Vitamin Deficiency Blood Test UK: Which Vitamins to Check & What Your Results Mean

Why test for vitamin deficiencies?

Vitamin and mineral deficiencies rarely announce themselves with a single dramatic symptom. Instead, they erode your energy, immunity, mood, and recovery slowly — often for months or years before you connect the dots. By the time fatigue, hair thinning, brain fog, or frequent infections drive you to a GP, the deficiency may have been building for a long time.

The NHS Health Check programme, offered every 5 years to adults aged 40–74, does not include nutritional screening. If you’re under 40, or between checks, your vitamin levels are essentially invisible unless you specifically ask. Even then, GPs typically only test one or two markers based on presenting symptoms — missing the broader picture.

A comprehensive vitamin deficiency blood test answers three questions:

1. Are your current supplements working? — up to 50% of people who take supplements don’t need them, while missing the ones they do. A blood test shows what’s actually low.
2. Could a deficiency explain your symptoms? — fatigue, poor concentration, mood changes, thinning hair, muscle weakness, and poor immunity all overlap with common deficiency patterns.
3. Are you in the “sufficient but not optimal” zone? — NHS reference ranges define the minimum to prevent disease. Functional ranges define the level at which your body performs best. The gap between the two is where most people live — technically “normal” but far from thriving.

10 symptoms that suggest a vitamin or mineral deficiency

None of these symptoms proves a deficiency on its own — but if you recognise three or more, a blood test is the fastest way to rule in or rule out a nutritional cause.

For a deeper dive into fatigue-related testing, see our always tired guide. For hair-specific markers, read our hair loss blood test guide.

Which vitamins and minerals should you check?

A thorough nutritional blood test covers 10 key markers across vitamins, minerals, and functional indicators. Here’s what each one tells you:

Vitamin D: the UK’s most common deficiency

The UK sits at 50–59°N latitude — north of the line where skin can synthesise meaningful vitamin D from sunlight between October and March. The NDNS reports that roughly 1 in 6 adults have serum 25(OH)D below 25 nmol/L (deficient) and up to 40% fall below 50 nmol/L (insufficient) during winter.

NHS reference range: >25 nmol/L is “sufficient.” But this threshold was set to prevent rickets and osteomalacia — the minimum for bone health, not the optimum for whole-body function.

Optimal range: most endocrinologists and longevity researchers target 75–125 nmol/L. The Endocrine Society Clinical Practice Guideline (2011) recommends maintaining levels above 75 nmol/L for optimal health outcomes.

What to do if low: PHE recommends 10 µg (400 IU) daily for all UK adults in autumn and winter. If your level is below 50 nmol/L, your GP or a private clinician may recommend a loading dose of 1,000–4,000 IU daily for 8–12 weeks, followed by maintenance. Vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol), and absorption improves when taken with fat.

For the full breakdown, read our vitamin D deficiency guide.

Vitamin B12: energy, nerves, and the grey zone

B12 is essential for red blood cell formation, neurological function, and DNA synthesis. Deficiency causes megaloblastic anaemia, peripheral neuropathy (tingling, numbness), cognitive impairment, and fatigue. The danger is that neurological damage can become irreversible if left untreated.

The grey zone problem: serum B12 between 148–300 pmol/L is technically “normal” by NHS standards but may still indicate functional deficiency. The British Society for Haematology (BSH) guidelines recommend further investigation with methylmalonic acid (MMA) or active B12 (holotranscobalamin) if serum B12 is below 300 pmol/L with symptoms present.

Who’s at risk: vegans and vegetarians (B12 is found almost exclusively in animal products), people over 50 (reduced intrinsic factor production), those on PPIs or metformin (both impair absorption), and anyone who has had bariatric surgery.

Optimal range: above 300 pmol/L (serum B12) or above 70 pmol/L (active B12). See our vitamin B12 blood test guide for a deep dive into the grey zone, MMA testing, and pernicious anaemia.

Folate (vitamin B9): more than a pregnancy vitamin

Folate is critical for DNA synthesis, amino acid metabolism, and the methylation cycle — a metabolic process that influences everything from neurotransmitter production to cardiovascular health. Most people associate folate exclusively with pregnancy, but low folate affects everyone.

The NDNS reports that approximately 5% of UK adults have clinical folate deficiency, with young women at highest risk. Low folate alongside low B12 causes megaloblastic anaemia — but folate deficiency also independently raises homocysteine levels, an emerging cardiovascular risk factor.

NHS range: >3.9 nmol/L is “sufficient.” Optimal range: above 20 nmol/L (serum folate) or above 340 nmol/L (red blood cell folate, which reflects 3-month status).

Note on MTHFR: roughly 10% of the UK population has a homozygous MTHFR C677T variant, which reduces the body’s ability to convert folic acid into its active form (5-methyltetrahydrofolate). If you have this variant, methylfolate supplements are more effective than standard folic acid.

Iron and ferritin: the three stages of deficiency

Iron deficiency is the most common nutritional deficiency worldwide, affecting roughly 2 billion people. In the UK, it disproportionately affects women of reproductive age, vegans, endurance athletes, and people with gut conditions that impair absorption.

Iron depletion progresses through three stages, and ferritin is the earliest warning signal:

1. Stage 1 — Iron depletion: ferritin drops below 30 µg/L. No symptoms yet, but stores are falling. Haemoglobin is still normal.
2. Stage 2 — Iron-deficient erythropoiesis: ferritin below 15 µg/L. Fatigue, exercise intolerance, and poor concentration emerge. Haemoglobin starts to dip.
3. Stage 3 — Iron-deficiency anaemia: haemoglobin below 120 g/L (women) or 130 g/L (men). Pronounced fatigue, pallor, breathlessness, palpitations. Requires treatment.

The catch: GPs typically only check haemoglobin (FBC), which stays normal through stages 1 and 2. You can be profoundly iron-depleted with a “normal” FBC. Ferritin is the test that catches the problem early.

NHS range: ferritin >15 µg/L is “normal.” Optimal range: 50–150 µg/L for women, 75–200 µg/L for men. Athletes should aim for the higher end.

For the complete iron panel breakdown including TIBC and transferrin saturation, see our iron deficiency blood test guide.

Magnesium: the overlooked mineral

Magnesium is involved in over 300 enzymatic reactions — including energy production, muscle contraction, nerve signalling, blood pressure regulation, and DNA repair. Despite this, it’s one of the least-tested minerals in routine NHS care.

The NDNS estimates that 11% of UK women and 16% of teenage girls fail to meet the lower reference nutrient intake for magnesium. True subclinical deficiency may be higher because serum magnesium (the standard test) only reflects 1% of total body magnesium — the rest is stored in bones and soft tissue.

Common depletors: chronic stress (cortisol drives urinary magnesium loss), alcohol, intense exercise, PPIs (proton pump inhibitors), and diets high in processed food. If you experience muscle cramps, poor sleep, anxiety, or heart palpitations, magnesium is worth checking.

NHS range: 0.70–1.00 mmol/L. Optimal range: 0.85–1.00 mmol/L. Levels below 0.85 mmol/L are associated with increased cardiovascular risk in meta-analyses published in BMC Medicine.

Forms that matter: magnesium glycinate or bisglycinate for sleep and anxiety, magnesium citrate for general supplementation, magnesium threonate (L-threonate) for cognitive function. Avoid magnesium oxide — it has poor bioavailability (roughly 4%) despite being the most commonly sold form.

Zinc, selenium, omega-3, and vitamin A

Zinc

Zinc supports immune function, wound healing, taste and smell perception, and testosterone production. The NDNS reports that 5% of UK men and 16% of UK women aged 19–64 have inadequate zinc intake. Vegetarians and vegans are at higher risk because phytates in grains and legumes inhibit zinc absorption. Optimal serum range: 11–18 µmol/L.

Selenium

Selenium is critical for thyroid hormone conversion (T4 → T3), antioxidant defence via glutathione peroxidase, and immune function. UK soil is selenium-poor compared to the US, which means UK-grown crops contain less selenium. The Lancet Planetary Health has flagged declining selenium in European populations. Optimal serum range: 1.14–1.90 µmol/L. Brazil nuts are the richest dietary source — two per day can maintain optimal levels.

Omega-3 index

The omega-3 index measures EPA and DHA as a percentage of red blood cell membranes — reflecting 2–3 months of intake rather than a snapshot. A 2021 analysis in the Journal of the American Heart Association found that an omega-3 index above 8% is associated with a 35% reduction in cardiac death risk compared to below 4%. Most UK adults who don’t eat oily fish twice weekly fall below 6%.

Vitamin A (retinol)

Vitamin A supports vision, skin integrity, and immune function. Clinical deficiency is rare in the UK (unlike globally, where it remains a leading cause of childhood blindness). However, subclinical insufficiency can contribute to dry skin, poor night vision, and impaired immune defence. Most comprehensive panels don’t routinely include it unless specifically requested. If you eat liver, dairy, or orange/yellow vegetables regularly, deficiency is unlikely.

NHS vs optimal ranges: the complete reference table

This table summarises the gap between NHS “normal” thresholds and the ranges associated with optimal health outcomes in published research. Your GP will typically only flag results outside the NHS column.

Optimal ranges compiled from Endocrine Society, BSH, BMJ Best Practice, and EFSA nutrient reference values. These are targets for healthy adults — your clinician may recommend different targets based on your medical history.

Who is most at risk of vitamin deficiency?

Some groups carry a significantly higher risk of nutritional deficiency due to diet, physiology, medication, or lifestyle factors:

• Vegans and vegetarians: B12 (animal-source only), iron (lower bioavailability from plant sources), zinc (phytate inhibition), omega-3 (no direct EPA/DHA from diet)
• Women of reproductive age: Iron (menstrual losses), folate (pregnancy preparation), vitamin D (higher requirements during pregnancy and breastfeeding)
• Over-50s: B12 (reduced intrinsic factor), vitamin D (decreased skin synthesis), magnesium (declining absorption efficiency)
• Endurance athletes: Iron (foot-strike haemolysis, sweat losses), magnesium (exercise-induced depletion), zinc (increased turnover)
• People on PPIs or metformin: B12 (both impair absorption), magnesium (PPIs reduce intestinal absorption), iron (PPIs raise gastric pH)
• People with gut conditions: Coeliac disease, Crohn's, and IBS can impair absorption of most micronutrients, particularly iron, B12, folate, and fat-soluble vitamins
• People on calorie-restricted diets or GLP-1 medications: Reduced food intake means reduced micronutrient intake across the board
• People with darker skin in the UK: Melanin reduces UVB-driven vitamin D synthesis; NDNS data shows higher deficiency prevalence in Black and South Asian populations

If you belong to two or more of these groups, annual nutritional screening is strongly advisable. The NICE guideline PH56 recommends vitamin D supplementation for all at-risk groups and testing when deficiency is suspected clinically.

NHS GP vs Helvy: what gets tested for vitamin deficiencies

If you ask your GP for a “vitamin check,” what you receive depends entirely on your symptoms, local commissioning guidelines, and budget. There is no standard NHS “nutritional panel.” Here’s a realistic comparison:

The most significant gaps are magnesium, zinc, selenium, active B12, and the omega-3 index — markers that are clinically important but almost never available through routine NHS care. A private nutritional panel tests them all in one go, from home, with results in days rather than weeks.

Supplementing based on results, not guesswork

The UK supplement market is worth over £500 million annually. Much of that spending is wasted — people taking nutrients they already have enough of because marketing told them to, while the nutrients they actually lack go unaddressed.

A blood test flips this model. Instead of “take everything and hope,” you get a targeted protocol:

• If vitamin D is below 50 nmol/L: 1,000–4,000 IU vitamin D3 daily with food containing fat. Retest in 3 months.
• If B12 is below 300 pmol/L with symptoms: methylcobalamin sublingual (1,000 µg daily) or hydroxocobalamin injections via GP. Retest at 8 weeks.
• If ferritin is below 50 µg/L: iron bisglycinate (better tolerated than ferrous sulphate) taken with vitamin C on an empty stomach. Retest at 12 weeks. Note: do NOT supplement iron without testing — excess iron is harmful.
• If magnesium is below 0.85 mmol/L: magnesium glycinate 200–400 mg before bed. Safe for most people. Retest at 3 months.
• If omega-3 index is below 8%: 2–3 g EPA+DHA from fish oil or algae-based supplement daily. Retest at 4 months (red blood cell turnover takes time).

For our evidence-based supplement tier list, read which supplements are actually worth taking.

Frequently asked questions