Biomarker Glossary

Vitamin D

Vitamin D is a fat soluble vitamin that supports bone health, immune function and mood. A vitamin D blood test measures the level of 25-hydroxy vitamin D, the main circulating form, in nmol/L. Deficiency is very common in the UK, particularly between October and April.

Also known as: 25-OH Vitamin D, Calcidiol, Vitamin D3

Vitamin D is most informative when read alongside related markers like Ferritin and Vitamin B12. A single number rarely tells the whole story. If your reading sits outside the typical range, share the full report with your GP or healthcare professional before drawing conclusions.

What is Vitamin D?

Vitamin D is technically a hormone, not just a vitamin. Your body makes it when sunlight (UVB) hits your skin, and you also get small amounts from foods such as oily fish, egg yolks and fortified cereals.

A vitamin D blood test measures 25-hydroxy vitamin D (25-OH vitamin D), the stored form of the vitamin and the best marker of your overall status. Results are reported in nmol/L in the UK (ng/mL in the US).

Vitamin D supports calcium absorption, bone strength, immune regulation, muscle function and mood. Low levels are linked to fatigue, low mood, frequent infections and, over the long term, weakened bones (osteomalacia in adults, rickets in children).

Why test Vitamin D?

  • Vitamin D deficiency is very common in the UK. Public Health England estimates that 1 in 5 adults have low levels, and the figure rises in winter and in people with darker skin, who cover their skin for cultural or religious reasons, or who spend most of their time indoors.
  • Symptoms (fatigue, low mood, muscle aches, bone pain, frequent colds) are vague and easily missed, which is why testing is the only reliable way to know your status.
  • If you take a vitamin D supplement, testing helps you find the right dose to reach the optimal range without overdoing it.
Vitamin D deficiency is very common in the UK. Public Health England estimates that 1 in 5 adults have low levels, and the figure rises in winter and in people with darker skin, who cover their skin for cultural or religious reasons, or who spend most of their time indoors.

Vitamin D normal range (UK)

Result What it means
Below 25 nmol/L Deficient (NHS: action required)
25 to 49 nmol/L Insufficient
50 to 75 nmol/L Adequate
75 to 125 nmol/L Optimal (often the target for general health)
Above 250 nmol/L High (toxicity risk if sustained, usually only from over supplementation)

Ranges are based on NHS and Royal Osteoporosis Society guidance. The "optimal" band is more contested. Many functional medicine practitioners and clinical studies use 75 to 125 nmol/L as a target, while NHS classifies anything above 50 nmol/L as adequate for bone health.

About these ranges. The ranges above are typical UK clinical lab ranges aligned to NHS and Royal College of Pathologists guidance. Your Chxhealth report will show the specific reference range used by our partner lab, Randox, for each marker. Lab ranges vary slightly between providers and assays. Always interpret your results in the context of the range printed on your own report.

What causes high Vitamin D?

  • High dose vitamin D supplementation, especially without testing
  • Granulomatous diseases such as sarcoidosis or tuberculosis
  • Some lymphomas
  • Excessive intake of cod liver oil

What causes low Vitamin D?

  • Limited sunlight exposure (winter months, working indoors, covered skin)
  • Darker skin (more melanin reduces UVB conversion)
  • Older age (skin is less efficient at making vitamin D)
  • Obesity (vitamin D is sequestered in fat tissue)
  • Diet low in oily fish, eggs and fortified foods
  • Malabsorption conditions (coeliac disease, Crohn's disease, gastric bypass)
  • Some medications (anticonvulsants, glucocorticoids)
  • Chronic kidney or liver disease

Symptoms of high Vitamin D

  • Nausea, vomiting or loss of appetite
  • Frequent urination and thirst (linked to high calcium)
  • Muscle weakness
  • Confusion
  • Kidney stones in chronic cases

Symptoms of low Vitamin D

  • Fatigue and low energy
  • Muscle aches and weakness
  • Bone pain or stiffness
  • Low mood, particularly in winter
  • Frequent colds or infections
  • Hair loss
  • Slow wound healing

How is Vitamin D tested?

Vitamin D is measured from a small blood sample, taken by finger prick at home or by a standard blood draw at a clinic.

No fasting is required. Hold any vitamin D supplements on the morning of the test for the most accurate reading.

Chxhealth samples are analysed by Randox, a UK laboratory accredited by UKAS. Results are typically returned in 3 to 5 working days.

How to support healthy Vitamin D levels

  • Spend short periods (10 to 20 minutes) of midday sun on bare arms or face during the months of April to September, without sunscreen.
  • Eat oily fish (salmon, mackerel, sardines) at least twice a week.
  • Include egg yolks and fortified foods such as some breakfast cereals and plant milks.
  • Between October and April, the NHS recommends a daily vitamin D supplement of 10 micrograms (400 IU) for all UK adults. People who are deficient often need higher doses, guided by testing.
  • Maintain a healthy body weight, as obesity reduces vitamin D bioavailability.

These are general lifestyle suggestions. Chxhealth is an information service. For personal medical advice, please speak to a healthcare professional.

Chxhealth panels that test Vitamin D

17 Chxhealth biomarker panels include Vitamin D. Each is analysed by Randox, a UK laboratory accredited by UKAS. Reports are delivered in 3 to 5 working days.

Vitamin D FAQs

What is the optimal vitamin D level?

The NHS considers levels above 50 nmol/L adequate for bone health. Many clinicians and researchers aim for 75 to 125 nmol/L as the optimal range for broader health, including immunity and mood. Levels above 250 nmol/L are not recommended.

How much vitamin D should I take?

If you are deficient (below 25 nmol/L), the NHS may prescribe a loading dose of 50,000 IU weekly for 6 to 8 weeks, then a maintenance dose. For general supplementation, 10 to 25 micrograms (400 to 1000 IU) per day is typical. Retest after 3 months to confirm you are in range.

Can I get enough vitamin D from sun alone in the UK?

Only between April and September. From October to April, the angle of UVB in the UK is too low for skin to make vitamin D, even on sunny days. This is why most UK adults are recommended to supplement through winter.

How often should I retest vitamin D?

If you supplement, retest after 3 months to confirm you have reached the optimal range. Once stable, annual testing (ideally late winter, when levels are lowest) is usually enough.

Is vitamin D the same as vitamin D3?

Vitamin D3 (cholecalciferol) is the form your body makes from sunlight and the form most supplements contain. Vitamin D2 (ergocalciferol) is from plant sources. Both raise blood levels of 25-OH vitamin D, which is what the test measures, but D3 is generally considered more effective.

Often tested with Vitamin D

The biomarkers below are commonly investigated alongside Vitamin D because they reveal connected aspects of the same physiological picture:

  • Ferritin: Both commonly low in unexplained fatigue, often paired.
  • Vitamin B12: Frequently tested together as part of a nutrient panel.
  • TSH: Thyroid function is the other classic fatigue cause to rule out.
  • CRP: Helps distinguish nutrient depletion from underlying inflammation.

Related reading

Choose the right test for your goal

If you are reading about Vitamin D because of a specific health goal, our buying guides walk you through which Chxhealth panels fit:

Sources and further reading

This page is informed by guidance from the NHS, NICE, Royal College of Pathologists and other UK authoritative bodies. For deeper detail or to verify the information, see:

About this page. Last reviewed: 13 May 2026. Next scheduled review: May 2027. This page has not yet been independently reviewed by a clinician. It is written from authoritative UK medical guidance (NHS, NICE, Royal College of Pathologists, peer-reviewed sources) but has not undergone formal clinical sign off.

Important. Chxhealth is a UK information service. We do not diagnose, treat or prescribe. The reference ranges and information on this page are general educational content and should not be used as a substitute for advice from a qualified healthcare professional. For any concerns about your health or results, please speak to your GP or another healthcare professional.