Vitamin D
25-Hydroxyvitamin D
What this test measures
Vitamin D helps your body absorb calcium and keeps your bones, teeth, and muscles healthy. You get vitamin D from sunlight, some foods, and supplements. This test checks if you have enough.
Understanding the numbers
Typical range: 50-125 nmol/L (adequate)
Below 25 nmol/L is deficient. 25-50 nmol/L is insufficient. Many people in the UK have low levels, especially in winter.
- Deficient: 0–25 nmol/L
- Insufficient: 25–50 nmol/L
- Adequate: 50–125 nmol/L
- High but safe: 125–220 nmol/L
- Toxic: 220–250 nmol/L
Key numbers
- 25 nmol/L — Below = deficient (loading dose)
- 50 nmol/L — Below = insufficient (maintenance)
- 220 nmol/L — Above = toxicity possible
NICE CKS / Royal Osteoporosis Society: below 25 nmol/L is deficient and needs loading-dose vitamin D (e.g. 300,000 IU over 6-10 weeks); 25-50 is insufficient and usually treated with maintenance 800-2000 IU/day; 50-125 is adequate. UK Department of Health recommends 10 µg (400 IU) daily for everyone in autumn/winter.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- You have plenty of vitamin D, which is usually fine
- Very high levels (above 250 nmol/L) are rare but can be harmful
- This usually only happens from taking very high dose supplements
- If your level is very high, you may need to reduce supplements
Questions you could ask your GP
- What dose of vitamin D supplement should I take, and for how long?
- Should I take vitamin D all year round, or just in winter?
- Do I need a bone density scan (DEXA) to check my bone health?
- When should I have my vitamin D level rechecked?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Severe deficiency — bone and muscle pain
Aarti had widespread muscle aches, low mood, and some bone tenderness. Vitamin D was 14 nmol/L; calcium low-normal, PTH raised.
What the GP said: “This is severe vitamin D deficiency, which can cause exactly the symptoms you have. We'll start loading-dose vitamin D — 50,000 IU weekly for six weeks — then switch to daily maintenance.”
What happened next: After loading and maintenance, her vitamin D was 82 and her pain had largely resolved.
Severe vitamin D deficiency causes real symptoms — bone pain, muscle aches, fatigue, low mood. Loading doses are needed to replete stores quickly, followed by daily maintenance.
Insufficient in winter, no symptoms
Ellis had a routine check in February. Vitamin D was 38 nmol/L; no symptoms.
What the GP said: “This is a very common UK winter pattern. Take 1000 IU a day through autumn and winter — you don't need loading. Over-the-counter supplements are fine.”
What happened next: Ellis took a daily supplement each winter. She didn't need further tests.
Many UK adults are 'insufficient' in winter. This doesn't always need heavy-handed treatment — daily maintenance 800-2000 IU through the dark months is usually enough.
Vitamin D and osteoporosis
Sharmila had a low-trauma wrist fracture. DEXA confirmed osteoporosis. Vitamin D 32, calcium normal.
What the GP said: “Before we start a bisphosphonate, we need your vitamin D above 50. Loading dose for four weeks, then maintenance 1000-2000 IU a day alongside calcium, and then alendronate.”
What happened next: Vitamin D rose to 78; alendronate was started and tolerated.
Vitamin D and calcium need to be replete before starting bone-protection medicines. Starting bisphosphonates in a vitamin D deficient patient can cause symptomatic hypocalcaemia.
Over-supplementation — high vitamin D
Toby had been taking 20,000 IU daily from the internet for a year. Vitamin D 235, calcium 2.8 (raised), kidney function mildly abnormal.
What the GP said: “This is vitamin D toxicity — you've been taking far too much. Stop it today. Your kidneys will recover as the vitamin D and calcium come down.”
What happened next: Three months off supplements, vitamin D was 110, calcium 2.5, kidney function normal.
High-dose internet supplements can cause vitamin D toxicity — raised calcium, kidney damage. Usual therapeutic maintenance is 800-2000 IU/day; anything dramatically higher needs medical oversight.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.