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Chronic tiredness is one of the top ten reasons UK adults visit their GP. Most of the time, no single scary cause is found, but there are a handful of common, treatable causes that show up on blood tests. If you’ve been feeling tired all the time for weeks or months, this guide covers which blood tests are worth asking for, what each one measures, and what the results usually mean.
This article isn’t a substitute for seeing your GP. If your fatigue is severe, sudden, or accompanied by other symptoms (weight loss, night sweats, breathlessness, chest pain), book a same-day appointment.
Here’s a quick-reference summary before the detail below:
| Marker | What it can point to | Test to ask for |
|---|---|---|
| Ferritin / iron studies | Iron deficiency, the most common cause in menstruating women | Iron Status Blood Test |
| B12 and folate | Fatigue, brain fog, tingling in hands/feet | Vitamin D, B12 & Folate Levels |
| TSH, Free T4, Free T3 | Underactive thyroid (hypothyroidism) | Thyroid Function Panel |
| Vitamin D | Tiredness, low mood, muscle aches (common in winter) | Vitamin D Blood Test |
| HbA1c and glucose | Pre-diabetes or type 2 diabetes | Diabetes Health Checks |
| Full Blood Count | Anaemia, infection/inflammation, blood disorders | Full Blood Count |
| Kidney and liver function | Chronic kidney or liver disease | Kidney Function (U&E) and Liver Function |
| Coeliac antibodies | Undiagnosed coeliac disease alongside iron deficiency | Coeliac screen (add-on) |
| Cortisol | Adrenal over- or under-activity (rare, but real) | Stress Hormone Blood Test or Stress & Adrenal Blood Test |
| Sex hormones | Low testosterone in men; perimenopause/menopause in women | Male Hormone Blood Test or Menopause Blood Test |
Iron deficiency is the single most common blood-test-findable cause of fatigue in UK women of menstruating age. It can also affect men and post-menopausal women, usually pointing to a different cause (blood loss from the gut, for instance).
The most useful marker is ferritin, the iron storage protein. Low ferritin is the earliest sign of iron running low, and it appears well before the full blood count shows anaemia.
Symptoms that often accompany low iron: exhaustion despite sleep, breathlessness on stairs, cold hands and feet, brittle nails, hair thinning, restless legs at night, unusual cravings (ice, dirt, ice cream).
What to ask for: an Iron Status Blood Test covering ferritin, iron, transferrin saturation, and TIBC. Or the Advanced Iron Blood Test if you want a fuller picture.
Low B12 causes fatigue, brain fog, tingling in the hands and feet, and (in severe cases) memory problems. It’s more common in vegetarians, vegans, older adults, people taking metformin, and people on long-term proton pump inhibitors (like omeprazole).
Folate (vitamin B9) has a similar role. Low folate can cause tiredness and can worsen the effects of low B12.
What to ask for: Vitamin D, B12 & Folate Levels as a bundled panel, or the individual B12 and Folate tests.
An underactive thyroid (hypothyroidism) is a classic cause of tiredness in the UK, particularly in women in their 40s and 50s. Symptoms often creep in over months.
Symptoms that often accompany hypothyroidism: tiredness that doesn’t lift with rest, weight gain despite no diet change, feeling cold, dry skin, constipation, low mood, heavy periods.
What to ask for: at minimum TSH. Ideally a full Thyroid Function Panel with TSH, Free T4, and Free T3.
Low vitamin D is very common in the UK, especially between October and March. It’s linked to tiredness, low mood, muscle aches, and bone pain. Public Health England advises the general UK population to consider a 10 μg (400 IU) supplement in winter.
What to ask for: Vitamin D Blood Test.
Fatigue is a common early symptom of raised blood sugar. HbA1c reflects average sugar over the previous 8 to 12 weeks and is the standard UK diagnostic test for type 2 diabetes and pre-diabetes.
Symptoms: fatigue, increased thirst, more frequent urination, blurred vision, slow-healing cuts, tingling in the feet.
What to ask for: Diabetes Health Checks, which includes glucose and HbA1c.
An FBC won’t explain every fatigue case but it will rule out or reveal several important ones: anaemia (low haemoglobin), infection or inflammation (raised white cells), some blood disorders (unusual platelet counts).
What to ask for: Full Blood Count.
Chronic kidney or liver disease can cause fatigue. These tests are quick to run and rule out or catch problems early.
What to ask for: Kidney Function (U&E) and Liver Function.
Undiagnosed coeliac disease is a surprisingly common cause of tiredness, iron deficiency, and vague gut symptoms. If iron studies show unexplained deficiency and there’s any GI symptom, the coeliac antibody test is worth adding.
Adrenal issues are much rarer than internet health forums suggest, but very high or very low cortisol does cause fatigue. A morning cortisol (measured before 10 AM) is the standard first-line test.
What to ask for: Stress Hormone Blood Test or the Stress & Adrenal Blood Test.
Rather than booking each test separately, most people are best off with a bundled panel. Swift Blood Tests offers a Tired All The Time Blood Test covering the main markers above in a single appointment for £160.
If you’d rather build your own, a reasonable starter set is:
| Test | What it covers |
|---|---|
| Full Blood Count | Anaemia, infection, blood disorders |
| Ferritin (or full iron studies) | Iron deficiency |
| Vitamin D | Deficiency, especially in winter |
| B12 and folate | Nervous system and red cell health |
| TSH, Free T4, Free T3 | Thyroid function |
| HbA1c | Diabetes and pre-diabetes |
| Liver function | Liver health |
| Kidney function | Kidney health |
This maps to roughly the same panel the NHS runs when your GP orders “TATT bloods” (TATT = Tired All The Time).
Around 40 to 50% of patients presenting with fatigue in UK primary care have normal blood tests. That doesn’t mean the fatigue isn’t real. The most common non-biochemical causes are:
If your blood tests come back clear and you’re still exhausted, the next step is usually a fuller conversation with your GP about sleep, mood, and lifestyle rather than more blood tests. The NHS guidance on tiredness and fatigue is a useful starting point.
Book a same-day GP appointment (or go to A&E) if your fatigue comes with any of:
Iron deficiency (low ferritin), low vitamin D, underactive thyroid, low B12, and pre-diabetes are the five most common blood-test-findable causes. Iron deficiency is the most common overall, especially in menstruating women.
Yes. Most UK private blood testing providers offer a bundled fatigue panel. Swift Blood Tests offers one for £160, covering thyroid, iron, vitamins, kidney, liver, blood count and HbA1c.
Standard panels come back in 24 to 48 hours from most UK private providers, including Swift Blood Tests. Some specialist add-ons like coeliac antibodies or cortisol take longer.
Yes, about 40 to 50% of fatigue cases in UK primary care have normal blood tests. Sleep, stress, mood and lifestyle are the most common non-biochemical causes.
Some markers do. HbA1c does not need fasting, but glucose does, and some providers prefer fasted iron studies. Book an early morning slot after an overnight fast and you’ll cover most cases.
Yes, most UK labs recommend stopping iron supplements for at least 24 hours before an iron blood test, ideally longer (some say 5 to 7 days). This gives a truer picture of your baseline iron status.
Low testosterone can cause fatigue, low libido, low mood, and poor recovery. A morning testosterone test is the standard first-line investigation. Not every case of tiredness in men is testosterone-related, most isn’t.
No. There is no single blood test for chronic fatigue syndrome (also called ME/CFS). Blood tests are used to rule out other causes. The NHS has detailed guidance on CFS/ME diagnosis that involves symptom criteria over months.
Swift Blood Tests offers a Tired All The Time Blood Test covering all the markers above for £160, with home visits across Essex, Bromley and Surrey. Results in 24 to 48 hours from a UKAS-accredited laboratory. Book online or call 01277 652067 to talk through which test is right for you.
