What Blood Tests Should You Get for Fatigue? A Complete Guide

If you've been persistently tired for more than four weeks, blood tests are the right starting point — but which ones you get matters enormously. A standard GP panel will catch severe anaemia and overt hypothyroidism. It will miss low-normal ferritin, vitamin D insufficiency, and borderline thyroid function: all common, all treatable causes of fatigue that regularly come back "normal."

The NHS recommends seeing a GP for persistent fatigue lasting more than four weeks, specifically to check for anaemia, diabetes, and thyroid problems. This guide covers those tests and the ones that often get missed.

The Most Important Test Most People Don't Get: Ferritin

The single most frequently missed cause of fatigue on blood tests is low ferritin — stored iron — in the absence of anaemia.

Standard blood tests check haemoglobin (the protein in red blood cells that carries oxygen). Haemoglobin can stay within normal range until iron stores are severely depleted. Ferritin, which reflects iron stores, falls to symptomatic levels weeks or months before haemoglobin drops — meaning you can have iron deficiency causing real, measurable fatigue while your haemoglobin is perfectly normal.

What levels mean:

| Ferritin (µg/L) | What it indicates | |---|---| | Below 15 | Confirmed iron deficiency | | 15–30 | Iron deficiency likely, symptoms expected | | 30–50 | Low-normal; symptoms possible in susceptible people | | 50–75 | Borderline; fatigue and restless legs associated at this level | | Above 100 | Adequate stores |

Many NHS labs report ferritin as "normal" at 20–30 µg/L because it falls within a broad reference range. But the research on fatigue symptoms consistently shows that levels below 50–75 µg/L can cause fatigue, poor concentration, and hair loss — even without anaemia.

What to ask for: Request "ferritin" specifically. It is not automatically included in a full blood count. If your GP only orders FBC (full blood count), you will not get ferritin.

Who is at risk: Women with heavy periods, vegetarians and vegans, people who train heavily, regular blood donors, anyone with a gut condition affecting absorption (coeliac disease, IBD, post-bariatric surgery). But low ferritin affects a wide range of people — if you haven't had it checked, it's worth requesting regardless.

Full Blood Count: What It Actually Shows

A full blood count (FBC) measures the components of your blood and is typically the starting point for any fatigue investigation.

What it tests:

• Haemoglobin — low haemoglobin indicates anaemia, which reduces oxygen delivery to tissues
• Mean corpuscular volume (MCV) — the size of red blood cells. Low MCV (microcytic) suggests iron deficiency. High MCV (macrocytic) suggests B12 or folate deficiency
• White blood cell count — elevated counts suggest infection or inflammation
• Platelet count — relevant if there are concerns about bleeding or clotting

A normal FBC does not rule out iron deficiency — it rules out anaemia caused by iron deficiency. These are different things. An FBC showing normal haemoglobin but low MCV is a prompt to check ferritin.

Thyroid Function: TSH and Free T4

The thyroid gland regulates metabolic rate across virtually every tissue. An underactive thyroid (hypothyroidism) produces a distinctive pattern of fatigue: persistent, present regardless of sleep, often accompanied by feeling cold, weight gain, dry skin, slower thinking, and constipation.

The primary test is TSH (thyroid stimulating hormone). High TSH means the pituitary is working harder to drive an underperforming thyroid — it rises before thyroid hormone levels fall. TSH is the most sensitive screening test.

Free T4 measures the available form of thyroxine (the main thyroid hormone). Free T4 falling while TSH rises confirms overt hypothyroidism.

Interpreting results:

| TSH | Free T4 | Interpretation | |---|---|---| | 0.4–4.0 mIU/L | Normal | Euthyroid (normal thyroid function) | | Above 4.0 | Normal | Subclinical hypothyroidism | | Above 4.0 | Low | Overt hypothyroidism | | Below 0.4 | Elevated | Hyperthyroidism |

The TSH "normal range" (typically 0.4–4.0 mIU/L) is wide. Some research suggests fatigue is more common at the higher end of normal (TSH 2.5–4.0), and many people feel better when treated to a TSH below 2.0. If your TSH comes back at 3.5 and your symptoms fit hypothyroidism, asking for a free T4 alongside it is reasonable.

Subclinical hypothyroidism (raised TSH with normal T4) is common — affecting around 5% of UK adults. It may or may not cause symptoms; treatment decisions depend on TSH level, symptom severity, and whether thyroid antibodies are positive.

Hashimoto's thyroiditis is the most common cause of hypothyroidism in the UK. Thyroid peroxidase antibodies (TPO Ab) confirm it. Testing isn't always done initially but is useful if TSH is borderline or repeatedly fluctuating.

Vitamin D: A Commonly Low, Easily Corrected Cause

Vitamin D deficiency is one of the most prevalent and most treatable causes of fatigue in the UK. Studies consistently show 20–30% of UK adults are deficient at any given time, rising above 40% in winter. The UK's latitude means meaningful vitamin D synthesis from sunlight is only possible from April to September, roughly between 11am and 3pm.

The test: 25-hydroxyvitamin D (written as 25(OH)D). This is the standard measure of vitamin D status.

UK units are nmol/L (not ng/mL used in some US guidelines):

| 25(OH)D nmol/L | Classification | |---|---| | Below 25 | Deficient — bone health affected, significant fatigue likely | | 25–50 | Insufficient — functional impairment begins | | 50–75 | Adequate per NHS guidance | | 75–150 | Optimal range in most research |

If your result comes back at 52 nmol/L and is reported as "normal," you are in the technically adequate-but-not-optimal range. Research on fatigue, muscle function, and immune regulation generally points to better outcomes at 75–125 nmol/L. A cautious trial of supplementation (1,000–2,000 IU D3 daily) is low-risk at this level.

Vitamin D causes fatigue through several mechanisms: impaired mitochondrial function, reduced muscle calcium transport (producing proximal muscle weakness and heavy legs), reduced serotonin synthesis, and increased inflammatory cytokine activity. See our full guide to vitamin D and fatigue for detail on mechanisms and dosing.

Blood Glucose: HbA1c and Fasting Glucose

Blood sugar dysregulation — including pre-diabetes and undiagnosed type 2 diabetes — causes fatigue through impaired glucose metabolism: cells either receive insufficient glucose (in insulin resistance, when glucose can't enter cells efficiently) or deal with the downstream inflammation of chronic hyperglycaemia.

HbA1c measures average blood glucose over the past two to three months by checking how much glucose has attached to haemoglobin. It is the standard screening test for diabetes and pre-diabetes, doesn't require fasting, and reflects long-term glucose control rather than a single snapshot.

Fasting glucose measures blood sugar after 8–12 hours without food. It's more sensitive for detecting early glucose abnormalities and is used alongside HbA1c for diagnosis.

Reference ranges:

• HbA1c below 42 mmol/mol (6%): normal
• HbA1c 42–47 mmol/mol (6–6.4%): pre-diabetes
• HbA1c 48+ mmol/mol (6.5%+): diabetes

Pre-diabetes is worth knowing about — it causes fatigue and is reversible with dietary changes and exercise at this stage.

Vitamin B12 and Folate

B12 and folate are both required for red blood cell formation and neurological function. Deficiency in either causes macrocytic anaemia (enlarged, fewer, less functional red blood cells) — visible as elevated MCV on a full blood count.

B12 deficiency causes fatigue with a neurological quality: tingling in hands and feet, poor memory, balance issues, and a distinctive cognitive "fog." It's particularly common in vegans and vegetarians (B12 comes almost exclusively from animal products), people over 60 (stomach acid required for absorption decreases with age), and people with pernicious anaemia or gut malabsorption.

Folate deficiency produces similar blood changes but without the neurological symptoms. It's associated with poor diet, alcohol excess, pregnancy, and some medications (including methotrexate).

Important nuance: "Normal" B12 on a standard lab report can still be functionally low. The lower end of the reference range (~180 pmol/L) is technically normal but associated with neurological symptoms in some people, particularly older adults. If symptoms fit and B12 is at the lower end of normal, methylmalonic acid or homocysteine testing can confirm functional deficiency.

Inflammatory Markers: CRP and ESR

C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are both markers of systemic inflammation. Fatigue is a core feature of inflammatory states — the same cytokine-mediated "sickness behaviour" that makes you want to rest during illness can persist as chronic low-grade inflammation.

Elevated CRP or ESR without an obvious cause prompts investigation for autoimmune conditions (rheumatoid arthritis, lupus, inflammatory bowel disease), chronic infections, and malignancy. These aren't common causes of fatigue but are important to exclude.

CRP is more specific and sensitive than ESR for acute inflammation. High-sensitivity CRP (hsCRP) measures lower levels and is used for cardiovascular risk assessment rather than infection/inflammation diagnosis.

Liver and Kidney Function

Liver function tests (LFTs) check enzymes including ALT, AST, and ALP, alongside albumin and bilirubin. Elevated liver enzymes can indicate fatty liver disease, alcohol-related liver disease, autoimmune hepatitis, or medication effects — all of which cause fatigue. Fatty liver disease in particular is increasingly common and frequently causes profound fatigue before any other symptoms appear.

Kidney function (urea and creatinine, included in a basic metabolic panel or urea and electrolytes/U&E) matters because kidney disease reduces the kidney's ability to produce erythropoietin, the hormone that signals bone marrow to produce red blood cells — leading to the anaemia of chronic kidney disease.

What GP Typically Orders vs What to Ask For

In practice, a standard "fatigue blood test" panel at an NHS GP appointment often includes:

• FBC (full blood count)
• TSH
• CRP
• HbA1c or fasting glucose
• Liver function tests
• U&E (kidney function)

What it typically doesn't include unless specifically requested:

• Ferritin — the most commonly missed
• Vitamin D — requires justification for NHS testing; often denied without risk factors
• Free T4 — only ordered if TSH is abnormal
• Vitamin B12 and folate — sometimes included, sometimes not

What to say to your GP: "I'd like to include ferritin specifically alongside the full blood count, and vitamin D if possible — I'm aware of my risk factors and would prefer to have a baseline." Most GPs will add these if asked directly. If vitamin D is refused on NHS grounds, a home fingerprick test costs £25–40 privately and is straightforward.

When to Go Private

Private blood tests are worth considering if your GP declines specific tests, if you want results quickly, or if you want to establish a personal baseline. Services like Medichecks, Thriva, and Blue Horizon offer finger-prick home test kits.

A comprehensive fatigue panel privately typically costs £80–150 and covers FBC, ferritin, thyroid (TSH + free T4), vitamin D, B12, folate, HbA1c, CRP, and liver/kidney function. This is often faster and more comprehensive than piecing tests together through GP appointments.

Sources

• NHS — Tiredness and Fatigue

Frequently Asked Questions

What is the most important blood test for fatigue?

Ferritin is the single most commonly missed test. Standard blood tests check haemoglobin, which can remain normal while iron stores (ferritin) fall to symptomatic levels. Ferritin below 30–75 µg/L can cause significant fatigue, hair loss, and poor concentration without any anaemia on a standard blood count. Always request ferritin specifically — it's not automatically included in a full blood count.

Which blood tests does the NHS recommend for persistent fatigue?

The NHS recommends checking for anaemia (full blood count), thyroid problems (TSH), and diabetes (HbA1c) as a starting point for unexplained fatigue lasting more than four weeks. In practice, most GPs will also include liver and kidney function and inflammatory markers (CRP). You may need to specifically request ferritin and vitamin D.

Can blood tests come back normal but still have a cause for fatigue?

Yes. A standard blood panel can miss low ferritin (iron stores), vitamin D insufficiency, borderline thyroid function, and early blood sugar dysregulation. If your blood tests came back "normal" but fatigue persists, ask whether ferritin and vitamin D were specifically tested, and ask for the actual numbers rather than just a "normal" result — many reference ranges are broad enough that a result can be technically normal but functionally suboptimal.

What should my ferritin level be to avoid fatigue?

Most research on fatigue symptoms suggests ferritin above 50–75 µg/L is needed to be consistently asymptomatic, though NHS reference ranges often accept anything above 10–15 µg/L as "normal." If your result is in the 20–50 µg/L range and you are fatigued, a trial of iron supplementation with a GP's agreement is reasonable.

How do I prepare for blood tests for fatigue?

For most fatigue tests, no special preparation is needed. If fasting glucose or HbA1c is included, you may be asked to fast for 8–12 hours beforehand. Take any usual medications as normal unless your GP advises otherwise. Staying hydrated is fine. Avoid intense exercise in the 24 hours before testing, as this can temporarily elevate inflammatory markers and lactate.