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. Vitamin D is involved in bone metabolism, immune function, and hormonal processes.
| Parameter | Value |
|---|---|
| Unit | ng/ml (nmol/l) |
| Reference Range | Lab-dependent, e.g. 30–100 ng/ml; the range on your report is what counts. Separately, the German DGE gives supply thresholds: below 12 ng/ml (30 nmol/l) deficiency, 20 ng/ml (50 nmol/l) and above good supply for bone health. |
Agree on your personal target range with your doctor.
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; the unit on your report is what counts.
↓ What a low value means
The lower limit on your report is set by your lab. The classification of the German DGE is a separate matter: it describes values below 12 ng/ml (30 nmol/l) as deficiency and values of 20 ng/ml (50 nmol/l) and above as good supply for bone health. Values in between are therefore not automatically a clinical deficiency. A marked, long-standing deficiency can impair calcium absorption, which raises the risk of bone softening (osteomalacia, rickets in children) and osteoporosis, and can come with muscle weakness. Links with susceptibility to infection, autoimmune disease or mood come mainly from observational studies and do not prove a cause; symptoms cannot be inferred from the value alone. Low values are more common with little sun exposure, in older age, with darker skin and with a vegan diet.
↑ What a high value means
Vitamin D toxicity with hypercalcemia (nausea, vomiting, weakness, kidney calcification) typically comes with sustained levels above 150 ng/ml (375 nmol/l). Toxicity almost exclusively occurs from uncontrolled supplementation — not from sun exposure.
✓ What can influence this value
The value is influenced by sun exposure (varies with the season and is lower in winter in Central Europe), skin type, time spent outdoors, and body fat. Preparations containing vitamin D, diet (fatty fish, egg yolks, fortified products, a minor contribution), diseases of the gut, liver, and kidneys, and certain medications can also change the value. The assessment is made by the treating doctor.
When to test
How often it is measured is decided by your doctor. The value varies with the season and usually reaches its low point at the end of winter (February–March). No fasting required. Supply status is assessed with 25-OH-D, not 1,25-OH-D, because active calcitriol does not reflect supply status.
Frequently asked questions
Does the 25-OH-D value indicate a suitable intake amount? +
No. The value describes your current supply status. How much a preparation changes the level depends, among other things, on the starting level and body weight. Whether and how much vitamin D is taken is decided by your doctor.
Why is vitamin D measured in the blood? +
Low vitamin D levels are common in winter in many regions. How much preparations raise the level varies considerably between individuals because absorption differs. Only a measurement shows your actual supply status.
Why is vitamin D3 often combined 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.
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. The unit on your lab report is what counts; mixing up the units leads to misinterpretation.
25-OH Vitamin D
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.