Vitamin D (25-OH): what it is, what it tells you and why we test it

Vitamin D helps your body absorb calcium and keep your bones strong. The test measures 25-OH vitamin D, the form that circulates in your blood and acts as your store. Your skin makes most of it in the sun, which is why it is often low in winter.

Why we test it

Vitamin D helps your gut absorb calcium from food. If you are short of it for a long time, your bones get weaker.

Your skin makes it in the sun, and few foods contain it. That is why it is often low in winter and in people who spend most of their time indoors, in Italy too.

If you feel well, you usually do not need to test it often. It matters more with bone, liver, kidney or gut conditions, a lot of extra weight, in pregnancy and breastfeeding, and in some older people. The number helps your doctor decide whether you need a supplement.

What it tells you, and what it does not

If it is low, your vitamin D store is short. Too little sun is usually behind it, especially in winter.

If it is high, high-dose supplements are usually behind it. Higher does not mean healthier.

On its own it does not show how your bones are doing. And low vitamin D often turns up in people with other illnesses, but that does not mean it causes them.

That is why it is read together with other tests:

• Calcium: Vitamin D helps you absorb it from food
• Alkaline phosphatase: It can rise when a long shortage affects your bones
• Magnesium: Your body uses it to activate vitamin D
• Haemoglobin: If you feel tired, anaemia may be the reason

Normal vitamin D levels

The lab decides what is normal. Many lab reports call 30 to 100 ng/mL sufficient. Your own report may print different limits, and sometimes splits the low end into insufficiency and deficiency.

Some reports use another unit, nmol/L, where the numbers are about two and a half times higher. So 30 ng/mL is 75 nmol/L, and 100 ng/mL is 250 nmol/L.

The limits on your report are the same for women and men, and they do not change with age.

In the Rosa report you see your vitamin D next to the rest of the panel, read together with calcium and alkaline phosphatase.

What your result means

Find the row for your result, in ng/mL. If your report is in nmol/L, divide the number by 2.5 first.

• Below your report’s limit, On many reports below 30: Low. Many lab reports call this insufficient. It is common in winter and in people who spend most of their time indoors. Your Rosa report shows what to do. If you need a supplement, your doctor sets the right dose for you.
• Within your report’s limits, On many reports 30 to 100: Normal. For your report, the store is enough. Higher does not mean better. In the Rosa report you see it next to the rest of the panel, read with calcium and alkaline phosphatase.
• Above your report’s limit, On many reports above 100: High. It usually comes from high-dose supplements. Too much vitamin D can raise your calcium. Talk to your doctor, who checks your calcium and tells you whether to stop the supplement.

If your vitamin D is low or high

• Low vitamin D: Very common, especially in winter. Too little sun is usually behind it.
• High vitamin D: It usually comes from high-dose supplements. Too much can raise your calcium.

Why vitamin D is low

Sunlight is usually the reason. The most common causes:

• Little winter sun: In winter the sun is low, and your skin makes little even on clear days.
• Lots of time indoors: People who work indoors, or cover most of their skin, make less.
• Darker skin: It takes more sun to make the same amount.
• A lot of extra weight: With more weight there is often less in the blood, and higher doses are needed to raise it.
• Age: Over the years your skin makes less, and people often spend more time indoors.
• Gut, liver or kidneys: Conditions that reduce absorption, such as coeliac disease, or liver and kidney disease.
• Some medicines: For example epilepsy medicines, or steroids taken for a long time. Tell your doctor.

Symptoms of vitamin D deficiency

Often none: many people with low vitamin D feel fine and find out from a blood test.

When the shortage is severe and lasts a long time, bones can weaken and ache, and muscles lose strength.

Tiredness is often blamed on vitamin D, but it has many other causes. If you have felt tired for weeks, talk to your doctor.

Vitamin D3 and supplements

Vitamin D3, or cholecalciferol, is the same vitamin D your skin makes in the sun, and it is the form in most supplements. It matters most for your bones. For other effects, for example on the heart or cancer, studies have not given clear answers.

A supplement makes most sense when a test shows your level is low, or at certain stages of life, such as pregnancy or old age. Your doctor sets the dose and how long to take it.

It is absorbed better with a meal that contains some fat. And more is not better: very high doses taken for a long time can raise the calcium in your blood too much.

How to raise your vitamin D: sun, food and supplements

If it is low, where to start:

• Sun: from spring to autumn, a little time outdoors with your arms and legs uncovered helps your skin make it. Without burning: sun protection still matters.
• Food: oily fish such as salmon, mackerel, herring and sardines, eggs and some fortified foods. Fruit and vegetables have almost none, and food alone is usually not enough.
• A vitamin D3 supplement, if your doctor recommends one: your Rosa report shows what to do, and your doctor sets the dose.
• If you take some water tablets for blood pressure, or have had kidney stones or high calcium, tell your doctor before you start.
• Do not go over the dose you were given, and do not stack several products that contain it.

What can skew the result

A few things change the number, or how you read it. It helps to know them first.

• The season: It is usually higher at the end of summer and lower at the end of winter. To compare two tests, keep the month in mind.
• A supplement: If you take one, the number reflects it. Tell your doctor, with the dose.
• The lab: Different methods can give slightly different numbers. To follow it over time, use the same lab.

When to worry

Talk to your doctor if:

• It is far below your report’s limit, even if you feel well.
• It is low and you have bone pain or muscle weakness.
• It is above your report’s limit: bring the list of supplements you take.
• You are pregnant or breastfeeding: ask your obstetrician whether to supplement.
• You have a gut, liver or kidney condition: ask how often to check it.
• You take vitamin D and feel sick, very thirsty or pass much more urine than usual: see a doctor the same day. With confusion or vomiting that will not stop, call 112 or go to the emergency department.

Frequently asked questions

What does 25-OH mean on my report? It is the form of vitamin D that circulates in your blood and acts as your store, and it is the one that is measured. When needed, your kidneys turn it into the active form.

How do I convert nmol/L to ng/mL? Divide by 2.5. For example, 75 nmol/L is 30 ng/mL, and 250 nmol/L is 100 ng/mL.

Do normal values change with age? The limits on your report do not: they are the same at every age. But over the years your skin makes less, so it is more likely to be low.

Can too much vitamin D be harmful? Yes, at very high doses taken for a long time: it can raise the calcium in your blood too much. High-dose supplements are usually behind it, not sun or food. That is why your doctor sets the dose.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If you take a supplement to correct it, your doctor decides when to check it again.

Sources

• Demay and colleagues, Endocrine Society clinical practice guideline on vitamin D for the prevention of disease. Journal of Clinical Endocrinology and Metabolism, 2024. PMID 38828931
• Ross and colleagues, the Institute of Medicine report on calcium and vitamin D needs: what is proven for bone, and what is not beyond it. Journal of Clinical Endocrinology and Metabolism, 2011. PMID 21118827
• Cesareo and colleagues, position statement of Italian clinical endocrinologists (AME) on vitamin D deficiency in adults, with values in ng/mL and nmol/L. Nutrients, 2018. PMID 29702603
• Webb and colleagues, how season and latitude change the vitamin D your skin makes, and why winter sun is not enough. Journal of Clinical Endocrinology and Metabolism, 1988. PMID 2839537