If you are tired despite solid training, clean nutrition and enough sleep, your blood values often say more than you think. The four values that most often explain fatigue in athletes are ferritin, TSH, vitamin B12 and vitamin D. A targeted blood test shows which one is at play for you.
I notice athletes almost always blame their fatigue on training first. More rest, more food, less volume. Sometimes that is right. But just as often the cause sits in a value that a standard draw skips, and then you keep guessing.
This guide walks through the most important values, links your complaint to the right marker and shows which test fits you.
Which blood values explain fatigue?
In athletes, fatigue usually comes from one of five corners: your iron store, your thyroid, your vitamins, your recovery hormones or your blood sugar. Each corner has one or two markers that say the most. The table below links your complaint to the value to check.
| Complaint or pattern | Check this blood value | Read on |
|---|---|---|
| Tired, pale, short of breath on exertion | Ferritin, hemoglobin | Iron deficiency in athletes |
| Tired, cold, slow recovery, weight up | TSH, free T4, free T3 | Slow thyroid symptoms |
| Tired, tingling, poor concentration | Vitamin B12, folate | Vitamin B12 and energy |
| Tired in winter, weak muscles | Vitamin D | Vitamin D deficiency |
| Cramps, poor sleep, restless | Magnesium | Magnesium for athletes |
| Tired after hard training blocks, irritable | Cortisol, CK, CRP | Tired despite training |
| Energy dips after meals | Glucose, HbA1c, insulin | Biomarker guide for athletes |
Below I work through the main corners, starting with the most underrated one.
Why am I tired despite good training and sleep?
If you do everything right and stay tired, that often points to a deficiency you cannot see from your lifestyle. Think of a low iron store, a thyroid that works slightly too slowly, or a vitamin B12 that has quietly dropped. You do not feel these values, but they set your energy.
Training makes it harder, not easier. Heavy endurance blocks raise your iron loss, and a strict diet can pinch your intake of B12 or iron without you noticing.
That is exactly why a blood test adds value here. You swap guessing for measuring.
One caveat first: a single value is a snapshot. You always read it next to your complaints, your training load and, where possible, an earlier measurement.
Blood count and inflammation: hemoglobin, hematocrit and CRP
Your blood count determines how much oxygen your blood can carry, and that touches your energy directly. Hemoglobin is the oxygen-carrying protein, hematocrit the percentage of red blood cells. If they are low, you notice it as shortness of breath and heavy legs.
Inflammation values tell another part of the story. A slightly raised CRP without a clear cause can point to low-grade inflammation that slows your recovery in the background.
For an athlete, timing counts. A high CRP right after a hard session comes from muscle damage and says little about a disease.
So plan your measurement at rest, not the day after a race. That way you read a cleaner picture.
Iron and ferritin: the most missed cause in athletes
Iron deficiency is the most missed cause of fatigue in athletes, and it starts long before you have anaemia. Ferritin is your iron store, and it can be low while your hemoglobin still looks normal. That phase is exactly what explains unexplained tiredness.
In a randomised trial, complaints of unexplained fatigue decreased in non-anaemic women with a ferritin around 50 or lower after iron supplementation (Verdon, 2003). That matters, because many labs already call a ferritin of 20 normal.
Endurance athletes carry extra risk. Through sweating, foot strike and inflammation after long sessions you lose more iron than a non-athlete.
For the details, read iron deficiency in athletes. Always ask for ferritin alongside iron, because without that number you miss the early phase.
Thyroid: TSH, free T4 and free T3
Your thyroid sets the pace of your metabolism, and a slow thyroid often shows first as tiredness, cold and slow recovery. TSH is the first signal, free T4 and free T3 add the rest. Together they show whether your engine gets up to speed.
The nuance matters. Research shows that complaints such as fatigue are hard to attribute to the thyroid alone, because they are so non-specific (Jansen, 2023). A deviating TSH is a clue, not a verdict.
For an athlete, the combination with your other values and your complaints counts most.
You can read more in slow thyroid symptoms. A deviating value you discuss with your GP, who weighs the whole picture.
Vitamin B12, vitamin D and folate
Vitamins steer your energy without you feeling a deficiency directly, and three of them explain fatigue most often: B12, vitamin D and folate. A deficiency creeps in and is regularly skipped on a standard draw. Measuring makes that blind spot visible.
Vitamin B12 touches your energy, your nerves and your concentration. Recognition and management of a deficiency are well described in the literature (Langan, 2017), and athletes on a plant-based diet carry particular risk.
Vitamin D plays a big role in our climate, certainly in winter, and a deficiency is more common than people think (Holick, 2007).
Dig into vitamin B12 and energy and vitamin D deficiency in athletes. Discuss supplementation before you dose high yourself.
Magnesium, blood sugar and cortisol: the recovery side
Fatigue is not only about deficiencies, but also about how well you recover. Magnesium touches your sleep and muscle relaxation, your blood sugar sets your daytime energy dips and cortisol shows how your body handles stress and training load. Together they draw your recovery picture.
Poor sleep from a low magnesium undermines your recovery, and then every session feels heavier. Read how that connects in magnesium for athletes.
Measure cortisol at a fixed moment, ideally in the morning, because it has a strong daily rhythm. A high value after a hard block is not strange, but a persistent pattern is a signal.
If you recognise tiredness after hard training blocks, read tired despite training.
What if your blood values are normal but you are still tired?
A result within the reference range does not rule out fatigue. Reference values are broad population averages, and your optimal value can sit just inside or just outside them. A ferritin of 25 is technically normal, but often too low for an endurance athlete.
According to Thuisarts and the NHG guidelines, a blood test is used in a targeted way for complaints, not as a loose check of everything at once. The RIVM also publishes population figures that help to put your own value in perspective.
My advice is simple. Follow the trend and watch your complaints.
A repeat measurement at a comparable moment says more than a single number. If the tiredness persists after eight weeks while your lifestyle is sound, discuss with your GP whether further testing makes sense.
Which blood test do you choose for persistent fatigue?
For unexplained fatigue you choose a test that covers the main corners at once: your blood count, your iron store, your thyroid, your vitamins and your blood sugar. That way you do not have to guess which single value to draw first. A broad starting panel gives you an overview in one go.
For most athletes a broad test is a logical starting point. Our 360 Health blood test combines most of these systems in one measurement.
If you first want to understand what each value means, use our biomarker guide for athletes as a reference.
If you prefer to test in a targeted way, pick the single marker that matches your complaint from the table at the top.
How often should you test your blood values?
For most athletes without complaints, once or twice a year is enough to track your baseline values. If you spot something and adjust, a retest after eight to twelve weeks is useful to see the effect. That way you build a trend instead of loose numbers.
Around heavy training blocks or a competition season an extra measurement can help. Your iron and your recovery markers move most then.
Preferably draw blood at a fixed time of day. Comparable conditions make your measurements more reliable.
Common mistakes when figuring out fatigue
Most mistakes sit not in the measurement, but in what you do with it. Anyone who takes their fatigue seriously often runs into the same traps.
The first mistake is looking only at iron and forgetting ferritin. Your early iron deficiency shows in your ferritin, not in a normal iron number.
The second mistake is being alarmed by a value within the range and doing nothing, or dosing high on your own. You read a value next to your complaints, and treatment you discuss with your doctor.
The third mistake is retesting too early. Give an adjustment eight to twelve weeks before you draw again.
Frequently asked questions
The questions I get back most from athletes who want to figure out their fatigue.
Which blood test do you do for fatigue? A useful starting panel covers your blood count, ferritin, TSH, vitamin B12, vitamin D and glucose. Which exactly you choose depends on your complaints and your sport.
Can a sports blood test be done without a referral? Yes, you can have a blood test done yourself. Every result is assessed by a BIG-registered doctor, and for treatment decisions you go to your GP.
Do I need to fast? For glucose and some other values yes, for most vitamins no. Preferably draw in the morning at a fixed moment.
What is a good ferritin for an athlete? Many athletes feel better with a ferritin well above the labs lower limit. A value that is technically normal can still be too low for an endurance athlete, so read it next to your complaints.
Does taking iron help against tiredness? Only if your iron store is genuinely low. With a low ferritin, supplementation can reduce the tiredness, but with a normal store it adds little and can even cause complaints.
Can fatigue also come from my hormones? Yes, alongside your thyroid, your testosterone and your cortisol can play a role. For athletes that is a reason to look wider than iron and vitamins when complaints persist.
Which doctor do I ask for a sports blood test? You can have a blood test done yourself without a referral. If you want the result weighed in treatment, your GP is the right point of contact, because they know your history.
References
- Verdon F, Burnand B, Fallab Stubi CL, et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ. 2003. PMID: 12763985.
- Langan RC, Goodbred AJ. Vitamin B12 Deficiency: Recognition and Management. American Family Physician. 2017. PMID: 28925645.
- Holick MF. Vitamin D deficiency. New England Journal of Medicine. 2007. PMID: 17634462.
- Jansen HI, Boelen A, Heijboer AC, et al. Hypothyroidism: The difficulty in attributing symptoms to their underlying cause. Frontiers in Endocrinology. 2023. PMID: 36814580.
- Thuisarts.nl / NHG. Blood testing. Accessed 2026.
- RIVM. Population figures and reference values. Accessed 2026.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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