Vitamin D

What the test measures 

Vitamin D is both a vitamin and a hormone. The body gets most of its vitamin D — up to 90% — from sunlight acting on the skin. A smaller amount comes from food, including oily fish, red meat, liver, egg yolk, and fortified foods such as cereals and margarine. 

Vitamin D goes through two conversion steps before it becomes fully active. First, the liver converts it to 25-hydroxyvitamin D (written as 25(OH)D). Then the kidneys convert that into the fully active form, 1,25-dihydroxyvitamin D. The routine blood test measures 25(OH)D, as this is the most reliable indicator of your overall vitamin D status. It stays in the blood longer and at higher concentrations than the active form. 

Vitamin D helps the gut absorb calcium and phosphate, which are essential for building and maintaining strong bones. Without enough vitamin D, bones can become soft and weak. It also plays an important role in muscle function. 

What your results mean 

Vitamin D results are measured in nanomoles per litre (nmol/​L). Reference ranges may vary slightly between laboratories but a general guide to interpretation can be found below. 

Your result should always be read alongside the reference range on your own laboratory report. 

Your doctor will interpret your result in the context of your symptoms, bone health, and any risk factors you have. 

If your 25(OH)D is below 25 nmol/​L — deficient 

A result below 25 nmol/​L means you are vitamin D deficient. This level is associated with a significant risk of bone disease. Rickets can develop in children at this level; osteomalacia (soft, painful bones) can occur in adults. Your doctor will recommend vitamin D supplements and may check your calcium and PTH levels too. The dose and duration of treatment will depend on how low your level is and your circumstances. 

If your 25(OH)D is between 25 and 50 nmol/​L — may be inadequate 

A result in this range may be insufficient for some people, particularly those with bone disease, fragility fractures, raised PTH, muscle symptoms, malabsorption conditions, or those taking anticonvulsant medicines or oral steroids. Your doctor will decide whether treatment is needed based on your individual situation. 

If your 25(OH)D is above 50 nmol/​L — sufficient 

A result above 50 nmol/​L is considered sufficient for almost everyone. If you are in this range, your doctor will reassure you and advise on how to maintain this level through sensible sun exposure, diet, and if needed a standard daily supplement. 

If your 25(OH)D is very high 

High levels of vitamin D almost always result from taking too many supplements. Very high levels can cause too much calcium to build up in the blood (hypercalcaemia), which can damage the kidneys and blood vessels. If your level is very high, your doctor will advise you to stop or reduce your supplement and recheck your level. Do not take high-dose vitamin D supplements without medical advice. 

The 1,25-dihydroxyvitamin D test 

This is a different, less commonly requested test. It measures the active form of vitamin D. It may be requested if your calcium is raised, or if you have a condition such as sarcoidosis or some lymphomas that can produce excess active vitamin D outside the kidneys. Low levels of 1,25-dihydroxyvitamin D may be seen in kidney disease. Your doctor will explain if this test has been requested for you and what the result means. 

If your level is in the deficient (e.g. below 25 nmol/​L), your doctor will prescribe or recommend a vitamin D supplement. The dose is usually higher at first to restore levels (a loading dose), followed by a lower maintenance dose. Your level will be rechecked after several months to confirm it has improved. 

If your level is in the borderline range (e.g. 25–50 nmol/​L), your doctor will review your symptoms and risk factors to decide whether treatment is needed. If you have bone disease or other risk factors, treatment is usually recommended. 

If your level is sufficient (above 50 nmol/​L), no treatment is usually needed. Your doctor may advise you to continue taking a standard daily supplement, especially during autumn and winter when sunlight is limited in the UK. 

If levels are very high, your doctor will review your supplement use and may arrange repeat testing once you have stopped or reduced the dose. 

• Sun exposure — levels are naturally higher in summer and lower in winter in the UK 
• Skin tone — people with darker skin produce less vitamin D from sunlight and may have lower levels 
• Age — the skin becomes less efficient at producing vitamin D with age 
• Body weight — vitamin D is fat-soluble and can be stored in body fat, meaning people with obesity may have lower circulating levels 
• Malabsorption conditions — conditions such as coeliac disease, Crohn’s disease, and cystic fibrosis reduce absorption of fat-soluble vitamins including vitamin D 
• Kidney disease — impairs the conversion of 25(OH)D to the active form 
• Liver disease — impairs the conversion of vitamin D to 25(OH)D 
• Certain medicines — anticonvulsants such as phenytoin, and some steroids, can lower vitamin D levels 
• Low magnesium — magnesium is needed for vitamin D metabolism; very low magnesium can make vitamin D supplementation less effective 

Vitamin D does not work alone. Your doctor may check: 

• Calcium — vitamin D controls how much calcium is absorbed from food. Abnormal calcium often prompts a vitamin D test. 
• Parathyroid hormone (PTH) — PTH rises when calcium is low, often as a result of vitamin D deficiency. Raised PTH alongside low vitamin D confirms the clinical picture. 
• Phosphate — low phosphate alongside low calcium and raised PTH can indicate vitamin D deficiency bone disease. 
• Magnesium — low magnesium can impair the body’s response to vitamin D and calcium supplementation. 
• Bone markers — may be requested to assess bone turnover in people with known bone disease. 

Vitamin D deficiency is very common in the UK. It can cause bone pain, muscle weakness, and in children, rickets. It is easily treated with supplements. Find out more about vitamin D on the NHS website.