Tired all the time? The blood tests that explain fatigue
Reviewed by the My London Pharmacy clinical team, Yvonne Lim, Pharmacist Independent Prescriber
·17 August 2026
·6 min read
Quick answer
Persistent fatigue has many possible physical causes, and a blood panel is often the fastest way to rule them in or out. The tests most commonly used include full blood count (anaemia), ferritin (iron stores), thyroid function, vitamin B12 and folate, vitamin D, and blood glucose or HbA1c. A normal panel does not mean nothing is wrong, it means the common physical causes are unlikely, which is still useful information.
Why fatigue is worth testing rather than guessing
Everyday tiredness usually has an obvious cause, a bad week, poor sleep, a stressful stretch, and resolves once that eases. Fatigue is different: a persistent, unrefreshing tiredness that does not lift with rest and starts affecting daily life. It is one of the most common reasons people see a clinician, and one of the least specific, the same feeling can come from a dozen different physical causes, several lifestyle factors, or both at once.
That is exactly why guessing is inefficient. Rather than trying supplements or changes one at a time, a blood panel checks the handful of common physical causes in one go, so you and your clinician know what you are actually dealing with before deciding what to do about it.
Full blood count: checking for anaemia
A full blood count (FBC) is usually the starting point. It measures your red blood cells, white blood cells and platelets, and is how anaemia, a shortage of healthy red blood cells to carry oxygen around the body, is identified. Anaemia is one of the most common physical causes of fatigue, particularly in women of reproductive age, and it produces a very recognisable pattern: tiredness, breathlessness on exertion, looking pale, and sometimes a fast heartbeat.
An FBC on its own tells you that anaemia is present, not always why, which is where the next test usually comes in.
Iron and ferritin: your iron stores
Ferritin reflects how much iron your body has in reserve, and low iron is the most common cause of anaemia worldwide, from diet, from heavy periods, or from ongoing blood loss elsewhere that is worth investigating if it does not have an obvious explanation. Ferritin can also sit low-normal without full-blown anaemia and still leave someone feeling drained, which is why it is often tested even when the full blood count looks otherwise unremarkable.
One practical note: ferritin rises with inflammation, so a normal or high reading during an infection or flare-up of an inflammatory condition does not always rule out low iron underneath it. Your clinician will interpret it alongside how you are feeling generally.
Thyroid function, vitamin B12 and folate
An underactive thyroid is a classic, easily missed cause of persistent fatigue, alongside weight gain, feeling cold and low mood, see our guide on how often to check your thyroid for the fuller picture. It is checked with a TSH blood test, sometimes alongside free T4.
Vitamin B12 and folate are both essential for making healthy red blood cells and for nerve function, and a deficiency in either produces fatigue that can be significant and, if left long enough, contributes to nerve symptoms such as tingling or numbness. B12 deficiency is more common than many people realise, particularly with a plant-based diet, after weight-loss surgery, or with certain digestive conditions that affect absorption.
Vitamin D and blood sugar
Vitamin D deficiency is extremely common in the UK, especially over autumn and winter, and low levels are linked with fatigue, low mood and aching muscles and bones, our guide to what your vitamin D result actually means explains how to read the number once you have it.
Blood glucose, checked either as a fasting glucose or HbA1c (your average blood sugar over roughly three months), picks up both undiagnosed diabetes and the earlier stage, prediabetes, both of which commonly cause fatigue, particularly after meals, well before any other symptoms appear.
What a normal result still tells you
A common frustration: the panel comes back entirely normal, and the fatigue has not gone away. That is not a wasted test. Ruling out anaemia, thyroid problems, vitamin deficiencies and blood sugar issues genuinely narrows things down, it points towards causes that are not visible on a standard blood panel, such as sleep quality, mental health, stress, medication side effects, or another underlying condition that needs a different kind of investigation.
A normal panel is a useful, evidence-based reason to broaden the conversation with a clinician rather than keep re-testing the same markers, or to consider a wider panel if your history suggests something more specific, hormone testing, for instance, if your cycle or libido has also changed.
The lifestyle factors blood tests don't capture
It is worth being upfront that a blood panel only checks physical markers, and several very common causes of fatigue sit outside what any test measures. Poor or insufficient sleep is one of the most under-recognised, not just short sleep, but disrupted or unrefreshing sleep, including undiagnosed conditions like sleep apnoea, which can leave someone exhausted despite spending eight hours in bed.
Chronic stress, low mood and anxiety are also major, genuinely physical contributors to fatigue, the tiredness they cause is not "just in your head", even though no blood test will show it. The same goes for excessive caffeine or alcohol use, an irregular schedule such as shift work, and simply not getting enough physical activity, which paradoxically tends to increase energy levels rather than deplete them. A clinician weighing up fatigue will usually ask about all of this alongside ordering blood tests, because the two pictures, biochemical and lifestyle, often need to be read together rather than treated as alternatives.
Getting tested
If fatigue has lasted more than a few weeks, is getting worse, or comes with other symptoms, unexplained weight change, heavy periods, breathlessness, or persistent low mood, it is worth having it checked rather than pushing through. Your GP can arrange NHS testing, and private testing is a convenient option when you want a fuller panel in one visit or a faster turnaround.
Our clinical team offers blood testing with results reviewed by a clinician, so you get a proper explanation of what the numbers mean for you, not just a printout. You can browse the full range in our private blood tests guide, or start with a conversation about which panel makes sense for your symptoms.
Frequently asked questions
What is the first blood test for unexplained tiredness?
A full blood count is usually the starting point, often alongside ferritin, thyroid function, vitamin B12, folate and vitamin D, together these cover the most common physical causes of fatigue.
Can low vitamin D really cause tiredness?
Yes. Vitamin D deficiency is common in the UK and is linked with fatigue, low mood and aching muscles, particularly in autumn and winter with less sunlight exposure.
What does it mean if my blood tests are all normal but I'm still exhausted?
It means the common physical causes checked by that panel are unlikely, which is useful information, it points towards other factors like sleep, stress, mental health or medication, worth discussing with a clinician.
Can blood sugar problems cause fatigue before other symptoms appear?
Yes. Fatigue, especially after meals, can be an early sign of prediabetes or diabetes, sometimes appearing before thirst, frequent urination or other classic symptoms.
How is B12 deficiency linked to fatigue?
B12 is essential for healthy red blood cells and nerve function, so low levels commonly cause fatigue and, if prolonged, can contribute to tingling or numbness.