You’ve been tired for weeks. You’ve tried going to bed earlier, cutting back on coffee and drinking more water, and you still wake up feeling flat. At some point the obvious question is: should I just get a blood test? Often the answer is yes. But it helps to know which tests a doctor normally orders, what each one is looking for, and what a “normal” result does and doesn’t tell you.
This article is part of our complete guide on how to increase your energy levels naturally. It covers when tiredness is worth checking, the standard first set of blood tests GPs use, the extra tests that are only useful in certain situations, and how to get the most out of the appointment. It is general information, not medical advice. Your own doctor is the right person to decide which tests you need.
When Should You See a Doctor About Tiredness?
Everyone has tired spells. Bad sleep, a stressful month or a cold can all leave you drained for a while, and that usually settles on its own. The NHS advises seeing a GP if you’ve felt tired for a few weeks without an obvious reason, if tiredness is affecting your daily life, or if it comes with other symptoms such as unexplained weight loss or changes in mood.
Waiting a few weeks is deliberate. A 2010 BMJ article on investigating fatigue in primary care (Hamilton, Watson and Round) pointed out that blood tests rarely find a hidden disease in people who are simply tired. The authors suggested testing for people who haven’t recovered after about a month, or sooner if the picture is unusual or there are warning signs.
Don’t wait if you have warning signs
See a doctor promptly, rather than waiting it out, if your tiredness comes with any of these:
- weight loss you can’t explain,
- night sweats or fevers,
- new lumps or swollen glands,
- breathlessness, chest pain or palpitations,
- bleeding that’s unusual for you (including very heavy periods or blood in your stool), or
- low mood, hopelessness or thoughts of harming yourself.
These don’t mean something serious is going on, but they change how quickly a doctor will want to look into it.
The Standard First Set of Blood Tests
In the UK, GPs follow NICE’s Clinical Knowledge Summary on tiredness and fatigue in adults. A 2025 audit of that guidance in the Annals of Clinical Biochemistry (Handjiev, Nobes and Murphy) set out the first-line tests it lists. Other countries’ guidance varies a little, but the core set is very similar. Here is what each test is checking for.
1. Full blood count (FBC)
The FBC measures your red cells, white cells and platelets. For tiredness, the main reason to do it is to look for anaemia (low haemoglobin), which reduces how much oxygen your blood carries. The size of your red cells also gives a clue to the cause: small cells suggest iron deficiency, and large cells suggest a lack of vitamin B12 or folate. Unusual white cell or platelet counts can point to infection, inflammation or, rarely, a blood disorder.
2. Ferritin (iron stores)
Ferritin reflects how much iron your body has in storage. It matters because your iron stores can run low long before your haemoglobin drops, so a normal FBC doesn’t rule out an iron problem. We cover this in detail in our article on low ferritin without anaemia, including the newer, higher cut-offs some guidelines now recommend. If your result is “within range” but close to the bottom, it’s reasonable to ask what level the lab uses as its cut-off.
3. Thyroid function (TSH, and sometimes T4)
An underactive thyroid is a classic cause of tiredness, along with feeling cold, weight gain, constipation, dry skin and low mood. The NHS notes that a blood test is the only accurate way to know whether your thyroid is the problem. The test measures thyroid-stimulating hormone (TSH), often alongside T4. A raised TSH with a low T4 points to an underactive thyroid. A raised TSH with a normal T4 may just mean a higher future risk, and your GP may suggest repeating the test later.
4. HbA1c (blood sugar)
HbA1c shows your average blood sugar over the past two to three months. It’s used to look for diabetes, which can cause tiredness along with thirst and passing more urine. In the UK, an HbA1c of 48 mmol/mol (6.5%) or above is in the diabetes range. In people without symptoms, a second test is normally done to confirm it. A result of 42–47 mmol/mol (6.0–6.4%) is a higher-risk range, often called prediabetes. The 2025 audit gives a sense of how useful this test can be: HbA1c was only requested in 2.8% of the tiredness test requests it reviewed, but 59% of those results came back in the diabetic range.
5. Kidney and liver function
Kidney function (U&Es, including creatinine and eGFR) and liver function tests (LFTs) check how these organs are working. Kidney disease, liver problems and salt imbalances can all cause fatigue, often with few other symptoms early on.
6. Inflammation markers (CRP or ESR)
C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR) go up when there’s inflammation or infection somewhere in the body. They don’t say where or why. But a raised result tells your doctor to look further, for example for an inflammatory or autoimmune condition.
7. Coeliac screen (tTG antibodies)
Coeliac disease is an autoimmune reaction to gluten. It can cause tiredness and iron deficiency, sometimes with few gut symptoms. The screening test is a blood test for tissue transglutaminase (tTG) antibodies. One practical point: keep eating gluten before the test. Coeliac UK recommends eating gluten in more than one meal every day for at least six weeks beforehand. Cutting it out first can make the result falsely negative.
Urine test
Many GPs also do a quick urine dipstick test for protein, blood and glucose. It’s an inexpensive extra check on the kidneys and blood sugar.
Tests That Are Only Useful in Certain Situations
Some tests get a lot of attention online but aren’t part of the routine first set. Your doctor may add them if your symptoms, diet or other results point that way.
Vitamin B12 and folate
The NHS lists tiredness and weakness among the main symptoms of B12 or folate deficiency anaemia. B12 deficiency can also cause pins and needles, numbness, balance problems or memory changes. These tests are worth asking about if your FBC shows large red cells, if you eat a vegan or mostly plant-based diet, if you’ve had stomach or bowel surgery, or if you have those nerve symptoms. The NHS also notes that some people have symptoms with apparently normal levels, and some have low levels with no symptoms. So the result needs to be read alongside how you feel, not on its own.
Vitamin D
This one surprises people. Vitamin D testing is not routinely recommended for tiredness. NHS guidance in Scotland states plainly that vitamin D “is not a test that is helpful in investigation of tiredness, chronic fatigue / fibromyalgia or non-specific aches and pains” when bone blood tests are normal. Testing is mainly for people with suspected bone problems (such as osteomalacia), low calcium or conditions that affect absorption. UK guidance already advises many adults to consider a daily vitamin D supplement in autumn and winter, which you can do without a test. (If you’re curious about the link with hair, see our article on vitamin D and hair fall.)
Others your doctor might consider
Depending on your symptoms, other possible tests include creatine kinase (for muscle problems), a bone profile (calcium), a glandular fever test after a recent sore throat, or a morning cortisol test. These are judgement calls, not tests to request “just in case.”
What If All Your Results Are Normal?
This is the most common outcome, and it’s useful information, not a dead end. In the VAMPIRE trial (Koch and colleagues, British Journal of General Practice, 2009), 325 patients with new, unexplained fatigue were followed across 91 Dutch general practices. Only about 8% had a physical illness that blood tests could detect. The same study found that ordering a larger set of tests mainly added false-positive results, which is one reason doctors start with a focused set rather than “test everything.”
Normal results mean the most common medical causes are much less likely. That points the search towards the things blood tests can’t measure:
- Sleep quality, not just hours. Sleep apnoea is common and usually undiagnosed, and it doesn’t show up in blood tests. See our guide to sleep apnoea signs, and our article on feeling tired even after 8 hours of sleep.
- Mood and stress. Depression and anxiety are among the most common causes of persistent tiredness, and they’re very treatable.
- Medicines. Antihistamines, some blood pressure tablets, antidepressants and other drugs can cause fatigue.
- Daily habits. Activity levels, alcohol, caffeine timing and how you eat all affect energy. Our pillar guide covers the evidence on each.
If you’re still exhausted months later despite normal tests, go back to your GP. Long-lasting fatigue that gets worse after exertion may need assessment for conditions such as ME/CFS, which is diagnosed clinically rather than with a single blood test.
How to Get the Most Out of Your Appointment
- Keep a two-week note of your sleep times, energy levels, periods (if relevant), and any other symptoms. Concrete details help a lot more than “I’m always tired.”
- Bring a list of everything you take, including supplements. High-dose biotin, for example, can interfere with some lab tests, including thyroid tests.
- Mention your diet if you’re vegan or vegetarian, or if you’ve cut out food groups such as gluten.
- Ask for the actual numbers, not just “normal,” so you can compare them over time.
- Ask about preparation. Some tests need fasting or a morning sample. Our article on what to know before a blood test covers the basics.
Frequently Asked Questions
What blood tests check for tiredness?
The usual first set is a full blood count, ferritin (iron stores), thyroid function (TSH), HbA1c (blood sugar), kidney and liver function, an inflammation marker (CRP or ESR) and a coeliac screen. B12, folate and other tests are added depending on your symptoms and results.
Should I get my vitamin D tested if I’m tired?
Usually not. NHS guidance says vitamin D testing isn’t helpful for investigating tiredness on its own. It’s mainly used when bone problems or absorption issues are suspected. Many adults are advised to take a vitamin D supplement in autumn and winter anyway, without testing.
Can a normal full blood count rule out iron deficiency?
No. Your iron stores (ferritin) can be low while your haemoglobin is still normal. If iron is a possible cause, ferritin needs to be measured separately.
How long should I be tired before getting a blood test?
The NHS suggests seeing a GP if you’ve been tired for a few weeks without an obvious reason, or sooner if it’s affecting your daily life or comes with symptoms such as weight loss, night sweats or changes in mood.
What does it mean if all my blood tests are normal but I’m still tired?
It’s the most common result. It makes the common medical causes less likely and points towards sleep quality, mood, stress, medicines and daily habits. If tiredness persists for months, go back to your doctor for a further review.
This article is for general information and is not medical advice. Which tests are right for you depends on your symptoms, history and examination. Speak to a qualified healthcare professional about persistent or unexplained tiredness, and seek urgent help for any of the warning signs listed above.
Sources
- NHS. Tiredness and fatigue (nhs.uk).
- NICE Clinical Knowledge Summaries. Tiredness/fatigue in adults (cks.nice.org.uk).
- Handjiev S, Nobes J, Murphy MJ. “Tired all the time”: what general practitioners request and find in patients with tiredness/fatigue – an audit against NICE Clinical Knowledge Summary of tiredness/fatigue in adults. Annals of Clinical Biochemistry, 2025;62(5). doi:10.1177/00045632251329175.
- Hamilton W, Watson J, Round A. Investigating fatigue in primary care. BMJ, 2010;341:c4259.
- Koch H, van Bokhoven MA, ter Riet G, et al. Ordering blood tests for patients with unexplained fatigue in general practice: what does it yield? Results of the VAMPIRE trial. British Journal of General Practice, 2009;59(561):e93–e100.
- NHS. Underactive thyroid (hypothyroidism): diagnosis (nhs.uk).
- NHS. Vitamin B12 or folate deficiency anaemia: symptoms and diagnosis (nhs.uk).
- Coeliac UK. Getting diagnosed (coeliac.org.uk).
- NHS Scotland Right Decisions. Vitamin D: prevention and treatment of deficiency in adults.
