Low Vitamin D Symptoms: What Your Blood Test Results Really Mean

Why Late Winter Is the Worst Time for Your Vitamin D

After months of limited sunlight, your body's vitamin D stores are at their lowest. National Diet and Nutrition Survey data consistently shows that blood levels of 25-hydroxyvitamin D drop to their lowest point during the January to March quarter, often falling by more than 20 nmol/L compared with late summer peaks [1]. For many people in the UK, that means entering spring already running on empty, without realising it.

How Common Is Vitamin D Deficiency in the UK?

More common than most people realise. A cross-sectional analysis of nearly 450,000 participants in the UK Biobank found that around 13% were clinically deficient (below 25 nmol/L), with considerably higher rates in participants of Asian ancestry (54%) and Black African ancestry (35%) compared with White European participants (12%) [2]. Levels were also lower in the northern UK than in the south, reflecting differences in UVB exposure, and lower in participants from more deprived areas.

These figures likely underestimate the true national picture. The UK Biobank cohort skews towards older, more affluent volunteers, and higher socioeconomic deprivation is independently associated with lower vitamin D status [2].

Diet alone doesn't solve the problem. The Scientific Advisory Committee on Nutrition (SACN) concluded that it's difficult to achieve the recommended 10 microgram daily intake from food sources alone, even with oily fish, eggs, and fortified foods in your diet [3]. This is why supplementation matters, particularly between October and March.

Common Symptoms of Low Vitamin D

Low vitamin D symptoms are often vague enough to be dismissed as "just feeling run down," which is exactly why so many cases go undiagnosed. The most common signs include:

• Persistent fatigue that doesn't improve with rest

• Generalised muscle aches and weakness, particularly in the thighs and upper arms

• Bone pain or tenderness, especially in the lower back, hips, or legs

• Low mood or irritability that tends to worsen during winter

• Frequent infections or slow wound healing

In more severe deficiency, you may notice muscle cramps, difficulty concentrating, or hair thinning. However, none of these symptoms are specific to vitamin D. They overlap with thyroid disorders, iron deficiency, B12 deficiency, and depression, among others. A blood test can help determine whether low vitamin D is a contributing factor, but your GP will interpret the result alongside your full clinical picture.

Why Does Vitamin D Matter?

Vitamin D is essential for calcium absorption and maintaining healthy bones, teeth, and muscles. Prolonged deficiency can lead to osteomalacia in adults (softening of the bones, causing deep bone pain and muscle weakness) and rickets in children.

Beyond the skeleton, vitamin D plays a role in regulating the immune system. There is evidence linking low vitamin D levels with increased susceptibility to respiratory tract infections [4]. Observational studies have also reported associations between low vitamin D and conditions including type 2 diabetes, cardiovascular disease, and certain cancers, but the SACN's 2016 review concluded that the evidence for non-musculoskeletal health outcomes was insufficient to draw firm conclusions [3]. Claims about causation in these areas remain contested, and supplementation trials have not consistently shown benefit beyond bone and muscle health.

Seasonal affective disorder (SAD) and low mood during winter have also been linked to vitamin D depletion, though the relationship is complex and likely involves other factors, including reduced daylight hours and lifestyle changes.

Who's Most at Risk of Low Vitamin D?

While anyone in the UK can become deficient, particularly over winter, certain groups face higher risk:

• People with darker skin tones (melanin reduces the skin's ability to produce vitamin D from sunlight)

• Those who spend most of their time indoors, including people who are housebound or in care homes

• People who cover most of their skin when outdoors

• Adults over 65 (the skin becomes less efficient at synthesising vitamin D with age)

• Pregnant and breastfeeding women

• People with obesity (vitamin D is stored and trapped in fat tissue, reducing its availability in the blood)

What Does a Vitamin D Blood Test Measure?

A vitamin D blood test measures 25-hydroxyvitamin D (also written as 25(OH)D), the main circulating form of vitamin D in your blood. This is considered the most accurate marker of your overall vitamin D status, as it reflects both what your skin produces from sunlight and what you absorb from food and supplements.

The standard UK reference ranges are:

• Below 25 nmol/L: Deficient (increased risk of bone disease)

• 25 to 50 nmol/L: Insufficient (may not be optimal for musculoskeletal health)

• Above 50 nmol/L: Adequate

A result in the "insufficient" range doesn't automatically mean something is wrong. A reading of 40 nmol/L in an otherwise healthy, asymptomatic person during February may simply reflect normal seasonal variation. Your GP will consider your symptoms, medical history, and the time of year when interpreting the result. Standard over-the-counter supplementation (10 micrograms daily) is usually sufficient for mild insufficiency, while confirmed deficiency (below 25 nmol/L) typically requires a higher loading dose prescribed by your GP [5].

Current UK Supplementation Guidance

The SACN recommends that everyone in the UK aged four and above should have a daily intake of 10 micrograms (400 IU) of vitamin D, throughout the year [3]. In practice, the NHS advises that most people can get enough vitamin D through sunlight and diet during spring and summer, but should consider a 10 microgram supplement during autumn and winter [4].

People in higher-risk groups (darker skin, limited sun exposure, over 65, pregnant) should consider supplementing all year round [4].

The Bigger Picture: Conditions That Mimic Low Vitamin D

In my clinical experience, I regularly see patients who are convinced their persistent tiredness must be vitamin D deficiency, only to discover through a broader blood panel that the real culprit is something else entirely. Conditions that closely mimic the symptoms of low vitamin D include:

• Hypothyroidism (underactive thyroid), which causes fatigue, weight gain, and low mood

• Iron deficiency, which produces tiredness, muscle weakness, and poor concentration even before you become formally anaemic

• B12 deficiency, which can cause fatigue, mood changes, and neurological symptoms like tingling

• Depression and anxiety, where physical symptoms like fatigue and body aches are common

Testing for vitamin D alone can miss these conditions. That's why a broader panel, covering thyroid function, iron stores, B12, and inflammatory markers, often gives a much clearer picture of what's going on.

When to See Your GP

A private blood test is a useful starting point, but you should see your GP if you experience any of the following:

• Persistent fatigue lasting more than four weeks that doesn't improve with rest

• Unexplained weight loss

• Severe or worsening bone pain

• Persistent low mood, particularly if accompanied by loss of interest in activities or thoughts of self-harm

• Symptoms of suspected adrenal insufficiency, including dizziness on standing, salt cravings, or skin darkening

• You are pregnant or breastfeeding and concerned about your vitamin D levels

If your Brooksby results show a 25(OH)D level below 25 nmol/L, take your report to your GP. Confirmed deficiency usually requires a higher-dose loading regimen that should be supervised by your doctor.

How Brooksby Medical Can Help

Brooksby Medical's Vitamin D Blood Test measures your 25(OH)D level from a simple finger-prick sample, with results reported within two to three working days and interpreted by a practising GP.

If you want a broader view, the Wellness Profile includes vitamin D alongside B12, folate, ferritin, magnesium, thyroid function, liver and kidney markers, and inflammation markers, giving you a clearer picture of your overall health as winter ends. For targeted investigation of fatigue, the Tiredness and Fatigue Blood Test covers the most common treatable causes.

References

1. Public Health England. National Diet and Nutrition Survey: Results from Years 9 to 11 (2016/2017 to 2018/2019). GOV.UK

2. Lin LY, Smeeth L, Langan S, Warren-Gash C. Distribution of vitamin D status in the UK: a cross-sectional analysis of UK Biobank. BMJ Open. 2021;11(1):e038503

3. Scientific Advisory Committee on Nutrition. Vitamin D and Health. 2016. GOV.UK

4. NHS. Vitamin D. NHS.uk

5. NICE Clinical Knowledge Summaries. Vitamin D deficiency in adults. NICE CKS

Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.

Medically reviewed: March 2026 | Next review due: March 2027

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.