monitored.
Can I supplement vitamin D without a blood test? +
Deficiency rates in winter are high (> 50% in many regions). A base dose of 1,000–2,000 IU/day is safe for most adults. For targeted dosing, a blood test is advisable because individual absorption differences are substantial.
Why combine vitamin D3 with K2? +
Vitamin D3 increases calcium absorption. Vitamin K2 (MK-7) activates matrix Gla-protein and osteocalcin, which direct calcium into bones and inhibit vascular calcification. The combination is recommended at doses above 2,000 IU/day.
What is the difference between ng/ml and nmol/l? +
Conversion factor 2.496: 40 ng/ml ≈ 100 nmol/l. US labs report ng/ml; many international studies use nmol/l. Always check the unit on your lab report to avoid misinterpretation.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.