Vitamin D blood test: what it measures and what your result means

What is vitamin D?

Your skin makes vitamin D when it's exposed to sunlight. In the UK, the sun is only strong enough to do this from about April to September. You also get small amounts from food, such as oily fish, eggs, red meat and fortified cereals.

Your liver turns vitamin D into 25-hydroxy vitamin D, which is what the test measures. It shows how much you've built up from sun, food and supplements over recent weeks.

Why is vitamin D tested?

• You have bone or muscle pain, or muscle weakness
• You spend little time outdoors, or usually cover your skin when outside
• You have darker skin, which makes less vitamin D from sunlight
• You have a condition that affects absorption, such as coeliac disease or Crohn's disease
• You take a vitamin D supplement and want to check your level
• You've been told you have thin bones (osteoporosis)

What does a low vitamin D result mean?

Low vitamin D means your stores are running short. Common reasons are:

• Not getting enough sunlight, particularly between October and March
• Darker skin, or covering most of your skin when outside
• Being older, as skin makes less vitamin D with age
• Being overweight
• Conditions that affect the gut, liver or kidneys
• Some medicines, including some for epilepsy

Many people with low vitamin D have no symptoms. Over time, very low levels can lead to bone pain and muscle weakness, and softened bones (osteomalacia in adults, rickets in children).

What does a high vitamin D result mean?

High vitamin D is uncommon. It almost always comes from taking high-dose supplements for a long time. Very high levels can raise calcium in the blood, which can cause thirst, nausea, tiredness and, in time, kidney problems.

What can affect your result?

• The time of year: levels are usually lowest in late winter and early spring
• Recent holidays in the sun, or starting a supplement
• You don't need to fast for a vitamin D test

When to see a GP

Your results report shows the laboratory's range and flags any result outside it. Speak to your GP if:

• Your vitamin D is flagged low and you have bone pain, muscle weakness or frequent falls
• Your vitamin D is flagged high
• You have a condition affecting your gut, liver or kidneys

Get urgent medical help if you take high-dose vitamin D and develop severe vomiting, confusion or extreme thirst.

How much vitamin D should I take?

The UK government advises that everyone should consider taking 10 micrograms (400 IU) of vitamin D a day in autumn and winter. People who rarely go outside, cover their skin, or have darker skin should consider taking it all year round. Don't take more than 100 micrograms (4,000 IU) a day unless a doctor advises it.

If your level is low, a GP or pharmacist may recommend a higher dose for a short time.