Low vitamin D on a blood test: what your level means
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 21 October 2026
Low vitamin D explained: ng/mL and nmol/L, what counts as low, who is at risk even in sunny Spain, why more is not better, and when to see a doctor.
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Vitamin D is one of the most common extra tests on a blood report. Many people are surprised to find it low while living in sunny Spain. This guide explains how to read the number, why it may be low, and why taking more is not always better.
- The test measures 25-hydroxy vitamin D. Spanish labs often use ng/mL; the UK uses nmol/L. Multiply ng/mL by 2.5 to get nmol/L.
- UK guidance treats under 25 nmol/L (10 ng/mL) as deficient. Many labs and Spanish experts flag under 20 ng/mL (50 nmol/L) as low.
- Low levels are common in winter, in people who cover their skin or stay indoors, and in people with darker skin.
- Too much vitamin D can cause harm. Spain's medicines agency has warned about high-strength products taken too often.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
What the test measures
Vitamin D helps your body use calcium. It keeps bones and muscles healthy. Most of it is made in the skin in sunlight. A smaller part comes from food, such as oily fish and eggs.
The blood test measures a form called 25-hydroxy vitamin D. On a Spanish report you may see 25-OH vitamina D or calcidiol.
Units and what counts as low
Labs report vitamin D in one of two units. To change ng/mL into nmol/L, multiply by 2.5.
| ng/mL | nmol/L | How it is often read |
|---|---|---|
| Under 10 | Under 25 | Deficient (UK guidance) |
| 10 to 20 | 25 to 50 | Low or "insufficient" in many guides |
| 20 to 30 | 50 to 75 | Enough for most healthy people |
Experts do not fully agree on the lines. Some labs flag anything under 30 ng/mL (75 nmol/L). Many guides, including Spanish ones, see 20 ng/mL as enough for most healthy adults. Your doctor will read it with your health and risks in mind.
Why it can be low, even in Spain
Sunny weather does not guarantee a normal level. Common reasons are:
- Winter, when the sun is too low for the skin to make much vitamin D.
- Working indoors, or covering most of the skin when outside.
- Regular high-factor sun cream, which is still the right choice for skin safety.
- Darker skin, which needs more sun to make the same amount.
- Older age, as the skin makes less.
- Being overweight.
- Gut conditions that reduce absorption, such as coeliac disease.
- Kidney or liver disease, and some medicines, such as some epilepsy medicines.
Symptoms
Most people with a low level feel nothing. When the level is very low for a long time, it can cause:
- Bone pain or aching.
- Muscle weakness, especially in the hips and thighs.
- Cramps or tingling, if calcium also falls.
Tiredness alone is rarely due to low vitamin D. If you feel tired, your doctor may check other causes too, such as iron, thyroid or blood sugar.
Have a vitamin D result you are not sure about?
You can book a video visit to go through the results you already have with a GP licensed in Spain, in English or Spanish.
Why more is not always better
Vitamin D is sold in many forms and strengths in Spain. Some are taken daily, others once a week, once a month or as a single dose. Mixing them up is easy.
The Spanish medicines agency, AEMPS, has warned about serious cases of high blood calcium from vitamin D overdose. Many happened when a product meant for monthly use was taken every day. High calcium can cause sickness, thirst, confusion and kidney damage.
So:
- Do not take more than one vitamin D product at once without telling your doctor.
- Read the label and check whether it is daily, weekly or monthly.
- Do not keep taking a high-strength product for months without a check.
What usually happens next
Your doctor will look at the level together with your age, diet, sun exposure, bones and other results. Common next steps are:
- Check calcium, phosphate and kidney function, if not done.
- Ask about falls, fractures, bone pain and medicines.
- Talk about safe sun, diet and whether a supplement makes sense for you.
- Agree if and when to repeat the test.
Routine repeat testing is not needed for most healthy people. Your doctor will say what suits you.
When to get urgent help
Seek urgent care, or call 112, if you take vitamin D and develop:
- Repeated vomiting, great thirst and passing a lot of urine.
- Confusion or great drowsiness.
- An irregular heartbeat.
These can be signs of high calcium. Bring the packs of everything you take.
Going through your results by video
Dr. Yelamo is a GP licensed in Spain. He can go through results you already have, by video, for adults. Visits run every day, 07:30–20:30 (Spain time). In the visit he will:
- Read the full report and convert units if needed.
- Compare it with past results, including ones from other countries.
- Ask about sun, diet, bones, medicines and any supplements you already take.
- Explain what the level means for you and what is usually done next.
- Tell you if you need a test or an exam in person.
He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.
Frequently asked questions
What vitamin D level is too low?
UK guidance treats under 25 nmol/L (10 ng/mL) as deficient. Many labs and Spanish guides flag under 20 ng/mL (50 nmol/L) as low. Experts do not fully agree, so your doctor reads it with your risks.
How do I convert vitamin D from ng/mL to nmol/L?
Multiply ng/mL by 2.5. For example, 20 ng/mL is 50 nmol/L, and 30 ng/mL is 75 nmol/L.
Why is my vitamin D low if I live in Spain?
Winter sun, indoor work, covering up, sun cream, darker skin, older age, weight and some gut or kidney conditions can all lower it, even in a sunny country.
Can you take too much vitamin D?
Yes. Too much can raise blood calcium and harm the kidneys. Spain's medicines agency has warned about high-strength products taken too often. Check the label and ask your doctor.
Can an online GP go through my vitamin D result?
Yes. Dr. Yelamo can go through results you already have by video, for adults. He explains them and tells you if anything else needs checking.
Sources
- NHS: Vitamin D
- NICE CKS: Vitamin D deficiency in adults
- AEMPS: Vitamin D, serious cases of high calcium from overdose (in Spanish)
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
