Vitamin D Test
A vitamin D test measures the level of 25-hydroxyvitamin D in your blood. It is used to assess bone health, immune function and overall wellbeing. Low vitamin D is common in the UK, particularly during winter, and can contribute to fatigue, muscle aches and reduced bone strength. This test is frequently included in health checks and symptom reviews.
What a Vitamin D Test Measures
A vitamin D test measures:
- 25-hydroxyvitamin D (25-OH-D) — the main circulating form used to assess vitamin D status.
This reflects vitamin D obtained from sunlight exposure, diet and supplements.
General ranges used by UK laboratories:
- Deficient: below 25 nmol/L
- Insufficient: 25 to 50 nmol/L
- Adequate: 50 nmol/L and above
Interpretation varies depending on symptoms, risk factors and wider health markers.
Why Vitamin D Tests Are Used
A vitamin D test helps identify:
- Vitamin D deficiency
- Causes of fatigue or muscle aches
- Bone and muscle health concerns
- Increased risk of low mood in winter
- Calcium or phosphate imbalance
- Contributing factors to bone pain or weakness
It is also commonly checked in people with long term conditions affecting absorption or metabolism.
Common Reasons to Have a Vitamin D Test
You may be advised to check vitamin D if you have:
- Tiredness or low energy
- Muscle aches or cramps
- Bone discomfort
- Low mood during winter
- Dietary insufficiency
- Low calcium on previous tests
- Darker skin tone (reduced UV absorption)
- Limited time outdoors
- Pregnancy or breastfeeding
- Known vitamin D deficiency
Vitamin D is also included in many routine health assessments.
Understanding Vitamin D Patterns
Low vitamin D
Often linked to limited sunlight exposure, poor absorption, winter months, low dietary intake or higher melanin levels in the skin.
Moderately low levels
Common in the UK. May contribute to tiredness and muscle aches, especially in winter.
High vitamin D
Usually due to excessive supplement use. Rare from sunlight exposure alone.
Temporary fluctuations
Illness, supplements and seasonal variation can influence vitamin D levels.
How the Test Is Performed
A small blood sample is taken from a vein in the arm. No fasting is required, and results are usually available within 24 to 48 hours.
When to Discuss Vitamin D Results with a GP
Speak to a GP if:
- Your vitamin D level is low
- You have ongoing fatigue or bone/muscle discomfort
- You are unsure about supplement dosing
- You have calcium imbalance
- You take medications affecting vitamin D metabolism
A GP can help determine the appropriate supplement strength and duration.
Related Tests
Vitamin D is often reviewed alongside:
For a wider overview of routine blood markers, you can visit our main Blood Tests section in the Knowledge Base.
If you would like to discuss your vitamin D results or symptoms related to low energy or bone health, you can book an online GP appointment for clear and personalised advice.
Last reviewed by Dr Zamiel Hussain, GMC registered GP
Updated: 8 December 2025