What Blood Tests Should You Have for Tiredness and Fatigue?

Feeling tired after a late night, an unusually demanding week or a period of stress is normal. Persistent fatigue is different. It may feel as though sleep no longer restores your energy, ordinary activities require more effort than they should, or your concentration and motivation have noticeably declined.

When tiredness continues, it is understandable to wonder whether a blood test could reveal the reason. Sometimes it can. Anaemia, iron deficiency, an underactive thyroid, diabetes, vitamin B12 deficiency and several other medical problems can all contribute to fatigue. Blood tests may help identify or rule out these conditions.

However, there is no single “fatigue blood test”, and the largest available testing panel is not automatically the best one. Fatigue is a broad symptom with physical, psychological, lifestyle and sleep-related causes. The most useful tests depend on your symptoms, medical history, medicines, diet, age, menstrual history and risk factors.

This guide explains which blood tests are commonly considered for tiredness and fatigue, what each test can reveal, when more specialised testing may be appropriate and when you should speak to a GP rather than relying on a private test.

Why tiredness does not always mean something is wrong with your blood

Fatigue is a symptom rather than a diagnosis. It describes a lack of physical or mental energy, but it does not tell you what is causing that lack of energy.

Blood tests are most useful when they are chosen to investigate a plausible cause. Before deciding which markers to check, it helps to consider the broader picture.

Common non-blood-related contributors to fatigue include:

• Too little sleep or poor-quality sleep
• Shift work or an irregular sleep schedule
• Sleep apnoea, especially when accompanied by loud snoring or morning headaches
• Stress, anxiety, low mood or burnout
• Recent infection or recovery from illness
• Long COVID or another post-viral condition
• Excessive exercise without adequate recovery
• Very little physical activity and resulting deconditioning
• Insufficient food intake or a highly restrictive diet
• Alcohol or recreational drug use
• Medicines that cause drowsiness or reduce alertness
• Pregnancy, caring responsibilities or major life changes

The NHS guidance on tiredness and fatigue recommends speaking to a GP when tiredness has lasted for several weeks, affects daily life or occurs alongside symptoms such as weight loss or mood changes. A GP may arrange blood tests for conditions including anaemia, diabetes or thyroid disease, but testing is only one part of the assessment.

It is also possible to have more than one cause. For example, someone may have mildly reduced iron stores while also sleeping poorly, experiencing stress and eating too little. Correcting the iron deficiency might help, but it may not completely resolve the fatigue.

This is why blood test results should be interpreted alongside symptoms rather than treated as a complete explanation on their own.

When fatigue needs prompt medical attention

Most episodes of tiredness are not medical emergencies. Nevertheless, new or persistent fatigue can occasionally accompany a condition that requires timely investigation.

Contact a GP promptly if fatigue is unexplained, is getting worse, or is accompanied by:

• Unintentional weight loss
• Persistent fever or night sweats
• Unusual bruising or bleeding
• Repeated infections
• Swollen lymph nodes
• Persistent abdominal pain or swelling
• A major change in bowel habits
• Blood in your stool or urine
• Shortness of breath during ordinary activity
• Palpitations, dizziness or fainting
• Marked muscle weakness
• New neurological symptoms
• Loss of appetite that does not improve
• Severe low mood or thoughts of self-harm

Persistent fatigue can sometimes be one of several symptoms prompting urgent investigation. For example, NICE advises clinicians to consider a very urgent full blood count in adults with persistent fatigue where a blood cancer such as leukaemia is a possibility. This does not mean that persistent tiredness usually indicates cancer; it means that the symptom should not be dismissed when it appears alongside concerning features.

Call 999 or seek emergency help for severe difficulty breathing, chest pain, collapse, confusion, signs of a stroke, vomiting blood, black stools or another immediately serious symptom.

A home blood test should not be used to delay urgent medical care.

Which blood tests are commonly considered for fatigue?

There is no universal panel that every tired person needs. A clinician usually starts with your history and symptoms, then selects tests capable of answering specific questions.

A broadly based initial investigation may include some of the following:

• Full blood count
• Ferritin and, where appropriate, additional iron studies
• Thyroid function tests
• HbA1c or blood glucose
• Kidney function and electrolytes
• Liver function tests
• Vitamin B12 and folate
• Inflammation markers such as CRP or ESR
• Coeliac disease antibodies
• Calcium and sometimes other bone-profile markers

Additional tests may be appropriate when symptoms or risk factors point towards a particular condition. These might include vitamin D, female or male hormone testing, creatine kinase, cortisol, infection tests or more detailed nutritional investigations.

That does not mean all of these should be ordered at once. NICE guidance cautions that testing people too early or without a clear clinical reason can produce incidental borderline results, leading to repeated tests, anxiety and investigations that may not improve care.

A useful panel is therefore not necessarily the panel with the most biomarkers. It is the one that covers the most credible explanations for your particular symptoms without including large numbers of unrelated markers.

Full blood count, ferritin and iron studies

Full blood count

A full blood count, often shortened to FBC, is one of the most frequently requested tests for persistent fatigue. It measures several groups of blood cells, including red blood cells, white blood cells and platelets.

For tiredness, particular attention is often paid to:

• Haemoglobin
• Red blood cell count
• Haematocrit
• Mean cell volume, or MCV
• Mean cell haemoglobin, or MCH
• White blood cell count and differential
• Platelet count

Low haemoglobin may indicate anaemia, meaning that the blood has a reduced capacity to carry oxygen. Anaemia can cause tiredness, weakness, breathlessness, headaches, dizziness and palpitations.

The MCV and other red-cell measurements can help indicate what type of anaemia may be present. Small red blood cells may occur with iron deficiency, while unusually large red blood cells can be associated with vitamin B12 or folate deficiency, alcohol use, liver disease, thyroid disease or certain medicines.

White blood cells and platelets provide additional information. Abnormal results can occur for many reasons, from a temporary infection to inflammation, medication effects or a haematological disorder. You can read more in our detailed guides to full blood count results, white blood cell counts and platelet results.

Ferritin

Ferritin is a protein that stores iron. It is frequently checked when fatigue may be related to iron deficiency, particularly in people who have heavy menstrual bleeding, follow a low-iron diet, donate blood regularly or have a condition affecting absorption.

It is possible to have depleted iron stores before haemoglobin falls below the laboratory range. In other words, someone can have iron deficiency without established anaemia. This is one reason why a normal full blood count does not always settle the question.

Symptoms associated with low iron stores may include:

• Tiredness
• Reduced exercise tolerance
• Breathlessness
• Headaches
• Restless legs
• Hair shedding
• Feeling cold
• Difficulty concentrating

Our guide to low ferritin with normal haemoglobin explains why this pattern occurs and what may need to be investigated.

Ferritin is not a perfect standalone measure. Because it can rise during inflammation, infection or liver disease, a result that appears normal or high does not always exclude iron deficiency. Depending on the circumstances, a clinician may also consider transferrin saturation, serum iron, transferrin or total iron-binding capacity.

A raised ferritin level is not proof of iron overload either. It can occur with inflammation, alcohol use, fatty liver disease, metabolic problems and other conditions. See our guide to high ferritin results for a fuller explanation.

Iron supplements should not be started simply because fatigue is present. Too much iron can be harmful, and unexplained iron deficiency may require investigation for blood loss or poor absorption rather than supplementation alone.

Thyroid, blood sugar, kidney and liver tests

Thyroid function

An underactive thyroid can cause persistent tiredness, feeling cold, constipation, dry skin, weight gain, slowed thinking, muscle aches and heavier or irregular periods. An overactive thyroid can also produce fatigue, although it is more often accompanied by symptoms such as palpitations, sweating, tremor, anxiety and unexplained weight loss.

A basic thyroid investigation usually measures thyroid-stimulating hormone, or TSH. Free T4 is commonly included or added when the TSH is abnormal. In selected circumstances, free T3 and thyroid antibodies may provide additional information.

Testing every thyroid marker available is rarely necessary as a first step. TSH and free T4 answer the main initial question for most people: is the thyroid producing too little or too much hormone?

Our thyroid blood test results guide explains common patterns involving TSH, free T4 and free T3.

HbA1c and glucose

Both high and low blood sugar can affect energy. Diabetes may cause tiredness alongside increased thirst, frequent urination, blurred vision, recurrent infections or unexplained weight loss. Prediabetes often causes no clear symptoms, although testing may still be appropriate when risk factors are present.

HbA1c estimates average blood glucose over the previous two to three months. It is commonly used to screen for and monitor type 2 diabetes.

A fasting glucose test provides a snapshot of blood sugar at the time the sample is taken. It can be useful in particular circumstances but varies more from one day to another.

People with symptoms suggestive of rapidly developing diabetes should seek medical advice promptly rather than waiting for a routine home test. Our guide to HbA1c and blood sugar results explains UK diagnostic ranges and common follow-up steps.

Kidney function and electrolytes

The kidneys filter waste from the blood and help regulate fluid balance, blood pressure and electrolytes. Kidney disease does not always cause symptoms in its early stages, but more advanced problems can contribute to tiredness, itching, swelling, nausea and reduced appetite.

Common kidney-related blood markers include:

• Creatinine
• Estimated glomerular filtration rate, or eGFR
• Urea
• Sodium
• Potassium

Electrolyte abnormalities can cause weakness, fatigue, cramps, confusion or heart-rhythm disturbances. Mild abnormalities may result from dehydration, medicines or a temporary illness, while more marked changes may require urgent assessment.

You can find a detailed explanation in our guides to kidney blood test results and urea and electrolyte results.

Liver function

Liver problems may cause fatigue, but tiredness alone does not strongly indicate liver disease. Other possible symptoms include discomfort in the upper right abdomen, itching, nausea, reduced appetite, dark urine, pale stools or jaundice.

A liver panel commonly includes markers such as:

• ALT
• ALP
• Bilirubin
• Albumin
• GGT

AST may also be included. These tests do not all measure the same thing. Some reflect irritation or injury to liver cells, some relate to bile flow, and others provide information about the liver’s ability to make proteins.

Abnormal liver enzymes can occur because of alcohol, fatty liver disease, medicines, infection, strenuous exercise and many other causes. Our guides to liver function results and ALT, AST, ALP and GGT explain how clinicians interpret common patterns.

Vitamin B12, folate, vitamin D and nutritional testing

Vitamin B12 and folate

Vitamin B12 and folate are needed for healthy blood-cell production, nervous-system function and numerous metabolic processes.

Deficiency can contribute to extreme tiredness, weakness, a sore tongue, mouth ulcers and changes in the full blood count. Vitamin B12 deficiency may also cause pins and needles, balance problems, cognitive changes or visual symptoms.

Importantly, neurological symptoms of B12 deficiency can occur without anaemia. A normal haemoglobin result does not always rule it out.

Testing may be particularly relevant for people who:

• Follow a vegan or highly restrictive diet
• Have coeliac disease or another absorption problem
• Have had stomach or bowel surgery
• Take metformin
• Use long-term acid-suppressing medicines
• Have an autoimmune condition
• Have unexplained neurological symptoms

The NHS provides more information about vitamin B12 and folate deficiency. You can also read our guide to B12 and folate blood test results.

Vitamin D

Vitamin D deficiency is common in the UK, particularly during autumn and winter. Low levels may be associated with muscle weakness, bone pain or generalised aches, and severe deficiency can affect bone health.

However, vitamin D is not a universal explanation for non-specific fatigue. Routine testing is not always needed, particularly when someone has no symptoms or risk factors and would already be advised to take a standard preventive supplement.

Testing may be more useful when there are symptoms of deficiency, a bone disorder, a malabsorption condition, significant kidney or liver disease, or another clinical reason to know the level.

Our vitamin D results guide explains deficiency, insufficiency, supplementation and repeat testing.

Other nutritional markers

Large commercial panels sometimes include magnesium, zinc, copper and other micronutrients. These can be valuable in selected circumstances, but they are not necessarily useful screening tests for everyone who feels tired.

Blood levels may not always reflect the body’s total stores, and borderline results can be difficult to interpret. The decision to test should ideally be based on diet, symptoms, medicines, digestive problems and the likelihood of a genuine deficiency.

A detailed dietary history is often at least as important as a long micronutrient panel.

Inflammation, coeliac disease and other medical causes

CRP and ESR

C-reactive protein, or CRP, and erythrocyte sedimentation rate, or ESR, are non-specific markers of inflammation.

They can rise with infections, inflammatory conditions, autoimmune disease, tissue injury and some cancers. Neither test identifies the cause by itself.

A normal result can be reassuring in some contexts, but it cannot exclude every inflammatory or serious condition. Conversely, a mildly raised result may be temporary and may not explain the fatigue.

Read our guides to CRP results and ESR results for more detail.

Coeliac disease antibodies

Coeliac disease is an autoimmune condition in which gluten triggers damage to the small intestine. It can cause diarrhoea, bloating, abdominal discomfort and weight loss, but some people mainly experience fatigue, anaemia, low ferritin or vitamin deficiencies.

The usual initial blood test includes tissue transglutaminase IgA, commonly written as tTG-IgA, together with total IgA. Other antibody tests may be used in specific situations.

You generally need to be eating gluten regularly before testing. Starting a gluten-free diet first can reduce antibody levels and make the result falsely reassuring.

Our guide to coeliac blood test results explains the common markers and what may happen after a positive or negative result.

Calcium and bone profile

Calcium abnormalities can cause fatigue, weakness, constipation, thirst, increased urination, confusion or muscle symptoms. A bone profile may include total calcium, adjusted calcium, albumin, phosphate and alkaline phosphatase.

Abnormal calcium should not usually be interpreted in isolation. Depending on the result, further testing might include parathyroid hormone, vitamin D, magnesium and kidney function.

Our guide to calcium and bone profile results covers these relationships in more depth.

Creatine kinase

Creatine kinase, or CK, is an enzyme found mainly in skeletal muscle. It may be checked when fatigue is accompanied by significant muscle pain, weakness or concern about muscle injury.

CK can rise temporarily after strenuous exercise, so an elevated result does not automatically indicate a muscle disease. The timing of the sample and recent activity matter.

When hormone, cortisol and infection tests may be appropriate

Testosterone and male hormones

Testosterone testing may be relevant for men with persistent fatigue alongside reduced libido, erectile difficulties, loss of morning erections, reduced muscle mass, infertility or other symptoms of testosterone deficiency.

Fatigue alone is not enough to diagnose low testosterone. Sleep deprivation, obesity, illness, calorie restriction, certain medicines and the timing of the sample can all affect the result.

Testosterone is generally measured in the morning. A low or borderline result usually needs to be repeated before a diagnosis is considered. Depending on the result, SHBG, calculated free testosterone, LH, FSH and prolactin may also be useful.

Our testosterone results guide explains why repeat morning testing and clinical symptoms matter.

Female hormones

Hormone testing may be appropriate when fatigue occurs alongside irregular periods, absent periods, fertility difficulties, hot flushes, symptoms of androgen excess or concern about early menopause.

Hormones such as FSH, LH, oestradiol and progesterone change during the menstrual cycle. A result can be difficult to interpret when the sample was taken at the wrong time or without knowing the cycle day.

In people aged over 45 with typical menopause symptoms, hormone blood tests are often not required to identify perimenopause or menopause. Testing may be more helpful in younger people, those with atypical symptoms or those whose menstrual pattern cannot be assessed.

Our guides to female hormone results and menopause blood tests explain when these measurements are and are not useful.

Cortisol

Cortisol testing is sometimes marketed as a general test for tiredness or “adrenal fatigue”. This requires caution.

Adrenal insufficiency is a recognised medical condition, but “adrenal fatigue” is not an established diagnosis. A random cortisol measurement taken at an arbitrary time may be difficult to interpret.

A clinician may arrange a morning cortisol test when symptoms and clinical findings suggest adrenal insufficiency. Features might include severe fatigue, weight loss, dizziness on standing, low blood pressure, nausea, abdominal symptoms, salt cravings or unusually darkened skin.

Abnormal or uncertain results may require specialist endocrine testing. A home cortisol test should not be treated as a standalone diagnosis.

Infection testing

Fatigue can persist after viral and other infections. However, broad screening for Epstein-Barr virus, cytomegalovirus, Lyme disease and multiple other infections is not routinely helpful without a compatible history or exposure.

Tests may show evidence of a past infection that has no connection to current symptoms. This can create confusion without providing an explanation.

Targeted infection testing is more appropriate when there are relevant symptoms, travel history, occupational risks, known exposures or examination findings.

How to choose the right fatigue blood test

The best starting point is not “Which panel contains the most markers?” but “Which explanations are reasonably possible in my situation?”

Consider the following questions:

• How long has the fatigue lasted?
• Did it begin suddenly or gradually?
• Is it constant or does it fluctuate?
• Does sleep improve it?
• Is there excessive daytime sleepiness?
• Are there changes in weight, appetite or bowel habits?
• Are periods heavy, irregular or absent?
• Is there breathlessness, dizziness or a racing heart?
• Are there muscle pains, joint symptoms or weakness?
• Has there been a recent infection?
• Is the diet restrictive?
• Are any medicines likely to cause fatigue?
• Is there significant stress, anxiety or low mood?
• Are there symptoms suggesting sleep apnoea?

For someone with heavy periods and breathlessness, an FBC and ferritin may be more immediately relevant than a large hormone panel. For someone with constipation, feeling cold and dry skin, thyroid testing may be a priority. For a person taking metformin with pins and needles, vitamin B12 deserves consideration.

A broader initial panel may be reasonable when fatigue has persisted and there is no clear direction. It might include FBC, ferritin, thyroid function, HbA1c, kidney function, liver function, B12, folate and an inflammation marker. Coeliac screening or other tests can then be added where appropriate.

Before ordering privately, check whether the panel:

• Includes the actual markers you need
• Uses an appropriate collection method
• Requires fasting or a morning sample
• Provides venous collection when the sample volume is large
• Includes interpretation by a qualified clinician
• Explains what to do with abnormal results
• Uses an accredited laboratory
• Has a clear policy for failed or insufficient samples

Our guides to finger-prick versus venous blood tests, the accuracy of home blood tests and NHS versus private blood testing can help you decide how and where to test.

Preparing for the test and interpreting the results

Preparation can affect the reliability of some blood tests. Read the instructions for your specific panel rather than assuming every test has the same requirements.

Depending on the markers being checked, you may be advised to:

• Fast for a specified period
• Provide a morning sample
• Avoid strenuous exercise beforehand
• Stay adequately hydrated
• Delay testing during an acute illness
• Record the day of your menstrual cycle
• Take the sample before a medication dose
• Discuss supplements that may interfere with laboratory tests

Do not stop prescribed medicines unless a healthcare professional has told you to do so.

Once the results arrive, avoid judging each number in isolation. Laboratory reference ranges describe values found in most of a reference population; they are not a perfect boundary between healthy and unhealthy.

A result just outside the range may be temporary, expected or clinically unimportant. A result within the range may still need attention when symptoms, trends or risk factors suggest a problem.

Interpretation should consider:

• How far the result is from the reference range
• Whether related markers show the same pattern
• Previous results and whether the value is changing
• Your age, sex, symptoms and medical history
• Medicines and supplements
• Recent exercise, dehydration or illness
• The collection method and sample quality

If a private test produces an abnormal result, do not simply keep buying larger panels in search of an answer. Some findings need confirmation, a physical examination, imaging, urine testing or another form of assessment.

Our guide to what to do after abnormal private blood test results explains when to contact a GP, when a repeat may be reasonable and which findings may require urgent attention.

What if all the blood tests are normal?

Normal blood tests do not mean the fatigue is imaginary, and they do not necessarily mean that nothing can be done.

Blood tests cannot directly diagnose or exclude many common causes of tiredness, including:

• Insomnia
• Sleep apnoea
• Stress and burnout
• Depression and anxiety
• Medication side effects
• Post-viral fatigue
• Long COVID
• Fibromyalgia
• ME/CFS
• Deconditioning
• Insufficient calorie intake

The NHS explains that there is no single blood test for ME/CFS. Blood and urine tests may instead be used to rule out conditions such as anaemia, thyroid disease, and liver or kidney problems.

ME/CFS should not be diagnosed solely because routine blood tests are normal. It involves a characteristic group of symptoms, including a substantial reduction in activity, unrefreshing sleep and post-exertional malaise, where symptoms worsen after physical or mental effort.

Equally, a normal panel should not bring the investigation to an end when there are concerning symptoms. Your GP may need to review your sleep, mental health, medicines, breathing, heart, nervous system, digestion or other areas that blood tests do not fully assess.

A symptom diary can be useful. Record your sleep, activity, meals, medicines, menstrual cycle, stress, associated symptoms and when fatigue is better or worse. Patterns that are difficult to remember during an appointment may become clearer on paper.

Frequently asked questions

What is the best blood test for tiredness?

There is no single best test. A common starting point may include a full blood count, ferritin, thyroid function, HbA1c, kidney function, liver function, vitamin B12 and folate. The right selection depends on your symptoms and risk factors.

Can you be iron deficient without being anaemic?

Yes. Ferritin can become low before haemoglobin falls below the laboratory range. This is sometimes described as iron deficiency without anaemia. It may be associated with fatigue, reduced exercise tolerance, restless legs or hair shedding, although these symptoms can have other causes.

Can vitamin B12 deficiency cause fatigue with a normal full blood count?

Yes. Vitamin B12 deficiency can cause neurological or cognitive symptoms without obvious anaemia or an abnormal haemoglobin result. B12 testing may be appropriate when there are relevant symptoms or risk factors.

Should everyone with fatigue have vitamin D tested?

No. Vitamin D testing is most useful when symptoms, medical conditions or risk factors make deficiency clinically important to identify. Testing every person with non-specific tiredness may not provide a useful explanation.

Can thyroid problems cause tiredness even if I have not gained weight?

Yes. Weight gain is only one possible feature of an underactive thyroid, and not everyone experiences it. Thyroid disease may also cause fatigue, feeling cold, constipation, dry skin, menstrual changes, muscle aches or changes in concentration.

Should I fast before a fatigue blood test?

It depends on the markers included. Many tests, including FBC, ferritin and thyroid function, do not routinely require fasting. Some glucose, lipid or specialised tests may have specific instructions. Follow the guidance supplied with your test.

Can exercise affect my blood test results?

Yes. Strenuous exercise can temporarily alter markers including creatine kinase, AST, ALT, inflammatory markers, creatinine and some hormone measurements. Avoid unusually intense exercise before testing when the instructions recommend it.

Can stress cause fatigue even when blood tests are normal?

Yes. Ongoing stress can disrupt sleep, concentration, appetite, mood and recovery. Stress-related fatigue is real, even when routine tests do not identify a physical abnormality. It is still important not to assume stress is the cause without considering other symptoms.

How long should tiredness last before I contact a GP?

Contact a GP when fatigue has lasted for several weeks, is not improving, affects normal activities or occurs with other unexplained symptoms. Seek advice sooner if the fatigue is severe or accompanied by weight loss, breathlessness, bleeding, fever, neurological symptoms or another concerning change.

Is a comprehensive private blood test better than targeted testing?

Not always. Comprehensive panels provide a broad overview and may be convenient when there is no clear direction. Targeted testing is often more useful when your symptoms point towards a specific possibility. Large panels can also produce borderline or incidental findings that require further interpretation.

What should I do if a private blood test is abnormal?

Read the clinical comments supplied with the result and check whether urgent action is recommended. Significant or unexplained abnormalities should usually be discussed with a GP or another appropriate clinician. Do not diagnose yourself or begin high-dose supplements solely from an isolated result.

Where can I order private blood tests for fatigue?

Private providers offer both targeted tests and broader health panels. Before ordering, compare the markers included, the collection method, laboratory standards and the support available after your results arrive. A range of relevant home and clinic-based blood tests can be viewed through Medichecks via All Health and Care. Use the discount code ALLHEALTH10 at checkout for an additional saving. This is an affiliate link, which means we may receive a commission if you make a purchase, at no additional cost to you.

Private testing can be useful when you want quicker access, a broader health overview or a specific marker that is not currently being offered through routine NHS care. It should complement appropriate medical advice rather than replace it, particularly when fatigue is persistent, severe or accompanied by other symptoms.