SYMPTOMS
Always Tired? The Blood Tests That Reveal Why
You're sleeping 7-8 hours. You're eating well. You're exercising. But you still wake up tired, hit a wall by 3pm, and can't shake the brain fog. Sound familiar? According to the NHS, around 1 in 5 people feel unusually tired at any given time. The answer is often hiding in your blood — and a standard GP blood test rarely looks deep enough to find it.
Why your GP says “everything's normal”
When you tell your GP you're tired, they'll typically run a full blood count (FBC) and maybe check your thyroid (TSH). If those come back “in range”, you're told everything is fine. But “in range” and “optimal” are very different things.
NHS reference ranges are designed to catch disease. They don't tell you if your iron stores are gradually depleting, if your vitamin D is low enough to impair energy production, or if your thyroid is sluggish but not quite failing. These are the grey zones where you feel terrible but technically aren't “sick”.
Here are the 8 biomarkers most commonly responsible for persistent fatigue — and what to do about each one.
The 8 biomarkers behind persistent tiredness
1. Iron & Ferritin →
HOW IT CAUSES FATIGUE
Iron is essential for haemoglobin — the protein in red blood cells that carries oxygen to every cell in your body. When iron is low, your cells literally don't get enough oxygen. Ferritin (iron stores) drops months before haemoglobin does, meaning you can feel exhausted long before you're technically anaemic.
WHAT TO LOOK FOR
Ferritin below 30 µg/L is associated with fatigue even when haemoglobin is normal. The NHS considers 12 µg/L as the lower limit. Performance ranges suggest 50-150 µg/L for optimal energy.
WHAT TO DO
If ferritin is low: iron bisglycinate (20-25mg daily) is the best-tolerated supplement form. Take with vitamin C (orange juice works) and away from tea, coffee, and dairy which inhibit absorption. Retest at 3 months.
2. Vitamin D →
HOW IT CAUSES FATIGUE
Vitamin D receptors exist in virtually every cell. Low levels impair mitochondrial function — your cellular energy factories. In the UK, Public Health England estimates that 1 in 5 adults are deficient, rising to nearly half during winter months.
WHAT TO LOOK FOR
The NHS considers 25 nmol/L as 'sufficient'. But research consistently links levels below 75 nmol/L with fatigue, low mood, and impaired recovery. Optimal: 100-150 nmol/L.
WHAT TO DO
Supplement with vitamin D3 (not D2): 2,000-4,000 IU daily for maintenance, up to 10,000 IU daily for 8-12 weeks if severely deficient (under medical guidance). Always take with fat — it's fat-soluble.
3. Vitamin B12 →
HOW IT CAUSES FATIGUE
B12 is critical for red blood cell formation and nerve function. Deficiency causes a type of anaemia where your red blood cells are too large and dysfunctional, reducing oxygen delivery. It also directly impairs neurological function, causing brain fog and poor concentration.
WHAT TO LOOK FOR
Serum B12 below 200 pg/mL is deficient. But symptoms often appear at levels below 400-500 pg/mL — well within the 'normal' NHS range. This is one of the most commonly missed causes of fatigue.
WHAT TO DO
Methylcobalamin (1,000mcg sublingual daily) is well-absorbed and bypasses any gut absorption issues. Vegans and vegetarians are at particular risk, as B12 is only found naturally in animal products.
4. Thyroid (TSH) →
HOW IT CAUSES FATIGUE
Your thyroid controls your metabolic rate — literally how fast your cells produce energy. When it's underactive (hypothyroidism), everything slows down: your energy, your metabolism, your recovery, even your thinking. Subclinical hypothyroidism (slightly elevated TSH) is extremely common and frequently dismissed.
WHAT TO LOOK FOR
NHS upper limit for TSH is 4.0-5.0 mIU/L (varies by lab). But TSH above 2.5 mIU/L is increasingly considered suboptimal. If you're tired and your TSH is 3.5, that could be the answer — even though it's 'in range'.
WHAT TO DO
Selenium (200mcg daily) supports thyroid hormone conversion. Ensure adequate iodine intake (dairy, fish, seaweed). If TSH is persistently above 4.0, discuss with your GP — you may benefit from levothyroxine.
5. Folate (Vitamin B9) →
HOW IT CAUSES FATIGUE
Folate works alongside B12 in red blood cell production and DNA synthesis. Deficiency causes the same megaloblastic anaemia as B12 deficiency — oversized, dysfunctional red blood cells that can't carry oxygen efficiently. It also impairs neurotransmitter production, affecting mood and mental energy.
WHAT TO LOOK FOR
Serum folate below 7 nmol/L is deficient. Optimal: 20-45 nmol/L. If you have the MTHFR gene variant (affects ~40% of the population), you may need higher intake.
WHAT TO DO
Methylfolate (5-MTHF) 400-800mcg daily. This is the active form that works regardless of MTHFR status. Eat more dark leafy greens, legumes, and fortified foods.
6. HbA1c (Blood Sugar) →
HOW IT CAUSES FATIGUE
HbA1c measures your average blood sugar over 3 months. When blood sugar regulation is impaired (insulin resistance or pre-diabetes), you experience energy crashes, post-meal sleepiness, and difficulty maintaining focus. An estimated 13.6 million people in England are pre-diabetic — most don't know.
WHAT TO LOOK FOR
HbA1c of 42-47 mmol/mol indicates pre-diabetes. But even levels at the upper end of 'normal' (38-41) can cause energy fluctuations. Optimal: below 36 mmol/mol.
WHAT TO DO
Reduce refined carbohydrates and sugars. Increase protein and fibre at meals to slow glucose absorption. Regular exercise (especially resistance training) improves insulin sensitivity dramatically. Chromium (200mcg daily) and berberine (500mg twice daily) have evidence for improving glucose regulation.
7. Magnesium →
HOW IT CAUSES FATIGUE
Magnesium is involved in over 300 enzymatic reactions including ATP (energy) production. It's also critical for sleep quality — and poor sleep is the most obvious cause of daytime fatigue. Magnesium deficiency is common because modern diets are lower in magnesium than historical diets, and stress depletes it rapidly.
WHAT TO LOOK FOR
Serum magnesium is a poor marker — only 1% of body magnesium is in the blood. Red blood cell magnesium is more accurate. If serum magnesium is below 0.85 mmol/L, deficiency is likely. But even 'normal' levels don't rule it out.
WHAT TO DO
Magnesium glycinate or threonate (300-400mg elemental magnesium daily, taken in the evening). Glycinate is best for sleep and relaxation. Threonate crosses the blood-brain barrier and may improve cognitive function. Avoid magnesium oxide — poorly absorbed.
8. hs-CRP (Inflammation) →
HOW IT CAUSES FATIGUE
Chronic low-grade inflammation is an underappreciated driver of fatigue. When your immune system is constantly activated — by poor diet, visceral fat, gut issues, or chronic stress — it produces inflammatory cytokines that directly cause tiredness, muscle aches, and brain fog. This is the same mechanism that makes you feel exhausted when you have the flu.
WHAT TO LOOK FOR
hs-CRP below 1.0 mg/L is ideal. Between 1.0-3.0 suggests moderate inflammation. Above 3.0 indicates significant inflammation (or acute infection). Many men with chronic fatigue have hs-CRP in the 1.5-3.0 range.
WHAT TO DO
Omega-3 fish oil (2-4g EPA+DHA daily) is the most evidence-based anti-inflammatory supplement. Address the root cause: reduce visceral fat, improve gut health (fibre, fermented foods), manage stress, and ensure adequate sleep.
The bottom line
Persistent tiredness is not normal. It's not “just stress” or “just getting older”. In most cases, there's a measurable biological explanation — and it's fixable once you know what it is.
A comprehensive blood test that covers these 8 markers gives you the data to stop guessing. Instead of cycling through random supplements or accepting that “this is just how I feel”, you can address the specific thing that's draining your energy.
Find out why you're tired
Our Essential panel (£129) covers all 8 of these fatigue-related biomarkers. Results in 5 days, with personalised recommendations.
Order Your TestMedical disclaimer: This content is for informational purposes only and does not constitute medical advice. Persistent fatigue can have many causes including sleep disorders, mental health conditions, and chronic diseases. If you are experiencing severe or worsening fatigue, please consult your GP. All Helvy blood tests are processed by UKAS-accredited NHS laboratories and reviewed by a GMC-registered doctor.
Last updated: April 2026 · By Helvy · Medically reviewed