Which blood tests are most important if I train a lot?

Ferritin and haemoglobin (Hb), Vitamin D, TSH, blood status, and creatinine. For men, testosterone is also relevant during high load and low energy intake.

Can training affect the test result?

Yes. Creatinine, ASAT, and CRP can be temporarily elevated for a couple of days after a hard session. Take the test after at least two quiet days for a representative result.

Why is ferritin more important than haemoglobin (Hb) for athletes?

Ferritin shows the iron stores and drops first. Haemoglobin (Hb) falls later, when the deficiency has already affected performance. Ferritin therefore catches the problem earlier.

My creatinine is high – are my kidneys bad?

Not necessarily. Creatinine is formed from muscle mass, so muscular and recently trained people can have higher levels without kidney function being impaired. The value should be interpreted together with eGFR and your body composition.

How often should I take a blood test as an active person?

Twice per year during structured training towards a goal, and more frequently if you have previously had iron deficiency or are being treated for it.

Do I need to fast before the test?

If the package includes blood lipids or fasting glucose, you must fast. Iron status, Vitamin D, and TSH do not require fasting, but take iron supplements only after sampling.

Health check for those who train heavily

If you train hard several times a week, the most relevant blood tests are ferritin and haemoglobin (Hb) (iron status and oxygen transport), Vitamin D, TSH, blood status, creatinine, as well as testosterone for men. Hard training increases iron loss and strains recovery, and iron deficiency is precisely the most common measurable cause of performance plateaus despite increased training volume. Take the test at least a couple of days after a hard session, otherwise several values may be temporarily affected.

Iron is the most common bottleneck

Endurance athletes lose more iron than others: through sweat, through microscopic bleeding in the gastrointestinal tract during long runs, and through the breakdown of red blood cells when the foot hits the ground. At the same time, the need increases, as training drives the production of new red blood cells.

The result is that ferritin can drop long before haemoglobin (Hb) does. Symptoms resemble overtraining: heavy body, worse intervals, longer recovery, and higher heart rate at the same pace. Therefore, ferritin should always be included when an active person checks their values – haemoglobin (Hb) alone is not enough.

Other markers that are extra relevant with heavy training

Besides iron, there are a few tests that often explain why performance does not develop as expected.

Vitamin D: low levels during the winter affect muscle function and injury risk.
TSH: a thyroid that is underactive causes fatigue and poorer recovery.
Creatinine and eGFR: values are interpreted differently in muscular people, as creatinine comes from muscle mass.
Testosterone in men: can drop during very high training loads combined with low energy intake.
CRP: an elevated value can suggest an ongoing infection – in which case hard sessions should wait.

How training affects the test result itself

A hard session affects several analyses for a couple of days. Creatinine and the liver test ASAT rise from muscle work, CRP can be slightly elevated, and blood volume changes temporarily from fluid loss, which can make haemoglobin (Hb) look higher than it is.

For a representative result: take the test in the morning after at least two quiet days, drink water as usual, and avoid taking it directly after a competition or a long session. If you track your values over time, the most important thing is that sampling takes place under the same conditions every time.

How often should an active person test themselves?

Twice per year is reasonable for those who train structured towards a goal – preferably once before the build-up period and once during the most intensive part of the season. If you have had iron deficiency before, more frequent monitoring of ferritin may be justified, and supplementary treatment should always be followed up with a new test.

Frequently asked questions

Which blood tests are most important if I train a lot?: Ferritin and haemoglobin (Hb), Vitamin D, TSH, blood status, and creatinine. For men, testosterone is also relevant during high load and low energy intake.
Can training affect the test result?: Yes. Creatinine, ASAT, and CRP can be temporarily elevated for a couple of days after a hard session. Take the test after at least two quiet days for a representative result.
Why is ferritin more important than haemoglobin (Hb) for athletes?: Ferritin shows the iron stores and drops first. Haemoglobin (Hb) falls later, when the deficiency has already affected performance. Ferritin therefore catches the problem earlier.
My creatinine is high – are my kidneys bad?: Not necessarily. Creatinine is formed from muscle mass, so muscular and recently trained people can have higher levels without kidney function being impaired. The value should be interpreted together with eGFR and your body composition.
How often should I take a blood test as an active person?: Twice per year during structured training towards a goal, and more frequently if you have previously had iron deficiency or are being treated for it.
Do I need to fast before the test?: If the package includes blood lipids or fasting glucose, you must fast. Iron status, Vitamin D, and TSH do not require fasting, but take iron supplements only after sampling.

Unsure which test suits you? Start the test guide – a few questions about age, sex and goals give a suggestion with three tests.

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Last updated: 2026-09-09