We all feel tired from time to time. A late night, a demanding week at work, or a period of increased stress can leave anyone feeling drained. But when tiredness becomes a constant companion — when you wake unrefreshed despite adequate sleep, when you struggle through the day regardless of how much coffee you drink, when fatigue interferes with your ability to function normally — it is time to pay attention.
Persistent tiredness is one of the most common reasons people visit their GP, yet it is also one of the most frequently dismissed symptoms. Too often, it is attributed to "just stress" or "getting older" without proper investigation. The truth is that chronic fatigue is your body's way of telling you something is wrong, and a simple blood test can often reveal the underlying cause.
When Is Tiredness a Red Flag?
Not all tiredness warrants concern. However, you should take your fatigue seriously if:
These patterns suggest something more than simple tiredness and deserve investigation.
Blood Conditions That Cause Fatigue
### Iron Deficiency Anaemia
Iron deficiency is the most common nutritional deficiency worldwide and a leading cause of fatigue. Iron is essential for producing haemoglobin, the protein in red blood cells that carries oxygen throughout your body. When iron levels are low, your tissues receive less oxygen, leading to profound tiredness.
Symptoms beyond fatigue:
Who is at risk:
Blood tests: Full blood count, serum iron, ferritin, transferrin saturation.
### Vitamin B12 Deficiency
Vitamin B12 is crucial for red blood cell production and nerve function. Deficiency develops slowly and can cause debilitating fatigue along with neurological symptoms.
Additional symptoms:
Who is at risk:
Blood tests: Vitamin B12, sometimes methylmalonic acid for borderline cases.
### Folate Deficiency
Folate (vitamin B9) works alongside B12 in red blood cell production. Deficiency causes a type of anaemia that leads to significant fatigue.
Blood tests: Serum folate, red cell folate.
Thyroid Disorders
Your thyroid gland controls your metabolism, and dysfunction is one of the most common causes of unexplained fatigue.
### Hypothyroidism (Underactive Thyroid)
When your thyroid produces insufficient hormones, everything slows down. Fatigue is often the first and most prominent symptom.
Additional symptoms:
Thyroid disorders are remarkably common, particularly in women. For more information, read our comprehensive guide to private blood tests for thyroid problems.
Blood tests: TSH, Free T4, Free T3, thyroid antibodies.
Diabetes and Blood Sugar Issues
Both diabetes and prediabetes can cause significant fatigue. When your body cannot properly use glucose for energy, tiredness results.
How blood sugar affects energy:
Additional symptoms:
For detailed information about diabetes testing, read our guide to understanding the A1C blood test.
Blood tests: Fasting glucose, HbA1c, fasting insulin.
Vitamin D Deficiency
Vitamin D deficiency is extremely common in the United Kingdom due to limited sunlight, and fatigue is a key symptom. Vitamin D plays roles in energy metabolism, muscle function, and immune health.
Additional symptoms:
Who is at risk:
Blood tests: 25-hydroxyvitamin D.
Inflammation and Infection
Chronic inflammation and hidden infections can cause profound fatigue as your immune system works overtime.
Conditions to consider:
Blood tests: C-reactive protein (CRP), ESR, full blood count. Learn more about inflammatory markers in our overview of the C-reactive protein test.
Kidney and Liver Function
Both kidney and liver problems can cause fatigue, often before other symptoms appear.
Why organ function matters:
For detailed information about liver health, read our guide to decoding your liver function tests.
Blood tests: Liver function tests, kidney function tests (creatinine, eGFR, urea).
Other Causes to Consider
Blood tests can also help identify or rule out:
The Importance of Comprehensive Testing
When investigating fatigue, a comprehensive blood panel is more valuable than single tests. Multiple conditions can coexist, and patterns across different markers often provide the clearest picture.
For an overview of what a thorough assessment includes, read our guide to what's included in a comprehensive health check.
Taking Action
Do not accept persistent tiredness as normal. While fatigue can have many causes, many of the most common are easily identified through blood testing and highly treatable.
Consider testing if you:
Fatigue Investigation in London
At Private Blood Tests London, we offer comprehensive fatigue panels at our CQC-registered clinic in South Kensington. Our tests cover the most common causes of tiredness, providing you with answers quickly and conveniently.
Explore our [thyroid tests](/thyroid-test), [full blood count](/full-blood-count-test), and [comprehensive health screening packages](/health-screening).
Stop accepting fatigue as your new normal. Book your blood test today and find out what your tiredness is really telling you.