Wellness . Symptoms guide
Persistent Fatigue: A Blood Test Roadmap
Quick answer
Feeling tired all the time, or fatigue that has lasted several weeks without an obvious explanation, deserves a structured work-up. The most common reversible drivers are iron deficiency (often before frank anaemia), vitamin D deficiency, B12 deficiency, thyroid dysfunction and low testosterone in men. A single comprehensive panel evaluates these key biological systems simultaneously, providing an immediate baseline for individuals looking to take a proactive, self-directed approach to their health.
This patient information is being clinically reviewed by our team. The factual content aligns with UK diagnostic frameworks, drawing on NHS primary care pathways, NICE clinical knowledge summaries (TATT), and British Society for Haematology guidelines.
What this might be
- Iron deficiency. Most commonly missed reversible cause; ferritin reveals it before anaemia appears.
- Thyroid dysfunction. Both directions cause fatigue.
- Vitamin D deficiency. Very common in the UK, especially winter to spring.
- B12 deficiency. Especially in vegan, vegetarian, or older patients.
- Low testosterone (in men). Both fatigue and low mood are common signs.
- Post-viral fatigue vs. systemic inflammation. True post-viral fatigue typically presents with normal inflammatory markers; checking CRP and ESR helps rule out active underlying inflammatory, autoimmune, or infectious conditions that mimic exhaustion.
When to seek urgent advice
If any of the following apply, please contact your GP or NHS 111 rather than waiting for private bloodwork.
- Unexplained weight loss
- Night sweats
- New persistent lumps
- New severe headache
- Blood in stool or urine
Common features that suggest this
- Tiredness lasting several weeks without an obvious explanation
- Not refreshed by sleep
- Reduced exercise tolerance
- Concurrent low mood, brain fog or hair changes
Recommended tests
Same-day appointments usually available at our Harley Street clinic, with results clinician-reviewed.
Low B12 is one of the causes worth ruling out: see B12 deficiency symptoms and testing.
Need a marker not in these panels? Build a custom panel and a GMC-registered clinician will design one for you.
Markers your clinician will commonly look at
These are the individual blood markers in the recommended panels above. Click any to read what it measures, its UK reference range, and what high or low values mean.
Testing advice
General Wellness and Hair Loss Essentials need a fast of at least 12 hours (water is fine, and encouraged). For The Hormone Specialist (9 to 12 hours) and the Adrenal Function Panel (8 to 12 hours), fasting is preferred but not essential. The Adrenal Function Panel must be drawn between 8am and 9am, and The Hormone Specialist between 8am and 10am.
Related reading
- Always Tired Despite Sleep? What Your Blood Can Tell You Sleeping enough but still exhausted? The treatable causes a blood test can find, why normal results can miss them, and when to see your GP instead.
- What Does Low Ferritin Feel Like? Symptoms, Causes and What Counts as Low What low ferritin feels like, what counts as low, what causes it and when to see a GP. Tiredness, hair shedding and brain fog can be linked to low ...
- Tired in Winter? Season or Something Treatable Flat since the clocks changed? What winter does to energy, mood and the cold, and the four checks worth doing: vitamin D, iron, B12 and thyroid.
- Is There a SAD Blood Test? What to Rule Out There is no blood test for SAD: a doctor diagnoses it. But thyroid, vitamin D, iron and B12 problems can look alike. What a test can rule out.
Common questions
Which panel covers most causes?
Our General Wellness panel is a comprehensive, foundational screen designed specifically to evaluate the primary biochemical and metabolic drivers of unexplained fatigue. If you are experiencing hair-specific changes alongside low energy, our Hair Loss Essentials panel integrates the necessary trichological markers without duplicating costs.
What if all my results come back normal?
A normal result provides highly valuable clinical clarity, as it effectively rules out the most common haematological, metabolic, and hormonal deficiencies linked to physical exhaustion. This allows you and your doctor to confidently narrow your focus onto other potential drivers, such as sleep architecture, lifestyle stressors, psychological factors, or rarer conditions that require a specialist diagnostic referral.
Related symptoms
Sources and further reading
This page provides general information only and is not a substitute for medical advice. A GMC-registered clinician will review your results and tailor any recommendations to you personally.
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