Fatigue Blood Test UK: 12 Markers That Explain Why You're Tired

Medically reviewed content. Last updated: February 2026.

Introduction: Why Am I Always So Tired?

If you find yourself asking “why am I so tired all the time?” you are far from alone. Research shows that 10–18% of UK adults experience persistent tiredness lasting a month or longer, and fatigue is the principal complaint in approximately 1 in 15 GP consultations. The medical shorthand for it — TATT, or “tired all the time” — is so common that it has its own acronym in clinical practice.

The frustrating reality is that tiredness has dozens of potential causes. Poor sleep, stress, and lifestyle factors play a role, but so do a range of measurable, treatable medical conditions. The good news: a well-chosen fatigue blood test can rule in — or rule out — the most common biological culprits, from iron deficiency and thyroid dysfunction to vitamin D depletion and blood sugar problems.

In this guide, we walk through the 12 blood markers that explain the majority of fatigue cases, what the results mean, and when you should see your GP versus arrange private testing.

The 12 Blood Markers That Explain Fatigue

NHS guidelines (NICE CKS “Tiredness/fatigue in adults”) recommend a core set of first-line investigations when a patient presents with unexplained tiredness. These typically include a full blood count, ferritin, thyroid function, liver and kidney function, CRP, and blood glucose markers. Additional investigations — such as vitamin D, vitamin B12, and folate — are recommended based on clinical judgement.

The 12 markers below cover all of these bases. Together, they form a comprehensive fatigue panel:

A full blood count (FBC) and kidney function markers (eGFR) are also essential components of a fatigue investigation — these are included in the panels discussed below alongside the 12 markers above.

1. Iron and Ferritin — The Most Common Cause of Fatigue

Iron is the mineral your body uses to make haemoglobin — the protein in red blood cells that carries oxygen from your lungs to every tissue and organ. When iron runs low, less oxygen reaches your cells, and the result is persistent, debilitating tiredness.

Ferritin is the storage form of iron. It is the single most useful marker for assessing whether your body has enough iron in reserve, and it is the first marker to drop when iron stores are being depleted — often months before you become anaemic.

How common is iron deficiency in the UK?

Very common. A 2025 study in Frontiers in Nutrition found that absolute iron deficiency (ferritin <30 ng/mL) affects 31.6% of UK women, compared to 2.9% of men. Among women aged 18–49, the prevalence exceeds 35%. At least 1 in 5 women of reproductive age in the UK are iron deficient according to National Diet and Nutrition Survey data.

The most common symptom reported by those with iron deficiency? Fatigue, affecting 31.5% of individuals with absolute iron deficiency.

The “normal” ferritin trap

One of the most important things to understand is that a “normal” ferritin result does not always mean optimal. Most UK labs use a lower reference limit of around 13–15 µg/L for ferritin. However, research consistently shows that fatigue symptoms improve when ferritin is raised above 50 µg/L. If your ferritin is 20 and your GP says it is “normal”, it may still be the reason you are exhausted.

Who is most at risk? Women with heavy periods, vegetarians and vegans, endurance athletes, people with coeliac disease, and anyone with chronic blood loss (e.g. regular blood donation or gastrointestinal conditions).

2. Vitamin D — The Winter Fatigue Factor

The UK sits at a latitude (50–60°N) where UVB rays are too weak to trigger meaningful vitamin D production in the skin between October and March. This is why vitamin D deficiency is so common here: approximately 1 in 5 UK adults has a serum 25(OH)D level below 25 nmol/L, the threshold the Scientific Advisory Committee on Nutrition (SACN) defines as deficient.

Recent private testing data suggests the problem may be even wider: nearly 50% of people tested had levels below the optimal range of 75–100 nmol/L.

How does vitamin D deficiency cause fatigue?

Vitamin D receptors are found in almost every tissue in the body, including muscles and the brain. Deficiency is associated with:

• Fatigue and general tiredness — one of the most consistently reported symptoms
• Muscle weakness and aches — particularly in the proximal muscles (thighs and upper arms)
• Low mood and seasonal depression — vitamin D influences serotonin production
• Impaired immune function — leading to more frequent infections, which themselves cause fatigue

SACN recommends that all UK adults take a 10 microgram (400 IU) supplement daily, especially during autumn and winter. People with darker skin, those who cover most of their skin, and those who spend little time outdoors are at even greater risk.

3. Vitamin B12 and Folate — Energy Metabolism Essentials

Vitamin B12 and folate (vitamin B9) work together in the methylation cycle, which is fundamental to energy production, red blood cell formation, DNA synthesis, and neurological function. Deficiency in either can cause fatigue that ranges from mild tiredness to debilitating exhaustion.

B12 deficiency: especially common in plant-based diets

Vitamin B12 is found almost exclusively in animal products. The EPIC-Oxford cohort study — one of the largest UK nutrition studies — found that 52% of male vegans and 7% of male vegetarians were B12 deficient (serum B12 <118 pmol/L). With approximately 3% of the UK population now identifying as vegetarian and 1% as vegan, this represents a substantial number of people at risk.

Supplementation makes a significant difference: vegans who supplement have a measurably better B12 status than vegetarians who do not, highlighting that awareness and action are the key factors.

Total B12 vs Active B12

Standard blood tests measure total vitamin B12, but only 10–30% of circulating B12 is in its active, usable form (holotranscobalamin). An active B12 test is a more sensitive early marker of deficiency. If your total B12 is borderline (between 150–300 ng/L), active B12 can clarify whether you are truly depleted.

4. Thyroid Function — The Silent Energy Drain

Your thyroid gland is the body’s metabolic thermostat. When it underperforms — a condition called hypothyroidism — every system slows down: your heart rate drops, your body temperature falls, and you feel profoundly tired, cold, and sluggish.

A comprehensive thyroid assessment requires three markers:

• TSH (thyroid-stimulating hormone) — the primary screening test. Elevated TSH signals that your pituitary gland is working harder to stimulate a sluggish thyroid.
• Free T4 — the main thyroid hormone. Low free T4 with elevated TSH confirms overt hypothyroidism.
• Free T3 — the most metabolically active thyroid hormone. Some people have normal T4 but poor conversion to T3, leading to persistent fatigue.

Subclinical hypothyroidism: the hidden problem

Subclinical hypothyroidism — where TSH is mildly elevated but T4 remains normal — affects an estimated 8–10% of the UK population, and its prevalence increases with age. In people over 60, approximately 10% have TSH values above the normal range. It is significantly more common in women.

Because T4 levels are still technically “normal”, subclinical hypothyroidism often goes undetected unless TSH is specifically tested. Symptoms are typically subtle: mild fatigue, slight weight gain, dry skin, and feeling cold. If your fatigue is persistent and unexplained, thyroid function should always be on the checklist.

For a deeper look at thyroid testing, see our full guide: Thyroid Blood Test UK: What to Test and What Results Mean.

5. Blood Sugar and HbA1c — Fatigue from Glucose Dysregulation

Your cells rely on glucose for fuel. When the system that regulates blood sugar starts to malfunction, fatigue is one of the earliest and most noticeable symptoms.

HbA1c (glycated haemoglobin) measures your average blood sugar over the previous 8–12 weeks. It is the gold standard for identifying pre-diabetes and type 2 diabetes.

The scale of the problem in the UK

According to Diabetes UK (February 2025), 6.3 million people in the UK have pre-diabetes and a further 4.6 million have a diabetes diagnosis — meaning over 12 million people, or 1 in 5 UK adults, are living with diabetes or pre-diabetes. Many are undiagnosed.

Pre-diabetes (HbA1c 42–47 mmol/mol) and early type 2 diabetes cause fatigue through several mechanisms:

• Insulin resistance means glucose cannot enter cells efficiently, leaving them starved of fuel.
• Post-meal blood sugar spikes followed by rapid drops (reactive hypoglycaemia) cause energy crashes, brain fog, irritability, and shakiness.
• Chronic hyperglycaemia triggers inflammation and oxidative stress, compounding fatigue.

6. Inflammation: CRP — Chronic Low-Grade Inflammation

C-reactive protein (CRP) is produced by the liver in response to inflammation. While a high CRP can indicate an acute infection, a persistently mildly elevated CRP often points to chronic low-grade inflammation — a common and under-recognised driver of fatigue.

What causes chronic inflammation?

• Autoimmune conditions such as rheumatoid arthritis, lupus, and inflammatory bowel disease
• Chronic infections (viral, bacterial, or parasitic)
• Obesity — adipose tissue produces pro-inflammatory cytokines
• Poor diet high in ultra-processed foods and refined sugar
• Chronic stress and poor sleep
• Post-viral fatigue, including long COVID

Symptoms of chronic inflammation are often subtle — fatigue, weight changes, joint pain, brain fog — and easy to dismiss. A CRP level below 5 mg/L is generally considered normal, but for cardiovascular risk assessment, a high-sensitivity CRP (hsCRP) test can detect inflammation at much lower thresholds. An hsCRP above 3 mg/L is considered elevated.

If your CRP is raised, it does not tell you what is causing the inflammation — but it tells you inflammation is present and warrants further investigation.

7. Liver and Kidney Function — Organs That Fail Silently

Both the liver and kidneys are remarkably resilient organs. They can lose significant function before producing obvious symptoms — and fatigue is often one of the first warning signs.

Liver function (ALT)

ALT (alanine aminotransferase) is an enzyme found primarily in liver cells. When liver cells are damaged — by alcohol, fatty liver disease, medications, or viral hepatitis — ALT leaks into the bloodstream, raising your levels.

Fatigue is the most common symptom of liver disease and frequently appears before more recognisable signs like jaundice or abdominal pain. Non-alcoholic fatty liver disease (NAFLD) is now the most common liver condition in the UK, affecting approximately 1 in 3 adults, and persistent unexplained tiredness is often its only early symptom.

Kidney function (eGFR)

Your kidneys filter waste products, regulate electrolytes, and produce erythropoietin (the hormone that stimulates red blood cell production). When kidney function declines, waste products accumulate, erythropoietin output drops, and anaemia develops — all of which cause fatigue.

eGFR (estimated glomerular filtration rate) is calculated from your creatinine blood test and indicates how well your kidneys are filtering. An eGFR below 60 mL/min sustained over three months indicates chronic kidney disease, which is often asymptomatic in its early stages.

8. Magnesium — The Overlooked Fatigue Marker

Magnesium is involved in over 300 enzymatic reactions in the body, including ATP (energy) production, protein synthesis, muscle and nerve function, and blood sugar regulation. Despite its importance, magnesium is one of the most commonly overlooked nutrients in fatigue investigations.

How common is magnesium deficiency?

Alarmingly common. Around 60% of the UK population fails to meet recommended magnesium intake. Approximately 15% of women and 10% of men fall below the Lower Reference Nutrient Intake, placing them at high risk of clinical deficiency. Among teenage girls, nearly 50% consume inadequate magnesium.

Symptoms of magnesium deficiency

• Fatigue and generalised weakness
• Muscle cramps, twitches, and spasms
• Sleep disturbance and insomnia
• Anxiety and irritability
• Heart palpitations and abnormal heart rhythms
• Headaches

Serum magnesium testing has an important limitation: only about 1% of the body’s magnesium is found in the blood, with the majority stored in bones and muscles. This means blood levels can appear normal even when total body stores are depleted. However, a low serum magnesium result is a strong indicator of deficiency and should always be acted on.

9. Testosterone — Not Just for Men

Testosterone plays a vital role in energy, motivation, muscle mass, mood, and cognitive function — in both men and women. Low testosterone is an underappreciated cause of fatigue, particularly in men over 40 and in women approaching or experiencing the menopause.

In men

Testosterone levels naturally decline from around age 30, at a rate of approximately 1–2% per year. Men with low testosterone commonly report feeling “flat” with reduced motivation, low energy, brain fog, poor concentration, and reduced physical performance. Low testosterone is also strongly associated with depression, which itself causes fatigue.

In women

Women produce testosterone in smaller quantities, but it is no less important for their energy and wellbeing. During perimenopause and menopause, testosterone levels drop significantly. Brain fog is the most commonly reported symptom among menopausal women, and an audit by Dr Louise Newson found that 39% of women reported improved cognition after testosterone was added to their HRT regimen.

For more on hormonal changes and fatigue in women, see our guide: Menopause Blood Test UK: What to Test and When.

10. Full Blood Count — Ruling Out Anaemia

A full blood count (FBC) is the foundation of any fatigue investigation. It measures your haemoglobin (the oxygen-carrying protein in red blood cells), red cell count, white cell count, and platelets. Anaemia — defined as haemoglobin below 130 g/L in men or 120 g/L in non-pregnant women — is a direct, measurable cause of fatigue.

Types of anaemia relevant to fatigue

An FBC alone does not tell you why you are anaemic, which is why it must be interpreted alongside ferritin, B12, folate, and CRP. Treating the underlying cause is essential — an iron supplement will correct iron-deficiency anaemia but will do nothing for B12-deficiency anaemia.

When to See Your GP vs Get Private Testing

Understanding when your tiredness warrants an urgent GP visit versus a private fatigue blood test is important.

See your GP urgently if you have:

• Unexplained weight loss (more than 5% of body weight without trying)
• Night sweats that soak your sheets
• Persistent fever without an obvious cause
• Lumps or swollen glands that do not resolve
• Blood in your stool or urine
• Severe breathlessness or chest pain
• Fatigue lasting more than 4 weeks combined with any of the above

These red flags can indicate serious underlying conditions — including cancers, chronic infections, and autoimmune diseases — that require prompt medical investigation.

When private testing makes sense

For the majority of people, persistent tiredness is caused by common, treatable deficiencies and imbalances. Private blood testing is a practical option when:

• You want a broader panel than the NHS typically offers (many GPs only test FBC and TSH initially)
• You have been told your results are “normal” but still feel exhausted (e.g. ferritin of 20, which is technically normal but functionally low)
• You want results quickly — NHS blood test waiting times can be several weeks
• You want to proactively check markers like vitamin D, B12, magnesium, and HbA1c that are not always included in NHS fatigue panels
• You have no red flag symptoms and want to understand and optimise your energy levels

For those who want an even more comprehensive investigation — including advanced hormonal markers, lipid profiles, and a wider metabolic panel — the Peak Insights 70 blood test covers 70 biomarkers across all major body systems.

Frequently Asked Questions

What blood tests should I ask for if I am tired all the time?

At minimum, ask for a full blood count, ferritin, thyroid function (TSH, free T4), vitamin D, vitamin B12, folate, HbA1c, CRP, liver function (including ALT), and kidney function (eGFR/creatinine). Adding magnesium and testosterone provides a more complete picture. NICE CKS guidelines support this panel as a first-line investigation for unexplained tiredness.

Can a blood test tell you why you are tired?

Yes, in many cases. Blood tests can identify common treatable causes of fatigue, including iron deficiency, thyroid dysfunction, vitamin deficiencies, blood sugar problems, inflammation, and organ impairment. However, blood tests may not identify every cause — conditions like sleep apnoea, depression, and chronic fatigue syndrome (ME/CFS) require different diagnostic approaches.

What is the most common blood test result that causes fatigue?

Iron deficiency (low ferritin) is the most common blood test finding in patients presenting with fatigue, particularly in women. It affects over 30% of UK women of reproductive age. Importantly, fatigue can occur even when ferritin is within the “normal” lab range — many experts recommend aiming for a ferritin level above 50 µg/L for optimal energy.

How much does a fatigue blood test cost in the UK?

Private fatigue blood tests in the UK typically range from £80 to £250, depending on the number of markers included. Comprehensive panels like the Core Health 45 (which covers all 12 key fatigue markers plus 33 additional biomarkers) offer better value than testing individual markers separately. NHS blood tests are free but may be limited in scope and involve longer waiting times.

Can low vitamin D make you feel extremely tired?

Yes. Vitamin D deficiency is strongly associated with fatigue, muscle weakness, and low mood. With 1 in 5 UK adults deficient (below 25 nmol/L) and nearly 50% below optimal levels, it is one of the most common nutritional causes of tiredness in the UK. The problem is worst between October and March when UVB sunlight is insufficient for skin production of vitamin D.

What thyroid level causes fatigue?

An elevated TSH (above approximately 4.5 mIU/L, depending on the laboratory) suggests your thyroid is underperforming. Even mildly elevated TSH — so-called subclinical hypothyroidism — can cause fatigue, affecting 8–10% of the population. Free T4 and free T3 should also be tested, as some people have poor T4-to-T3 conversion despite normal TSH levels, which causes persistent tiredness.

Should I get a fatigue blood test if I feel tired after COVID?

Yes. Post-COVID fatigue can be caused by residual inflammation (detectable via CRP), iron deficiency, thyroid dysfunction, or blood sugar changes — all of which are identifiable through blood tests. While some post-COVID fatigue resolves on its own, a blood test can rule out treatable contributing factors and give you a clearer picture of what your body needs to recover.

How long do fatigue blood test results take?

With private testing through Lola Health, results are typically available within 2 working days of your sample arriving at the laboratory. A professional phlebotomist visits your home or workplace to collect the blood — no GP appointment or hospital visit required. NHS blood test turnaround times vary by area and can take 1–3 weeks.