In short: Blood tests cannot make you faster, but they can uncover treatable problems that quietly hold athletes back, such as low iron stores, low vitamin D, an underactive thyroid or the effects of under-fuelling. The most useful core checks are usually a full blood count, ferritin, vitamin D, thyroid function, vitamin B12 and folate, plus kidney and liver tests. Results always need interpreting alongside your symptoms and training load, because hard exercise itself can shift several markers.

This guide, written by a Ready Health prescribing pharmacist, explains which blood tests are most relevant for people who train hard, what each one shows, how to prepare so your results are accurate, and when symptoms need a medical review rather than a change to your training plan.

Why would an athlete need a blood test?

Regular training places repeated demands on muscles, bones, blood and hormones. When energy, performance or recovery drop for no obvious reason, there is sometimes a medical cause underneath. A blood test can help to:

find deficiencies (for example iron, vitamin D or B12) that can cause tiredness and reduced endurance
check for conditions such as anaemia, thyroid disease or diabetes that may present as poor performance
give a personal baseline, so future results can be compared with what is normal for you
support investigation of possible under-fuelling (low energy availability) when combined with a clinical assessment

Blood tests are a tool for health, not for performance enhancement. Ready Health does not provide tests or advice aimed at supporting the use of performance-enhancing drugs.

Which blood tests are most useful for athletes?

Test	What it looks at	Why it matters in sport
Full blood count (FBC)	Haemoglobin, red cells, white cells, platelets	Can show anaemia or signs of infection. Endurance athletes can have slightly diluted haemoglobin because training expands plasma volume, so context matters.
Ferritin (iron stores)	How much iron your body has stored	Low iron stores are common in endurance athletes, people who menstruate, vegetarians and vegans, and can cause fatigue before anaemia develops. Ferritin also rises with inflammation, which can mask low stores.
Vitamin D	25-hydroxy vitamin D level	Important for bone and muscle health. Levels in the UK commonly fall over autumn and winter, especially in indoor athletes.
Thyroid function (TSH, free T4)	How well the thyroid is working	An underactive or overactive thyroid can affect energy, weight, heart rate and recovery.
Vitamin B12 and folate	Vitamins needed to make healthy red blood cells and support nerves	Low levels can cause tiredness, breathlessness and pins and needles. People on plant-based diets are at higher risk of low B12.
Kidney function (U&E, eGFR)	Creatinine, sodium, potassium	Useful for hydration and kidney health. Muscular athletes and those taking creatine supplements may have higher creatinine without kidney disease.
Liver function (LFT)	Liver enzymes and proteins	Some enzymes (ALT, AST) also come from muscle, so a hard session can raise them temporarily.
HbA1c and cholesterol	Average blood sugar and blood fats	Being fit does not rule out diabetes risk or inherited high cholesterol, so these remain part of a general health check.

Additional tests that may be relevant

CRP (C-reactive protein): a general marker of inflammation or infection. It is not specific, and can rise briefly after very intense exercise.
Creatine kinase (CK): a muscle enzyme that is often high after heavy training. It is mainly useful when a clinician suspects significant muscle injury.
Hormone tests: where under-fuelling is suspected, a clinician may check sex hormones (for example oestradiol, or testosterone and LH in men), prolactin or cortisol, alongside a full history.
Coeliac screen: worth considering if you have low iron or B12 with digestive symptoms.

What is RED-S and how do blood tests help?

Relative Energy Deficiency in Sport (RED-S) happens when the energy you take in is not enough to cover both training and normal body functions. It can affect anyone who exercises, of any sex, age or level, and can occur even at a normal body weight. Warning signs include missed or stopped periods, loss of morning erections in men, repeated stress fractures or injuries, frequent illness, low mood and poor recovery.

Blood tests can support the assessment, but normal results do not rule RED-S out, and hormonal contraception can mask changes in periods and hormone levels. If you recognise these signs, speak to a GP or sports medicine clinician.

How often should athletes have blood tests?

There is no single UK schedule for healthy recreational athletes. A sensible approach is:

a baseline set of tests when you are well and training normally
repeat tests when a clinician advises, for example to check a deficiency is responding to treatment
testing sooner if you develop symptoms such as persistent tiredness, breathlessness, repeated illness or a sudden fall in performance

Routine repeat testing every few months is not necessary for most people without symptoms or a known problem.

How should I prepare for a blood test as an athlete?

Avoid very hard or unusually long training sessions for 24 to 48 hours beforehand where possible, as these can temporarily raise muscle enzymes, creatinine and inflammatory markers.
Drink water normally so you are well hydrated.
Follow any fasting instructions you are given.
Tell the clinician about supplements you take. Biotin can interfere with some laboratory tests, including thyroid tests, and creatine can raise creatinine.
Try to test at a similar time of day and training phase if you want to compare results over time.

When should I see a doctor rather than adjust my training?

Seek medical advice promptly rather than training through it if you have:

chest pain, fainting, palpitations or unusual breathlessness during exercise (call 999 for chest pain or collapse)
dark or cola-coloured urine after exercise, especially with severe muscle pain
unexplained weight loss, night sweats or ongoing fever
missed periods for three months or more, or repeated stress fractures
tiredness that does not improve with rest

Do not start iron or high-dose vitamin supplements without testing and advice. Too much iron can be harmful, and supplements can hide the underlying cause of a problem.

Blood tests for athletes in Wigan and Standish: what to expect

At Ready Health in Standish, near Wigan, you can book a private blood test appointment. A clinician will talk through your symptoms and training, help choose tests that are relevant, take the sample and review the results with you. If anything needs further investigation or treatment, we will explain your options, which may include referral back to your NHS GP.

Frequently asked questions

What blood tests should athletes have?

A typical starting point is a full blood count, ferritin, vitamin D, thyroid function, vitamin B12 and folate, and kidney and liver function. Other tests depend on your symptoms and history.

Can a blood test explain why I feel tired all the time?

Sometimes. Low iron, low B12 or folate, thyroid problems and diabetes can all cause tiredness. However, overtraining, poor sleep, stress and not eating enough are also common causes, and these do not always show up on a blood test.

Can exercise affect my blood test results?

Yes. Hard training can temporarily raise creatine kinase, liver enzymes (ALT and AST), creatinine and CRP, and endurance training can slightly lower haemoglobin by increasing plasma volume. Tell your clinician about recent training.

Should athletes take iron supplements?

Only if a test shows low iron and a clinician recommends it. Taking iron when you do not need it can cause side effects and, over time, iron overload.

Key takeaways

Blood tests can identify treatable causes of fatigue and poor recovery, but they do not replace a clinical assessment.
Core tests include FBC, ferritin, vitamin D, thyroid function, B12 and folate, and kidney and liver function.
Hard exercise, creatine and biotin supplements can all affect results, so prepare and tell your clinician.
Missed periods, repeated injuries and frequent illness can be signs of RED-S and need medical advice.
Do not self-treat with iron or high-dose supplements without testing.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you train regularly and want to understand your health in more detail, book a blood test appointment at Ready Health in Standish and a clinician will help you choose the right tests and explain your results.

Blood Tests for Athletes: Which Checks Matter and Why