Blood Tests for Fatigue: Which Tests to Get and What They Actually Tell You

Fatigue is one of the most common reasons people visit their GP. It's also one of the hardest to pin down, because dozens of conditions can cause it and many of them overlap. The good news is that a well-chosen blood panel can rule in or rule out the most treatable causes quickly. This article explains what that panel should include, where the genuine gaps are, and how to interpret results that come back "normal."

What your GP will check

GPs investigating fatigue follow NICE Clinical Knowledge Summaries, which recommend a broad set of first-line bloods. The standard panel typically includes:

• Full blood count (FBC) to check haemoglobin, red cell indices, and white cell count

• Thyroid function (TSH), sometimes with free T4

• Liver function tests (LFTs), including ALT

• Renal function (U&Es), including eGFR and creatinine

• C-reactive protein (CRP) or another inflammatory marker

• HbA1c or fasting glucose for diabetes screening

• Coeliac serology (tTG antibodies) if clinically indicated

That's a solid starting point and it covers a lot of ground. A 2025 audit of nearly 17,000 primary care fatigue requests found that GPs were requesting FBC in 89% of cases, renal function in 83%, liver function and TSH in 80%, and CRP in 66% [Sherwood et al., 2025]. The system works well for the markers it includes.

But there are genuine gaps.

Where the gaps actually are

The same audit found that ferritin was requested in only 9.4% of fatigue investigations, and coeliac serology in just 3.2% [Sherwood et al., 2025]. Those are striking numbers, because ferritin is arguably the single most clinically useful marker in a fatigue workup, particularly for premenopausal women.

Ferritin measures your iron stores. Your haemoglobin can sit comfortably in the normal range while your ferritin is on the floor. A ferritin of 15 is technically "within range" at most labs, but many patients with levels below 30 report significant fatigue. The British Society for Haematology and recent evidence (Sholzberg et al., 2025) support a treatment threshold of 30 µg/L and a target above 50 µg/L. When ferritin isn't included on the request form, this gets missed.

Vitamin D isn't part of the standard NICE fatigue panel either, despite good evidence linking deficiency to tiredness, muscle weakness, and low mood. SACN data shows that a significant proportion of UK adults are deficient by the end of winter [SACN, 2016]. It's a cheap, simple test that often isn't requested unless the patient specifically asks.

Vitamin B12 and folate are recommended by NICE on a case-by-case basis rather than as routine first-line tests. B12 deficiency causes fatigue, brain fog, and neurological symptoms. It's more common in vegetarians, vegans, older adults, and people taking metformin or proton pump inhibitors. NICE guidance (NG239, 2024) puts the deficiency threshold at a total B12 below 133 pmol/L, with an indeterminate zone above that where clinical judgement matters.

Free T4 is the other gap. Many labs will run TSH alone as the first-line thyroid test, which is reasonable as a screening tool. But a normal TSH with a low-normal free T4 is a pattern that can cause symptoms, and you won't see it if free T4 isn't on the form.

The full panel worth checking

If you're persistently tired and want to cover the treatable causes properly, these are the markers a thorough investigation should include:

• Ferritin. Iron stores. The most commonly missed cause of fatigue in premenopausal women. Look for levels above 30, not just "within range."

• Full blood count (FBC). Haemoglobin, red cell indices, white cells. Rules out anaemia and flags blood disorders.

• TSH and free T4. Thyroid function. TSH alone is a good screening test, but adding free T4 shows how much active hormone your thyroid is producing.

• Vitamin D. Deficiency is extremely common in the UK, particularly between October and April. Causes fatigue, muscle pain, and low mood.

• Vitamin B12 and folate. B12 deficiency can present with fatigue and neurological symptoms even before the blood count looks abnormal.

• HbA1c. Average blood sugar over two to three months. Fatigue is an early symptom of pre-diabetes, which affects around 1 in 7 UK adults [NICE PH38, 2012].

• CRP. Inflammation marker. If raised, it suggests your body is dealing with something, whether infection, autoimmune activity, or chronic low-grade inflammation.

• Liver and kidney function (ALT, eGFR, creatinine). Organ dysfunction causes fatigue long before other symptoms appear.

• Coeliac serology (tTG). Coeliac disease is underdiagnosed and can present with fatigue and iron deficiency without obvious gut symptoms. Only reliable if you're still eating gluten regularly [NICE NG20, 2015].

Most of these overlap with what your GP would request anyway. The additions that make this a more complete fatigue panel are ferritin, vitamin D, B12 and folate, free T4, and coeliac serology.

When your results come back "normal"

Normal results are useful information. They mean the most common treatable causes have been ruled out, and that matters.

If all the markers above are genuinely normal, the next step is usually a conversation with your GP about sleep quality, stress, medication side effects, and lifestyle factors. Blood tests can't measure sleep debt or burnout.

But "normal" deserves a closer look sometimes. A ferritin of 28, a vitamin D of 32, and a TSH of 4.1 might all fall within the reference range individually. Together, they paint a picture of someone whose body is running on low reserves. One of the things I see regularly in practice is fatigue driven by two or three borderline results at the same time, none of them dramatic on their own but all pulling in the same direction.

That's where interpretation matters more than flagging. An automated system will mark each result as "normal" and move on. A GP reading those numbers together will see the pattern.

How Brooksby Medical can help

We offer several tests that cover the fatigue panel above, depending on how thorough you want to be:

• Iron Profile Blood Test (£45) — Ferritin, serum iron, TIBC, and transferrin saturation. The essential iron panel for fatigue investigation.

• Nutrients and Energy Profile (£99) — B12, folate, ferritin, vitamin D, and iron studies. Covers the nutritional causes of fatigue in one test.

• Advanced Thyroid and Sex Hormones Check (£129) — TSH, free T4, free T3, plus key hormones. For when thyroid function is suspected but a basic TSH was normal.

• The Wellness Profile (£129) — Our most complete panel. Covers all 8 core markers plus liver, kidney, cholesterol, diabetes risk, and inflammation. If you want one test that covers everything, this is the one.

• Vitamin D (£59) — A standalone vitamin D test if you suspect deficiency but don't need a full panel.

Every result includes a GP-written report explaining what your numbers mean together, not just whether each one falls inside a reference range.

When to see your GP

Blood tests are one part of the picture. If your fatigue is severe, getting worse, or accompanied by unexplained weight loss, night sweats, or new lumps, see your GP promptly. These symptoms need clinical assessment, not just blood work.

If you've already had bloods through your GP and everything came back normal but you're still struggling, that's worth a follow-up conversation too. Persistent fatigue lasting more than three months may need a different type of assessment, and your GP can guide that process.

Medically reviewed: April 2026 | Next review due: April 2027

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.

Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.

References

1. NICE Clinical Knowledge Summaries. Tiredness/fatigue in adults. NICE CKS, 2021

2. Sherwood L, et al. 'Tired all the time': What general practitioners request and find in patients with tiredness/fatigue. Ann Clin Biochem. 2025

3. Sholzberg M, et al. Iron deficiency and iron deficiency anaemia. 2025

4. Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. SACN, 2016

5. NICE. Vitamin B12 deficiency in over 16s: diagnosis and management. Guideline NG239, 2024

6. NICE. Coeliac disease: recognition, assessment and management. Guideline NG20, 2015

7. NICE. Type 2 diabetes: prevention in people at high risk. Public health guideline PH38, 2012

8. Vaucher P, et al. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin. CMAJ. 2012;184(11):1247-1254

9. Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021;70(11):2030-2051