Low vitamin D: how to read your 25-hydroxy vitamin D result

Vitamin D is measured as 25-hydroxy vitamin D. UK labs report it in nmol/L, US labs in ng/mL: multiply ng/mL by 2.5 to get nmol/L. In the UK, below 25 nmol/L (10 ng/mL) is deficiency and 25–50 nmol/L may be too low for some people. In the US, below 12 ng/mL (30 nmol/L) is deficiency and 20 ng/mL (50 nmol/L) or more is generally considered adequate for bone health.

Updated: 2026-10-26

Reference ranges

Category	Reference range
Deficient	below 25–30 (below 10–12) nmol/L (ng/mL in brackets)
Possibly inadequate	25–50 (10–20) nmol/L (ng/mL in brackets)
Adequate for most people	50 or more (20 or more) nmol/L (ng/mL in brackets)
Higher than needed, possible harm	above about 125 (above 50) nmol/L (ng/mL in brackets)

UK (NHS, SACN) and US (National Academies, NIH) cut-offs differ slightly; your lab may print another range. 1 ng/mL = 2.5 nmol/L.

Vitamin D 20 ng/mL or 50 nmol/L: what it means

These two numbers are the same level in different units, and they are the most searched results for a reason: they sit right on the line. Under the US National Academies' approach, 20 ng/mL is enough for bone health in most people. The UK uses 25 nmol/L as the deficiency threshold and considers 25–50 nmol/L possibly inadequate. Some labs and older guidance use 30 ng/mL (75 nmol/L) as the target, which is why one report may flag 50 nmol/L as "insufficient" and another call it fine.

In practice, a result around 20 ng/mL / 50 nmol/L in someone without bone problems or symptoms is usually considered acceptable or borderline, and the doctor's advice depends on your risk factors and the season.

Converting vitamin D units

ng/mL to nmol/L: multiply by 2.5. 12 ng/mL ≈ 30 nmol/L; 20 ng/mL ≈ 50 nmol/L; 30 ng/mL ≈ 75 nmol/L.
nmol/L to ng/mL: divide by 2.5. 25 nmol/L ≈ 10 ng/mL; 40 nmol/L ≈ 16 ng/mL; 75 nmol/L ≈ 30 ng/mL.
Some reports write µg/L, which is the same as ng/mL.
"25-OH vitamin D", "25(OH)D", "calcidiol" and "vitamin D total" all refer to the same test. Results may be split into D2 and D3; the total is what matters.

Why vitamin D is low

Season and latitude: in the UK and northern US, the sun is too weak to make vitamin D in the skin from about October to March. Levels are usually lowest in late winter.
Little time outdoors or skin mostly covered.
Darker skin, which needs more sun exposure to make the same amount.
Higher body weight, as vitamin D is stored in fat tissue.
Older age, pregnancy and breastfeeding.
Absorption problems: coeliac disease, inflammatory bowel disease, weight-loss surgery.
Liver or kidney disease and some medicines that speed up vitamin D breakdown.

Low vitamin D symptoms

Most people with mildly low vitamin D have no symptoms. When deficiency is more marked and lasts a long time, it can cause bone pain or tenderness, muscle weakness (for example difficulty climbing stairs or getting up from a chair) and, in children, rickets. In adults, long-standing deficiency can lead to softening of the bones (osteomalacia).

Tiredness, low mood and hair loss are often blamed on low vitamin D. Studies have not shown a clear link in most cases, so if these are your main symptoms, it is worth looking at other causes too, such as iron, thyroid function and sleep.

When to see a doctor

Discuss your result with your GP or doctor, who will advise based on your level, symptoms and risk factors. In the UK the NHS also advises everyone to consider vitamin D in autumn and winter, and some groups all year round. See a doctor sooner if you have:

bone pain, a waddling walk or muscle weakness that makes everyday movements hard;
a fracture after a minor fall;
muscle cramps, tingling around the mouth or in the fingers, or twitching, which may point to low calcium and need urgent assessment;
a very high vitamin D result, especially with thirst, nausea, confusion or passing a lot of urine, which can occur with excessive supplement use.

Sources

Written using public information from:

NHS (National Health Service, UK): Vitamin D
Scientific Advisory Committee on Nutrition (SACN, UK): Vitamin D and health
NIH Office of Dietary Supplements (US): Vitamin D fact sheet for health professionals
National Academies of Sciences, Engineering, and Medicine (US): Dietary Reference Intakes for calcium and vitamin D
Royal Osteoporosis Society (UK)

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Frequently asked questions

Is vitamin D 20 ng/mL low?

It is borderline. US guidance generally considers 20 ng/mL adequate for bone health, while some labs use a target of 30 ng/mL. In UK units it equals 50 nmol/L, the point the NHS treats as sufficient for most people.

Is 50 nmol/L vitamin D normal?

For most people, yes. UK guidance treats under 25 nmol/L as deficiency and 25 to 50 as possibly inadequate, so 50 is at the lower end of sufficient.

How do I convert vitamin D from nmol/L to ng/mL?

Divide by 2.5. For example, 60 nmol/L is about 24 ng/mL. To go the other way, multiply ng/mL by 2.5.

What are the symptoms of low vitamin D in women?

The same as in men: often none, and with more severe deficiency bone pain and muscle weakness. Women after the menopause have a higher risk of bone thinning, so doctors pay more attention to vitamin D in this group.

How long does it take to raise vitamin D levels?

Usually several weeks to a few months, depending on the starting level and what your doctor recommends. A repeat test is often done after about three months.

This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.