Low vitamin D: what your blood test result means and what to do next

What does the vitamin D test measure?

The test is called 25-OH vitamin D (25-hydroxyvitamin D). It measures the form of vitamin D circulating in your blood and shows how much you have received in recent weeks: from the sun, from food and from supplements, if you take any.

Vitamin D helps the body use calcium and phosphorus, so it matters for bones and muscles. Most of it is made in the skin, in direct sunlight. Less comes from food: oily fish (salmon, mackerel, sardines), eggs and fortified foods.

The result comes in ng/mL or nmol/L, depending on the lab. To convert, multiply ng/mL by 2.5. For example, 20 ng/mL is 50 nmol/L.

What counts as low vitamin D, in numbers?

There is no single threshold, and that confuses many people. The US National Institutes of Health (NIH) use the thresholds below for adults. Many labs, however, use a higher desired range of 30–100 ng/mL.

So the same value, say 24 ng/mL, can be “enough” by NIH standards and “below range” on your report. That is not a mistake. They are two ways of reading the same number. What matters for you is your lab's range and what your doctor says after seeing the rest of your results.

Why does vitamin D drop in autumn and winter?

In much of Europe, from October to March the sun is too low for the skin to make vitamin D. The UK's National Health Service (NHS) says the same: in the colder months, almost nobody can make enough vitamin D from sunlight.

Levels drop more easily if you:

• spend a lot of time indoors or work in an office;
• wear clothes that cover almost all your skin;
• have darker skin;
• are over 65 (the skin makes less vitamin D);
• carry extra weight (vitamin D is stored in fat);
• have a condition that affects absorption, such as coeliac disease or inflammatory bowel disease;
• take certain medicines, for example some anticonvulsants.

If you already have your report, upload it to your free MentorSano account. You see vitamin D next to your other results, in plain words, with the explanation checked by a pharmacist.

What symptoms can it cause?

Most often, none. Many people only find out from a blood test. When the deficiency is large and lasts, you may notice:

• bone or muscle pain;
• muscle weakness, especially when climbing stairs or getting up from a chair;
• tiredness.

These signs have many other possible causes, from too little sleep to low iron or thyroid problems. That is why vitamin D is read together with your other results and how you feel, not on its own.

What should you do if your vitamin D is low?

The steps below are general. What suits you is decided by your doctor, who sees the whole picture.

Step	Why
Show the result to your GP	They decide whether you need a supplement, at what dose and for how long.
Ask about calcium, phosphorus or PTH	With a very low value, your doctor may want to see these too.
Get some sun in spring and summer	A few minutes a day with arms and face uncovered, without burning.
Eat oily fish once or twice a week	It is one of the few food sources.
Repeat the test when your doctor says	Usually after a few months, to see whether the value has risen.

The NHS recommends 10 micrograms (400 IU) of vitamin D a day for adults and children over one in the colder months. The dose for a low value may be different and is set by your doctor. The Endocrine Society's 2024 guideline does not recommend routine testing in healthy adults under 75, only when there is a reason.

Can vitamin D be too high?

Yes. Above 50 ng/mL (125 nmol/L), the NIH says unwanted effects can appear. Very high values almost always come from high-dose supplements taken for a long time without monitoring. Sunlight does not cause excess. That is why a high dose is not taken “just in case”, and the test is repeated when your doctor asks for it.

Which foods contain vitamin D?

Only a little vitamin D comes from food, but some foods help more than others:

• oily fish: salmon, mackerel, herring, sardines, trout;
• egg yolks;
• liver;
• mushrooms exposed to UV light (it says so on the pack);
• foods fortified with vitamin D: some milks, yoghurts, cereals or plant drinks.

Even with a good diet, in winter it is hard to get what you need from food alone. That is why, in the colder months, your doctor may recommend a supplement, especially for people in the higher-risk groups.