Vitamin D (25-OH)

What is vitamin D (25-OH)?

Vitamin D is a fat-soluble vitamin that the body produces in the skin when exposed to ultraviolet B (UVB) sunlight, and that can also be obtained — in smaller amounts — from dietary sources such as oily fish, egg yolks, and fortified foods. To become biologically active, vitamin D undergoes two conversion steps: first in the liver (producing 25-hydroxyvitamin D, also written 25(OH)D or calcidiol), then in the kidneys (producing the active hormone calcitriol).

The blood test for vitamin D measures 25-hydroxyvitamin D, the main circulating form and the most reliable index of overall vitamin D status. It is the form with the longest half-life in the body and the one recommended by clinical guidelines for assessment.

Vitamin D plays a central role in calcium and phosphate metabolism and is therefore critical for maintaining strong bones and teeth. Beyond the skeleton, vitamin D receptors are found in nearly every tissue in the body, and low levels have been associated with increased risk of infections, autoimmune disease, cardiovascular disease, depression, and certain cancers — though causality is still under active investigation in many of these areas.

Vitamin D deficiency is extraordinarily common in the UK. The country's latitude means UVB radiation is insufficient for cutaneous synthesis between October and March, and many people have low levels by winter's end regardless of diet. The NHS advises that everyone in the UK consider supplementing with 10 micrograms (400 IU) daily, particularly in autumn and winter.

Normal reference range (UK thresholds)

• Sufficient: above 50 nmol/L
• Insufficient: 25–50 nmol/L
• Deficient: below 25 nmol/L

Important: Reference ranges vary between laboratories and depend on individual factors including age, sex, pregnancy status, and medication. Always interpret your result in the context of a clinician review. The Vesey reviews every result before release.

What low vitamin D may indicate

Low vitamin D is one of the most common nutritional deficiencies worldwide. Possible causes and consequences include:

• Insufficient sun exposure — indoor workers, shift workers, those who cover their skin, and darker skin tones that require more UVB to synthesise adequate vitamin D
• Poor dietary intake — particularly in those who avoid oily fish and dairy
• Malabsorption — coeliac disease, Crohn's disease, and following bariatric surgery impair fat-soluble vitamin absorption
• Obesity — vitamin D is sequestered in adipose tissue and less available to the bloodstream
• Chronic kidney or liver disease — impairs the conversion steps that activate vitamin D
• Older age — skin synthesis efficiency decreases with age

Clinical effects of deficiency include bone pain, muscle weakness, fatigue, low mood, frequent infections, and — at severe levels — osteomalacia (softening of bones) in adults or rickets in children.

What high vitamin D may indicate

Vitamin D toxicity (hypervitaminosis D) is rare and almost never caused by sun exposure alone. It is essentially always the result of excessive supplementation, particularly with high-dose preparations. Consequences include:

• Hypercalcaemia — elevated blood calcium causing nausea, vomiting, constipation, weakness, and confusion
• Nephrocalcinosis — calcium deposits in the kidneys, potentially leading to kidney damage
• Kidney stones — a recognised complication of prolonged excessive supplementation

Testing vitamin D before starting high-dose supplementation and monitoring during treatment is strongly recommended to prevent over-correction.

Get tested at The Vesey, Sutton Coldfield

Vitamin D is measured within the following panels at The Vesey Private Hospital:

• Nutritional Health panel — vitamin D, B12, folate, ferritin, and more
• Bone Health panel — vitamin D, calcium, phosphate, and bone markers
• Vitamins & Minerals panel — comprehensive micronutrient screen
• Lifestyle Screen — broad health check including vitamin D

Results are reviewed by a clinician before secure release, typically within 24 hours. Appointments available Monday–Saturday.

Frequently asked questions

What is a normal vitamin D level in the UK?

In the UK, a 25-OH vitamin D level above 50 nmol/L is considered sufficient. A level of 25–50 nmol/L is insufficient, and below 25 nmol/L is deficient. Severe deficiency is often defined as below 12.5 nmol/L.

What are the symptoms of vitamin D deficiency?

Symptoms include bone pain, muscle weakness, fatigue, low mood, and increased susceptibility to infections. Prolonged severe deficiency causes rickets in children and osteomalacia in adults.

Who is most at risk of vitamin D deficiency in the UK?

Those most at risk include indoor workers, older adults, people with darker skin tones, those who cover their skin for cultural or religious reasons, people with obesity, and those with malabsorption conditions. The majority of the UK population has insufficient levels by the end of winter.

Can you have too much vitamin D?

Yes, though toxicity is rare and almost always caused by excessive supplementation rather than sun exposure. It causes hypercalcaemia with symptoms including nausea, vomiting, confusion, and kidney damage. Testing before high-dose supplementation is strongly recommended.

How long does it take to correct vitamin D deficiency?

With adequate supplementation, vitamin D levels typically take 3–6 months to rise to the sufficient range. The dose required depends on the starting level and individual factors. A repeat test after 3 months of supplementation allows your clinician to assess the response and adjust the dose if needed.