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) and serum ferritin - to check haemoglobin, red cell indices, and white cell count and look for evidence of iron deficiency
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
Creatine kinase - to assess for a neuromuscular cause
Bone chemistry calcium and phosphate — to assess for metabolic bone disease.
It also NICE CKS also recommend that clincians to juse clinical judgement when ordering:
Vitamin D level — if the person is at risk of deficiency.
Vitamin B12 and folate levels — if there is macrocytic anaemia
Viral serology and other tests — if there is a history of infection, the person is at risk, and/or there is a possibility of other latent infection
9am cortisol level — to exclude adrenal insufficiency.
N-terminal pro-B-type natriuretic peptide level (NT-pro-BNP) — to assess the likelihood of heart failure.
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.
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 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.
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
NICE Clinical Knowledge Summaries. Tiredness/fatigue in adults. NICE CKS, 2021
Sherwood L, et al. 'Tired all the time': What general practitioners request and find in patients with tiredness/fatigue. Ann Clin Biochem. 2025
Sholzberg M, et al. Iron deficiency and iron deficiency anaemia. 2025
Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. SACN, 2016
NICE. Vitamin B12 deficiency in over 16s: diagnosis and management. Guideline NG239, 2024
NICE. Coeliac disease: recognition, assessment and management. Guideline NG20, 2015
NICE. Type 2 diabetes: prevention in people at high risk. Public health guideline PH38, 2012
Vaucher P, et al. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin. CMAJ. 2012;184(11):1247-1254
Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021;70(11):2030-2051
Medical disclaimer. This article is for informational purposes 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, history, and clinical picture. If you have concerns about your health, please consult your GP.