Blood Test for Tiredness and Fatigue: What to Check

Fatigue is the single most common reason people visit their GP in the UK. Around 1 in 5 adults report feeling unusually tired at any given time, and 1 in 10 say they experience persistent fatigue. Yet the standard GP response — a basic full blood count and perhaps a TSH test — barely scratches the surface of what could be going wrong.

If your GP has told you your blood results are "normal" but you still feel exhausted, that doesn't mean nothing is wrong. It means the tests weren't thorough enough. Here's what actually needs checking — and why the standard approach misses so much.

What Your GP Tests vs What You Actually Need

When you visit your GP complaining of tiredness, you'll typically get:

• Full Blood Count (FBC) — checks for anaemia by measuring haemoglobin, red and white blood cells
• TSH — a single thyroid marker, often the only one tested
• Sometimes a random glucose or basic liver function

What you actually need tested:

• Ferritin — your iron stores (not just haemoglobin)
• TSH + Free T3 + Free T4 — the full thyroid picture
• Vitamin D — deficiency is rampant in the UK
• Active B12 — the usable form, not just total B12
• Folate — works alongside B12
• HbA1c — screens for diabetes and insulin resistance
• CRP — detects chronic inflammation

The gap between these two lists explains why millions of people in the UK are told they're fine when they clearly aren't.

Ferritin: The Most Overlooked Cause of Fatigue

The NHS considers a ferritin level of 12 µg/L as the lower limit of normal. If yours comes back at 13, you'll be told everything looks fine. But here's what that misses: ferritin needs to be above 50 µg/L — and ideally above 70 — for your body to have adequate iron stores for energy production.

Iron is essential for making haemoglobin, which carries oxygen to every cell in your body. But it's also critical for mitochondrial function — the energy-producing machinery inside your cells. When ferritin drops below 50, many people experience fatigue, brain fog, restless legs, and exercise intolerance, even though their haemoglobin is technically normal.

This is called iron depletion without anaemia, and it's extraordinarily common in women of reproductive age. Studies suggest up to 30% of British women have ferritin below 30 µg/L. Most have never been told there's a problem.

A standard FBC won't catch this. It only flags anaemia — the final stage of iron deficiency, by which point you're already severely depleted. You need ferritin measured directly.

Thyroid Function: Why TSH Alone Isn't Enough

Thyroid dysfunction is the second most common hormonal cause of fatigue after iron deficiency. The thyroid gland controls your metabolic rate — when it underperforms, everything slows down. You feel cold, tired, sluggish, and your weight creeps up.

GPs typically only test TSH (thyroid-stimulating hormone). The NHS reference range for TSH is roughly 0.27–4.2 mIU/L. If yours is 3.8, you'll be told it's normal. But a growing body of evidence suggests that optimal TSH sits between 0.5 and 2.5, and that levels above 2.5 — while technically within range — may indicate early thyroid dysfunction.

More importantly, TSH alone doesn't tell you what the thyroid is actually producing. You need:

• Free T4 (FT4) — the inactive hormone your thyroid produces directly
• Free T3 (FT3) — the active hormone that cells actually use for energy

Some people have normal TSH but poor T4-to-T3 conversion, meaning their cells aren't getting enough active thyroid hormone. This is invisible on a TSH-only test. It's one of the most commonly missed causes of persistent fatigue.

Vitamin D: The UK's Silent Epidemic

Public Health England estimates that 1 in 5 adults in the UK has a vitamin D level below 25 nmol/L — the threshold for clinical deficiency. During winter months, that figure rises sharply because the UK doesn't get enough UVB sunlight between October and March for skin to produce vitamin D.

Deficiency causes fatigue, muscle weakness, low mood, bone pain, and impaired immune function. The optimal range is above 75 nmol/L, with many functional medicine practitioners aiming for 100–150 nmol/L.

If your vitamin D hasn't been checked, and you live in the UK, there's a reasonable chance it's low — particularly if you spend most of your day indoors, have darker skin, or don't supplement.

Active B12 and Folate

Vitamin B12 is essential for red blood cell production and neurological function. Deficiency causes fatigue, weakness, numbness, tingling, and cognitive problems. The standard test measures total serum B12, but this includes both active and inactive forms. You can have a "normal" total B12 while your active B12 — the form your cells can actually use — is deficient.

Active B12 (also called holotranscobalamin) is a far more reliable marker. If it falls below 37 pmol/L, you're deficient regardless of what your total B12 says.

Folate works alongside B12 in methylation and red blood cell production. Low folate amplifies the effects of B12 deficiency and independently causes fatigue and anaemia. Both should be tested together.

HbA1c: Fatigue as an Early Sign of Blood Sugar Problems

Persistent tiredness — particularly after meals, or with afternoon energy crashes — can signal insulin resistance or early type 2 diabetes. HbA1c measures your average blood glucose over the past 2–3 months:

• Below 42 mmol/mol — normal
• 42–47 mmol/mol — pre-diabetes
• 48+ mmol/mol — diabetes

Around 13.6 million people in the UK are estimated to be at increased risk of type 2 diabetes. Many are in the pre-diabetic range without knowing it, experiencing fatigue, brain fog, and sugar cravings as early symptoms. A simple HbA1c test can catch this years before it becomes full-blown diabetes.

CRP: When Fatigue Is Driven by Inflammation

C-reactive protein (CRP) is produced by the liver in response to inflammation. Elevated CRP — even mildly elevated, say above 3 mg/L — is associated with fatigue, low mood, and reduced motivation. Chronic low-grade inflammation can be driven by poor sleep, stress, excess body fat, food sensitivities, or undiagnosed autoimmune conditions.

If your CRP is persistently raised and you can't explain it, that's a signal worth investigating further.

Which Blood Test Should You Order?

If fatigue is your primary concern, the Core Health 45 blood test (£120) covers every marker discussed in this article: ferritin, iron studies, full thyroid panel (TSH, FT3, FT4), vitamin D, active B12, folate, HbA1c, CRP, and a complete blood count. It's the most thorough option for investigating tiredness.

If you suspect iron or B12 deficiency specifically, the Blood Health 6 test (£89) focuses on ferritin, iron, transferrin saturation, B12, folate, and FBC — a targeted panel for nutritional causes of fatigue.

For the most complete picture, the Peak Insights 70 test (£185) measures 70 biomarkers including everything above plus liver function, kidney function, lipids, and hormones.

Stop accepting "normal" results when you don't feel normal. Order a blood test that actually checks what matters, get your results in 2–3 working days, and find out what's really behind your tiredness.