Blood tests for endurance athletes
For runners, cyclists, triathletes, rowers and anyone else who trains for endurance, at any level.
10 tests explainedAI-verified against cited sources. Awaiting expert reviewLast reviewed 2026-10-19
Endurance training changes some blood tests in ways that can look odd on a standard report. The watery part of the blood expands, muscle enzymes rise after hard sessions, and iron needs go up. Eating too little for the training load can also lower sex hormones and thyroid hormone. These tests help you and your clinician sort normal training effects from things worth a closer look.
The tests that matter most
Most important first. Each one links to its full page, where the note for this group sits near the top.
Ferritin
Ferritin shows how much iron the body has in reserve. In athletes, stores can run low from eating too little, and from higher needs with altitude training, menstrual blood loss and foot strike (red cells breaking up from the pounding of running).
Sources: British Journal of Sports Medicine (2018) and Australian Institute of Sport
- Men, 20–59
- 30–400 ng/mL
- Men, 60+
- 31–409 ng/mL
Hemoglobin
Endurance training expands plasma, the watery part of blood, so hemoglobin and hematocrit can read a little lower even though the body has more red cells in total. This is sometimes called sports anemia and isn't anemia in the medical sense. Iron deficiency and under-fuelling can lower hemoglobin too.
Sources: Frontiers in Physiology (2013) and British Journal of Sports Medicine (2023)
- Men, 16+
- 13–17.7 g/dL
- Women, 16+
- 11.1–15.9 g/dL
Transferrin Saturation
Transferrin saturation shows how much iron is moving in the blood right now. The Australian Institute of Sport lists it with ferritin and hemoglobin as the minimum iron checks for athletes.
Sources: Australian Institute of Sport and British Journal of Sports Medicine (2018)
- Usual adult range
- 15–55 %
Vitamin D (25-Hydroxy)
Many athletes run low on vitamin D at some times of the year. How much sun reaches the skin and skin type both play a part.
- Usual adult range
- 30–100 ng/mL
Creatine Kinase (CK), Total
CK leaks out of muscle after hard training and races, so a high result is common in athletes. People who do a lot of hard training tend to run higher even at rest, and some people rise much more than others after the same session.
- Men, 19–49
- 26–366 U/L
- Women, 19–49
- 20–239 U/L
Testosterone, Total
In men, eating too little for the training load (low energy availability), for a long time or by a lot, can lower testosterone. The IOC lists low testosterone as a sign of Relative Energy Deficiency in Sport (REDs), the health effects of under-fuelling.
- Men, 20+
- 264–916 ng/dL
- Men, 18–19
- 150–785 ng/dL
Estradiol (E2)
In women, under-fuelling is linked to lower estrogen and to missed or stopped periods. Periods that stop are one of the main signs the IOC uses when it looks for REDs, once other causes have been ruled out.
- Usual adult range
- 7.6–42.6 pg/mL
Free T3 (Free Triiodothyronine)
Free T3 is an active thyroid hormone, and under-fuelling can lower it. The IOC counts a T3 in the lowest quarter of the lab's range, or below it, as a sign of REDs.
- Usual adult range
- 2–4.4 pg/mL
LDL Cholesterol
Under-fuelling is linked to higher total and LDL cholesterol. The IOC counts total or LDL cholesterol above the lab's range as a secondary sign of REDs.
- Usual adult range
- up to 99 mg/dL
Cortisol
The IOC lists a raised resting morning cortisol as a possible sign of REDs. It is a newer sign that the IOC doesn't yet count in its score. Physical stress, steroid medicines and some other conditions can raise it too, so it is read alongside everything else.
Ranges shown are the usual adult ranges labs print. The range on your own report is the one your result is judged against.
Getting results you can compare
- For iron tests, the Australian Institute of Sport suggests a morning draw, only light to moderate activity in the 24 hours before, no muscle-damaging sessions in the 2 to 3 days before, and no signs of illness.
Source: Australian Institute of Sport
- Arrive well hydrated. Losing fluid during exercise concentrates the blood and pushes hematocrit up for a while, so a draw straight after a long session can read high.
Sources: Frontiers in Physiology (2013) and Australian Institute of Sport
- After hard muscle work, CK can keep rising for hours or days. In healthy people it usually settles within about 7 to 9 days. Telling your clinician about recent training and races helps them read the result.
Good to know
- REDs is judged from many signs together, such as periods, bone health, mood and several blood tests. Each of these can have other causes, so one result on its own doesn't mean REDs.
- Experts haven't agreed on the vitamin D level that counts as low or as enough, and supplement guidelines for athletes aren't set yet.
- The IOC notes that periods and sex hormone results can't be read this way in people using hormonal birth control, and thyroid results can't in people taking thyroid medicine.
Questions you could ask
- How often should we recheck my ferritin during the season?
- Could my training or how much I eat around training explain any of these results?
- When in my training week is the best time for my next blood draw?
- Which of these results should we read together rather than one at a time?
Sources
- 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs), British Journal of Sports Medicine (2023)
- IOC consensus statement: dietary supplements and the high-performance athlete, British Journal of Sports Medicine (2018)
- Iron: how and when do I use it?, Australian Institute of Sport
- Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells, Frontiers in Physiology (2013)
- Creatine-kinase- and exercise-related muscle damage implications for muscle performance and recovery, Journal of Nutrition and Metabolism (2012)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-19. How we build and check these pages.
Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.