What it measures
- 25-hydroxy vitamin D
Why vitamin D matters
Vitamin D helps your body absorb calcium for strong bones and teeth, and supports your muscles and immune system. Most of it is made in your skin from sunlight, and in the UK the sun isn’t strong enough to make vitamin D from about October to March. That’s why low levels are so common here.
Understanding your vitamin D result
Results are measured as 25-hydroxy vitamin D in nmol/L:
Level | What it means |
|---|---|
Below 25 nmol/L | Deficient |
25–50 nmol/L | May be too low for some people |
Above 50 nmol/L | Enough for most people |
The NHS advises everyone in the UK to consider a daily 10 microgram supplement in autumn and winter. If your result is low, share it with your GP, who may recommend a higher dose for a period.
Who it’s for
- Tiredness, aches or muscle weakness
- Little time outdoors, or covering your skin
- Darker skin, which makes less vitamin D from sunlight
- Pregnancy, breastfeeding or planning a pregnancy
- Checking whether a supplement is working
How to prepare
- Drink plenty of water beforehand; it makes the sample quicker and easier to take.
- You don’t need to fast.
Vitamin D Blood Test: your questions
What are the symptoms of low vitamin D?
Often there are none. Some people notice tiredness, aches, muscle weakness or bone pain. Severe, long-term deficiency can soften the bones.
When is the best time to test vitamin D?
Any time. Testing in late winter or early spring shows your lowest level of the year, while testing a few months after starting a supplement shows whether it’s working.
Information on this page is general and doesn’t replace advice from your GP, midwife or consultant. If you feel unwell or are worried about symptoms, contact your GP, call NHS 111 or, in an emergency, 999.
