Feeling tired after a long day is normal. But what if you wake up tired, struggle to concentrate, feel low on energy throughout the day, or need several cups of tea or coffee just to get through your routine?
Persistent tiredness can have many causes. Poor sleep, stress, an unhealthy routine and lack of physical activity can all contribute. However, certain medical conditions and nutritional deficiencies can also cause fatigue—and some of these can be investigated with simple blood tests.
The right tests depend on your symptoms, medical history, age, diet and other risk factors. A blood test cannot identify every possible cause of fatigue, but it can help investigate several common medical conditions.
What Causes Constant Tiredness?
Fatigue is a symptom, not a diagnosis.
Some common causes or contributing factors include:
- Iron deficiency or anaemia
- Thyroid disorders
- Vitamin B12 deficiency
- Vitamin D deficiency
- Blood sugar abnormalities
- Certain infections or inflammatory conditions
- Liver or kidney problems
- Poor-quality sleep
- Stress
- Certain medications
- An unhealthy or physically inactive lifestyle
This is why simply taking a vitamin supplement because you feel tired may not solve the underlying problem.
Which Blood Tests Can Help Investigate Fatigue?
There isn’t one single “fatigue blood test.”
Depending on your symptoms and medical history, a healthcare professional may recommend different tests.
1. Complete Blood Count (CBC)
A CBC test is commonly used when investigating unexplained fatigue.
A Complete Blood Count measures different types of blood cells, including:
- Red blood cells
- White blood cells
- Platelets
It can help identify abnormalities such as anaemia and provide useful information about your overall blood health.
Why Might a CBC Be Relevant to Tiredness?
Red blood cells contain haemoglobin, which helps carry oxygen throughout the body. When haemoglobin is low, you may experience symptoms such as:
- Tiredness
- Weakness
- Dizziness
- Shortness of breath
A CBC may be particularly useful when fatigue is accompanied by:
- Pale skin
- Weakness
- Dizziness
- Shortness of breath
- Headaches
- Reduced stamina
- Unusual tiredness during physical activity
However, a CBC result should always be interpreted alongside your symptoms and other clinical findings.
2. Ferritin and Iron Tests
Iron deficiency can contribute to tiredness and may occur even before significant anaemia develops.
Ferritin is a blood test that helps assess the body’s stored iron. Depending on the situation, a healthcare professional may also recommend additional iron studies.
Iron-related testing may be particularly relevant if tiredness occurs alongside:
- Heavy menstrual bleeding
- Pale skin
- Hair loss
- Reduced exercise tolerance
- Dizziness
- Weakness
- A diet that is low in iron
Important: Don’t start taking iron supplements simply because you feel tired. Testing can help determine whether iron deficiency is actually present.
3. Thyroid Tests — TSH and T4
Your thyroid gland plays an important role in regulating your body’s metabolism.
An underactive thyroid, also known as hypothyroidism, can cause fatigue. It may also be associated with symptoms such as:
- Unexplained weight gain
- Feeling unusually cold
- Constipation
- Dry skin
- Changes in hair
A TSH test is commonly used as an initial test to assess thyroid function. Depending on the result and your symptoms, additional testing such as free T4 may be recommended.
If you’re experiencing persistent tiredness together with unexplained weight changes or other thyroid-related symptoms, discuss thyroid testing with a healthcare professional.
4. Vitamin B12 Test
Vitamin B12 is important for healthy blood cells and normal nerve function.
Low vitamin B12 levels can be associated with fatigue and weakness. Some people may also experience neurological symptoms such as tingling or numbness.
A B12 test may be worth discussing with a healthcare professional if you:
- Follow a vegetarian or vegan diet
- Have certain digestive or absorption problems
- Take certain long-term medications
- Experience tingling or numbness
- Have unexplained fatigue
- Have previously been diagnosed with low B12
A B12 result should be interpreted according to the laboratory’s reference range and your overall health.
5. Vitamin D Test
Vitamin D is another nutrient that may be tested when deficiency is suspected.
People with limited sun exposure, certain dietary patterns or other risk factors may be more likely to have low vitamin D levels.
However, not every case of tiredness is caused by vitamin D deficiency.
If fatigue continues despite a normal vitamin D result, other possible causes should also be considered.
6. Blood Sugar Tests and HbA1c
Blood sugar abnormalities can sometimes be associated with tiredness.
Depending on your symptoms and risk factors, a healthcare professional may recommend tests such as:
- Fasting blood glucose
- HbA1c
- Other glucose-related tests when appropriate
HbA1c provides information about your average blood glucose levels over the previous few months and generally does not require fasting.
Blood sugar testing may be particularly relevant if tiredness occurs alongside:
- Increased thirst
- Frequent urination
- Unexplained weight changes
- A family history of diabetes
- Previous abnormal blood sugar results
- Other symptoms associated with high or low blood sugar
Other Blood Tests Your Doctor May Consider
The best approach isn’t necessarily to order every available blood test.
Depending on your symptoms and medical history, a healthcare professional may consider additional tests such as:
| Test | What it may help assess |
|---|---|
| Liver Function Tests (LFT) | Liver health |
| Kidney Function Tests (KFT) | Kidney function |
| Electrolytes | Electrolyte balance when clinically indicated |
| CRP or ESR | Inflammation when suspected |
| Folate | Folate status in selected cases |
| Other tests | Based on symptoms and medical history |
The exact combination of tests should be decided based on your individual situation.
Do You Need to Fast Before a Blood Test?
It depends on which tests you are having.
Not every blood test requires fasting.
For example, thyroid testing generally does not require fasting unless your healthcare professional gives you specific instructions.
If your testing includes a fasting blood glucose test, you may be instructed not to eat or drink certain things for a specific period before the test.
Always follow the preparation instructions provided by your doctor or diagnostic laboratory.
If you’re unsure whether you need to fast, ask the laboratory before your appointment.
What If My Blood Tests Are Normal But I’m Still Tired?
This is an important question.
Normal blood test results don’t mean that your fatigue isn’t real.
Blood tests can investigate several medical causes of tiredness, but they cannot identify every possible cause.
If your initial results are normal, a healthcare professional may consider other factors such as:
- Sleep quality
- Sleep apnoea
- Stress
- Mental health
- Lifestyle
- Medication side effects
- Recent infections
- Physical activity levels
- Other underlying medical conditions
For example, persistent tiredness accompanied by loud snoring, choking or gasping during sleep may indicate that your sleep quality needs further evaluation.
So, if your blood reports are normal but you’re still experiencing significant fatigue, don’t simply keep repeating the same tests. Discuss your symptoms with a healthcare professional.
When Should You See a Doctor About Constant Tiredness?
Consider speaking with a healthcare professional if your tiredness:
- Continues for several weeks
- Is affecting your work or daily activities
- Keeps returning without an obvious explanation
- Occurs despite getting enough sleep
- Is accompanied by unexplained weight changes
- Occurs with dizziness, weakness or breathlessness
- Is accompanied by other persistent symptoms
Seek Urgent Medical Attention If:
Fatigue occurs with severe or sudden symptoms such as:
- Chest pain
- Severe difficulty breathing
- Fainting
- Significant bleeding
- Sudden neurological symptoms
Frequently Asked Questions
What blood test should I get if I am always tired?
There isn’t one universal blood test for tiredness. Depending on your symptoms, a healthcare professional may consider tests such as CBC, ferritin and iron studies, TSH, vitamin B12, vitamin D, blood glucose or HbA1c.
Can a blood test show why I am tired?
Blood tests can help investigate several possible causes of fatigue, including anaemia, iron deficiency, thyroid disorders, certain vitamin deficiencies and blood sugar abnormalities. However, blood tests cannot identify every possible cause of tiredness.
Is CBC enough to find the cause of fatigue?
Not always. A CBC can provide useful information about your blood cells and may help identify anaemia, but fatigue can have many different causes. Additional testing may be appropriate depending on your symptoms.
Should I get a Vitamin B12 test if I feel tired?
It may be appropriate in certain situations, particularly if fatigue occurs alongside symptoms or risk factors associated with B12 deficiency. Speak with a healthcare professional to determine whether testing is appropriate for you.
Can thyroid problems cause tiredness?
Yes. An underactive thyroid can cause fatigue and may also be associated with symptoms such as weight gain, feeling cold, constipation and changes in skin or hair. Thyroid blood tests can help investigate thyroid function.
Do I need to fast for a thyroid test?
Usually, thyroid blood tests do not require fasting. However, always follow the preparation instructions provided by your healthcare professional or diagnostic laboratory.
Can low iron cause tiredness?
Yes. Iron deficiency can be associated with fatigue, particularly when it results in anaemia. Ferritin and other iron-related tests can help assess iron status.
What if my blood tests are normal but I’m still tired?
Normal results can make certain medical conditions less likely, but they don’t explain every case of fatigue. Sleep, lifestyle, stress, medications and other medical conditions may also need to be considered.
How do I know which blood tests I need?
The right tests depend on your symptoms, age, medical history, diet, medications and other risk factors. Rather than ordering every available test, discuss your symptoms with a healthcare professional who can recommend appropriate investigations.
The Bottom Line
Feeling tired occasionally is normal. Feeling exhausted day after day shouldn’t simply be dismissed as “weakness” or automatically blamed on a vitamin deficiency.
Blood tests can help investigate several possible contributors to fatigue, including:
- Anaemia
- Iron deficiency
- Thyroid problems
- Vitamin deficiencies
- Blood sugar abnormalities
The right approach is targeted testing based on your symptoms—not simply ordering the largest possible health package.
If you’re experiencing persistent, unexplained fatigue, speak with a qualified healthcare professional about which tests may be appropriate for you.
Remember: A blood report is an important diagnostic tool, but it is not a diagnosis by itself.
