What we measure
This test looks for signs of health issues that commonly cause fatigue, including thyroid problems and a deficiency in iron or vitamin D. If you've been feeling tired a lot lately, this test can help you figure out whether there may be an underlying health reason.
Thyroid-stimulating hormone (TSH)
TSH is produced by the brain and tells the thyroid gland how many hormones it should make. If TSH levels are abnormal, it can indicate that the thyroid isn’t working as it should be, which can cause symptoms of fatigue.
Iron
Iron is essential for making a protein called haemoglobin, which transports oxygen around the body. If you don’t have enough iron, your muscles don’t get the oxygen they need to work normally—and this can make you feel tired.
Total iron binding capacity (TIBC)
TIBC is a measure of how many proteins in the body are available to attach (“bind”) themselves to iron, so that they can transport it to our muscles and tissues. This can tell us how well iron is moving through the body.
Ferritin
A protein that stores iron within the body for future use. Ferritin levels can indicate how much iron you’ve got stored, and whether this is adequate for your body to function normally.
Transferrin saturation
This is a measure of how much of your transferrin, the main protein that attaches to iron and carries it around the body, is actually attached to iron. A low transferrin saturation can be a sign of iron deficiency.
Vitamin D
Vitamin D is an essential nutrient that contributes to immunity, mood, and muscle health—and has been linked to fatigue when its levels are low.
C-reactive protein (CRP)
CRP is a protein produced by the liver in response to inflammation in the body, which is thought to cause fatigue. Elevated CRP can also be a sign of various health problems, including autoimmune disease.
Full blood count with 5-part white cell differential
This is a way of identifying the different cells within your blood, and how many of you have of each in relation to each other. It can help spot signs of certain illnesses that may cause fatigue.

This test can help indicate health issues that are causing symptoms of fatigue.
Feeling tired from time-to-time is perfectly normal—and unavoidable for most of us. But when you’ve been feeling constantly tired for more than four weeks, something else might be going on.
Fatigue is a symptom of many different diseases, but there are some conditions it’s more strongly associated with. Thyroid disease is one example: the thyroid regulates how the body uses energy, so when it’s not working as it should this tends to make us feel lethargic and tired.
Another common cause of fatigue is an iron deficiency. Iron plays a crucial role in transporting oxygen around the body, which your muscles use as energy. Without enough iron, your body doesn’t get the energy it needs to function—and you feel tired as a result.
This test can help you to figure out whether your fatigue is being caused by a health problem. It looks for signs of disease and nutritional deficiencies, and can be a useful first step in finding out what’s happening inside your body.
Your results:
what to expect
Your results will tell you how much of each substance measured in present in your blood, and whether this falls within a normal range.
If any of your readings are abnormal (i.e., outside of the normal range), it can be a sign of a health problem that commonly causes tiredness.
Think of your results as a general overview of what’s going on inside your body: they can tell you what looks to be in order and where there may be an issue. You and your doctor can use these insights to help decide what you should do next—which may be to make certain lifestyle changes, for example, or to investigate further.

Need help? Speak with an expert
Don’t quite understand your results or simply want to see a doctor? We’ve got you sorted.
Speak to a GP
Book an online consultation with one of our GPs. We offer consultations in both English and Chinese.
Just fill out an online form and find a time that’s convenient.
Get a private prescription
Need to get your medication? Our GPs can write private prescriptions.
Tell the GP what the problem is during your consultation and they’ll take it from there. Prescriptions can be picked up at your local pharmacy.
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FAQs
Nope, you don’t need to abstain from food or drink ahead of this test.
Because this test measures thyroid function, you should avoid taking the vitamin B supplement biotin for at least 48 hours before the test. Biotin can cause false results in thyroid blood tests.
If your results show that you’ve got abnormal levels of one or more of the substances measured, this can indicate that you’ve got a health problem. But this isn’t a diagnosis.
Only a doctor can confirm whether you have a disease or nutritional deficiency. You should speak with them about your results—they’ll be able to explain what your readings mean for your health, decide whether it’s best to run further tests, and advise on any treatment options.
Symptoms may differ depending on whether your thyroid is producing too many hormones (hyperthyroidism) or too few (hypothyroidism).
Some common signs of hyperthyroidism include:
- Feeling tired all the time
- Anxiety, irritability, and nervousness
- Feeling hyperactive, like you can’t stay still and have a lot of nervous energy
- Mood swings
- Trouble sleeping
- Muscle weakness
Some symptoms of hypothyroidism are:
- Tiredness
- Weight gain
- Constipation
- Depression
- Muscle aches and weakness
- Being sensitive to the cold
Yes, this test is accurate in measuring the substances in your blood that may be related to fatigue. Your results are not a diagnosis in themselves, though—only a doctor can diagnose you with disease.
All of our tests follow the highest quality standards for health services: we’re CQC regulated, which is a quality marker for health services you can trust, UKAS accredited, and have our tests validated by registered doctors and clinical scientists.
The test won’t take longer than a few minutes.
For a phlebotomy, you’ll simply need to go to your local pharmacy and have them draw your blood.
Yes, absolutely. You can take this test at any point during your menstrual cycle.
