Vitamin D — 25-Hydroxyvitamin D
The 25-hydroxy vitamin D test measures how much vitamin D is in your body. How to read your result against the published levels, and what moves it.
The 25-hydroxy vitamin D test is the most accurate way to measure how much vitamin D is in your body — vitamin D your skin makes in sunlight and gets from food and supplements, and that your bones and immune system need.
Watch the 28-second version
Your skin makes vitamin D in sunlight — and clouds, age, darker skin and sunscreen all cut how much.
Transcript & sources → All videosMr. Age is AgeGen's editorial character. Facts come from the protocol above.
The biomarker
- Name: Vitamin D — 25-Hydroxyvitamin D (25-OH)
- Units: ng/mL
- Reference levels: many experts recommend 20–40 ng/mL; others recommend 30–50 ng/mL (ranges vary by lab)
How to read your result
| Value (ng/mL) | Interpretation |
|---|---|
| < 12 | At risk of vitamin D deficiency, per the Food and Nutrition Board (NIH ODS) |
| 12–20 | Some people are potentially at risk of inadequacy, per the Food and Nutrition Board (NIH ODS) |
| 20–50 | Adequate for bone health and overall health in most people, per the Food and Nutrition Board (NIH ODS) |
| > 50 | Can be associated with adverse effects, per the Food and Nutrition Board (NIH ODS) — discuss with your clinician, and do not keep taking vitamin D supplements without their advice |
What moves the needle
- Sunlight. Your body makes vitamin D when bare skin is exposed to the sun; clouds, smog, older age, darker skin and sunscreen reduce how much it makes, and skin does not make it through a window.
- Diet. Fatty fish such as trout, salmon, tuna and mackerel are among the best natural sources; egg yolks have small amounts, and many foods are fortified.
- Supplements. The right amount varies by person — discuss with a clinician before starting.
Why this test is worth asking for
- It shows whether you have too much or too little vitamin D in your blood.
- A higher-than-normal level is most commonly caused by taking too much vitamin D and can raise blood calcium (hypercalcemia).
- Experts do not agree on a single target, so read your number against your lab's range and ask your clinician what it means for you.
Testing first is the whole point — a finger-prick kit gets you the number before anything changes.
Vitamin D (25-OH) Home Test
A finger-prick home test for 25-hydroxyvitamin D, the marker clinicians use to judge vitamin-D status.
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Related protocols
- Vitamin B12 + Methylmalonic Acid (MMA)
- Zinc — Immune Mineral Status
- Urine ACR — Albumin-to-Creatinine Kidney Marker
Sources
- MedlinePlus — Vitamin D Test
- MedlinePlus — 25-hydroxy vitamin D test (Medical Encyclopedia)
- NIH Office of Dietary Supplements — Vitamin D Health Professional Fact Sheet
- NIH Office of Dietary Supplements — Vitamin D Fact Sheet for Consumers
AgeGen lab guides are educational only. We do not provide medical diagnosis, prescribe brands, or recommend specific doses. Talk to a licensed clinician before changing your supplement or medication routine.
