What Causes Unexplained Fatigue? A Guide to Common Causes and Which Tests Can Help
Feeling exhausted all the time, even after a good night’s sleep? Constant, unexplained tiredness is one of the most common things people ask us about – and while lifestyle plays a role, many people discover there’s a specific, identifiable, and often very treatable cause underneath it.
This guide covers the most common causes of unexplained fatigue, and which blood tests can help identify what’s actually going on.
If your symptoms are severe, sudden, or getting worse, please seek urgent medical advice rather than relying on this guide.
Find out what’s causing your fatigue
Book a blood profile test or speak with our clinical team today.
Common causes of unexplained fatigue
Iron deficiency or low ferritin
Low iron stores are one of the top causes of unexplained fatigue. Even if your haemoglobin is normal, low ferritin can leave you feeling drained and weak — this is especially common in women with heavy periods.
Vitamin B12 deficiency
B12 is essential for energy production and nerve function. Deficiency is common in vegetarians, vegans, and older adults, and can cause tiredness, brain fog, and tingling sensations (pins and needles).
Vitamin D deficiency
Especially common in the UK, particularly during autumn and winter. Low vitamin D is strongly linked to fatigue, muscle aches, and low mood.
Underactive thyroid (hypothyroidism)
An underactive thyroid slows your metabolism, causing persistent tiredness, weight gain, and feeling the cold more than usual.
Diabetes or prediabetes
Fluctuating or elevated blood sugar can cause significant fatigue. An HbA1c test can detect this early, often before other symptoms appear.
Anaemia
Reduced red blood cells mean less oxygen reaches your tissues, leaving you feeling exhausted. A full blood count can reveal this.
Hidden inflammation
Non-specific, persistent tiredness can sometimes be linked to chronic, low-grade inflammation in the body — even when you don’t feel obviously unwell otherwise.
Poor sleep quality
Even a full 8 hours doesn’t always mean restorative sleep — conditions like hay fever, sleep apnoea, or restless legs can quietly disrupt sleep quality without you fully realising it.
Stress and anxiety
Chronic stress keeps your body in a heightened state, gradually draining your energy reserves over time.
Which blood test might help?
The right test depends on your symptoms, but here’s a general guide to what each one looks at:
If you’re not sure which applies to you, a full tiredness blood profile offers a wider check across several of these markers at once, for a more complete picture. Ask our team which option best fits your symptoms when you book.
Frequently Asked Questions
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