What Blood Tests Should I Take for Constant Fatigue? A UK Clinical Guide
Everyone feels tired sometimes. But feeling exhausted most days, for weeks or months at a time, is not something to normalise. Chronic fatigue is one of the most common reasons people book a blood test — and it is a genuinely useful starting point. Blood tests can identify iron deficiency, thyroid disease, undiagnosed diabetes, vitamin deficiencies and inflammation, all of which are treatable causes of tiredness.
However, fatigue also has many causes that cannot be diagnosed on a blood test — including poor sleep quality, obstructive sleep apnoea, chronic stress, depression, anxiety, medication side effects and simple overwork. This guide explains the blood tests that are genuinely worth considering if you are constantly tired, the tests that a GP would typically arrange first, and — importantly — the symptoms that mean you should see a doctor rather than order a home blood test.
Why “constant fatigue” is more complicated than it looks
Fatigue is a symptom, not a diagnosis. It has an unusually long list of possible causes, which is why a single “fatigue test” does not really exist. Broadly, the causes fall into a few categories:
- Nutritional and haematological — iron deficiency (with or without anaemia), vitamin B12 or folate deficiency, low vitamin D.
- Endocrine — an underactive or overactive thyroid, diabetes and prediabetes, low testosterone in men, perimenopause and menopause in women, adrenal problems.
- Sleep-related — chronic short sleep, poor sleep quality, obstructive sleep apnoea, shift work.
- Mental health — depression and anxiety are among the most common causes of chronic tiredness.
- Chronic disease — kidney disease, liver disease, heart failure, autoimmune conditions such as coeliac disease, and inflammatory conditions.
- Infection and post-infection — glandular fever (EBV), post-viral fatigue and long COVID.
- Medications — sedating antihistamines, beta-blockers, some antidepressants, opioids, statins in some people, and others.
- Lifestyle — high alcohol intake, overtraining, under-eating, high caffeine intake, chronic dehydration.
- Chronic fatigue syndrome / ME — a recognised condition diagnosed only after other causes have been excluded.
Because the list is so broad, a good approach is: address the obvious lifestyle factors, exclude the common medical causes with appropriate tests, and involve a clinician if the fatigue is severe, prolonged or accompanied by other symptoms.
When fatigue needs a GP, not a home blood test
Some symptoms suggest that fatigue may be a sign of something that needs proper clinical assessment rather than self-directed testing. Speak to your GP promptly — or in an emergency, call 999 or attend A&E — if your tiredness is accompanied by any of the following:
- Unintentional weight loss
- Drenching night sweats or persistent fever
- New or worsening breathlessness, especially at rest or when lying flat
- Chest pain or new palpitations
- Coughing up blood, blood in stool or urine, or unusually heavy or prolonged periods
- New or unexplained lumps, or swollen glands that do not settle
- Severe headaches, confusion, or new neurological symptoms (weakness, numbness, vision changes)
- Yellowing of the skin or eyes
- Fatigue following a head injury or serious infection
- Suicidal thoughts or a significant drop in mood
Also see a GP if fatigue has continued for more than four weeks with no obvious cause, if it is stopping you from doing normal activities, or if it is getting steadily worse.
Blood tests worth considering for chronic fatigue
UK guidance (including NICE guidance on tiredness and on ME/CFS) recommends that a basic panel of blood tests is arranged for anyone with unexplained ongoing fatigue, largely to exclude common treatable causes. The exact panel a clinician orders depends on your history, but a sensible baseline typically includes the tests below.
Full blood count (FBC)
Measures haemoglobin, red cells, white cells and platelets. Identifies anaemia (of any cause), signs of infection or inflammation, and — occasionally — bone marrow problems. It is arguably the most important single test for tiredness.
Ferritin and iron studies
Ferritin reflects iron stores and is the earliest marker to fall in iron deficiency. You can be iron deficient with a normal haemoglobin (see our iron deficiency vs anaemia guide). Ferritin can also be raised by inflammation, which needs to be taken into account when interpreting the result.
Thyroid function (TSH, and free T4 if indicated)
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) commonly cause fatigue. Hypothyroidism can also cause weight gain, cold intolerance, dry skin, low mood and constipation. Thyroid problems are common, easily missed and easily treated once identified. For more on this, see Thyroid Explained.
Vitamin B12 and folate
Deficiency of either can cause fatigue, brain fog, mood changes and — in the case of B12 — pins and needles or memory problems. Risk factors include vegetarian or vegan diets, older age, long-term use of proton pump inhibitors or metformin, and gastrointestinal conditions.
Vitamin D
Vitamin D deficiency is very common in the UK, particularly from October to March. Symptoms can include fatigue, muscle aches and low mood. Read more in Vitamin D: Why Testing Matters More Than Ever.
HbA1c (blood sugar)
HbA1c reflects average blood glucose over the previous 2–3 months. Undiagnosed or poorly controlled diabetes and prediabetes are common, and fatigue is a frequent symptom.
Kidney and liver function
Chronic kidney disease and liver disease can cause fatigue, and both are often silent in their earlier stages. Routine testing includes urea, creatinine, eGFR and liver enzymes.
C-reactive protein (CRP)
A general marker of inflammation. A raised CRP does not tell you what is inflamed, but it flags that something may need further evaluation.
Hormones — when relevant
- Men: low testosterone can cause fatigue, low mood, reduced libido and reduced motivation. See Signs of Low Testosterone in Men.
- Women: perimenopause and menopause commonly cause fatigue, disrupted sleep, mood changes and brain fog. Hormone testing needs careful timing and interpretation and is best guided by symptoms.
Cortisol
Cortisol can be measured in blood, saliva or urine and is sometimes assessed when clinicians suspect an adrenal problem. It is not a routine “stress test” — see our guide on cortisol for more.
Which test is right for you?
The right blood test depends on your symptoms, history and what you already know. As a rough guide:
If you suspect iron or anaemia
If you have heavy periods, are pregnant or breastfeeding, follow a plant-based diet, train endurance sports, or feel breathless and dizzy alongside fatigue, iron is a reasonable first area to investigate. The Anaemia Profile combines a full blood count with iron studies, ferritin, B12 and folate. For a focused iron-only check, the Iron Blood Test is a more targeted option.
If you are unsure what is causing your fatigue
When symptoms are non-specific and you want a broader baseline, the Vitality Blood Test covers the vitamins, minerals and inflammation markers most commonly linked to low energy — ferritin, iron studies, vitamin D3, vitamin B12, magnesium and CRP. It is designed to answer the “is a common deficiency limiting me?” question in one test.
If you want a full picture
If you would prefer a comprehensive workup — including thyroid function, kidney and liver markers, HbA1c, lipids, hormones and full haematology — the Health Optimisation Blood Test is a broader panel that covers the more common medical causes of chronic fatigue in a single sample.
If you train regularly
Fatigue in athletes and regular gym-goers is often a combination of iron turnover, undereating, inflammation and inadequate recovery. The Competitive Athlete Blood Test provides a wider performance-focused assessment.
If you would like a hand narrowing it down
Our Find My Test tool asks a short set of questions to point you towards the most relevant panel for your symptoms, and you can always speak to a specialist before ordering anything if you would rather have a clinical opinion first.
Things a blood test won’t tell you (but that matter)
Blood tests are useful, but they will not detect some of the most common causes of chronic fatigue. Before or alongside testing, it is worth honestly reviewing:
- Sleep quantity and quality. Consistently fewer than seven hours a night, disturbed sleep, loud snoring, morning headaches or witnessed pauses in breathing may point to obstructive sleep apnoea, which is common, under-diagnosed and treatable.
- Mood. Depression and anxiety very commonly present as fatigue and low motivation before they present as sadness. If you are also low, tearful, losing interest in things, sleeping badly, or having thoughts of self-harm, please talk to your GP.
- Alcohol intake. Even amounts within recommended limits noticeably worsen sleep quality and next-day energy for many people.
- Caffeine timing. Caffeine has a long half-life; afternoon coffee is a common, quiet cause of poor sleep and morning fatigue.
- Nutrition and eating patterns. Under-eating, especially in combination with training, is a common driver of persistent tiredness.
- Training load. Chronic under-recovery (“overreaching”) is often misread as low testosterone or low iron.
- Medications and supplements. Sedating antihistamines, beta-blockers, opioid painkillers, some antidepressants and some blood pressure medications can all cause fatigue.
- Stress and workload. Prolonged psychological stress genuinely lowers energy and cognitive performance; it is not “just in your head.”
Addressing these factors alongside investigating medical causes is usually what actually resolves chronic fatigue.
What if my blood tests come back normal?
Normal blood tests are useful information — they meaningfully reduce the likelihood of the common medical causes of fatigue and are a reasonable prompt to look more carefully at sleep, mood, lifestyle and psychological factors, or to widen the workup. Persistent, unexplained fatigue with normal initial tests is a good reason to see your GP, who may consider further investigations, referral to a specialist, or assessment for post-viral fatigue, long COVID or ME/CFS.
A blood test is a snapshot. It is not a substitute for a full clinical assessment when symptoms are ongoing.
How often should you retest?
There is no single correct answer — it depends on what was found and what you are doing about it. Some general principles:
- After starting treatment for a deficiency, retesting is usually done after an interval agreed with your clinician, once treatment has had time to work.
- If a level was borderline, retesting some weeks or months later, alongside symptom review, is often reasonable.
- If tests were normal and symptoms persist, repeating the same tests immediately is rarely helpful — reviewing the picture with a clinician usually is.
- For people training intensely or managing a known condition, periodic monitoring at a cadence guided by a clinician is sensible.
Frequently asked questions
What is the best blood test for tiredness?
There is no single “best” test. For most people the sensible baseline includes a full blood count, ferritin, thyroid function, vitamin B12, folate, vitamin D, HbA1c, kidney and liver function, CRP, and a coeliac screen. The right panel depends on your symptoms and history.
Can a blood test tell me why I’m always tired?
Sometimes, yes — particularly when there is a treatable medical cause such as iron deficiency, thyroid disease or a vitamin deficiency. Often, however, chronic fatigue is multifactorial and involves sleep, mood, stress and lifestyle as well as medical factors. Blood tests are one part of the picture.
Should I get a thyroid test if I’m tired?
For most people with unexplained ongoing tiredness, yes. Thyroid dysfunction is common, easily missed and easily treated. TSH is the standard first-line test.
Can low vitamin D cause severe fatigue?
Vitamin D deficiency has been associated with fatigue, low mood and muscle aches. Deficiency is common in the UK, particularly through the winter. Testing and correcting a low level is straightforward.
What blood tests should men have for fatigue?
The general fatigue panel above applies. In addition, men with reduced libido, erectile dysfunction, loss of morning erections or shrinking testes should consider testosterone testing — see our guide on the signs of low testosterone in men for more.
What blood tests should women have for fatigue?
The general panel above applies, with particular attention to iron and ferritin if periods are heavy. In women approaching or in menopause, symptoms including fatigue, poor sleep, brain fog and mood changes may be hormonal — a discussion with a clinician can help decide whether hormone testing adds useful information.
Can stress alone cause chronic tiredness?
Yes. Prolonged psychological stress can meaningfully reduce energy, disrupt sleep and impair concentration, even in the absence of any measurable blood abnormality.
What if I’ve had long COVID or a viral illness?
Post-viral fatigue, including long COVID, is well recognised. Blood tests are used mainly to rule out other causes rather than to diagnose the condition itself. If fatigue is persistent after a viral illness, discuss it with your GP — supportive management pathways are available.
Is chronic fatigue the same as ME/CFS?
No. Chronic fatigue is a symptom that can have many causes. Myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) is a specific clinical diagnosis made only after other causes have been excluded, using UK diagnostic criteria (NICE NG206). It requires clinical assessment.
Medical disclaimer
This article is for general information and education. It is not medical advice and is not a substitute for consultation with a qualified healthcare professional. Do not start, stop or change any treatment on the basis of this content. If you have severe or worsening symptoms, unexplained weight loss, chest pain, breathlessness at rest, blood loss, persistent fever or thoughts of self-harm, contact your GP or NHS 111 promptly, or in an emergency call 999 or attend A&E.
References
- NHS. Why am I tired all the time? NHS website.
- NICE Clinical Knowledge Summary. Tiredness/fatigue in adults.
- NICE Guideline NG206. Myalgic encephalomyelitis (or encephalopathy)/chronic fatigue syndrome: diagnosis and management.
- NICE Clinical Knowledge Summary. Anaemia — iron deficiency.
- NICE Clinical Knowledge Summary. Hypothyroidism.
- NICE Guideline NG188. COVID-19 rapid guideline: managing the long-term effects of COVID-19.