Can Blood Tests Detect Overtraining? What the Evidence Actually Shows
Table of Contents
Key Takeaways: Can Blood Tests Detect Overtraining
- Overtraining syndrome (OTS) remains a clinical diagnosis. There is no single blood test or biomarker that can definitively confirm or exclude it.
- The athlete’s history, symptom progression, recovery, and performance changes are usually more informative than any individual laboratory result.
- Resting biomarkers such as testosterone, cortisol, creatine kinase (CK), and the testosterone-to-cortisol ratio may provide supportive information but have limited diagnostic value when interpreted in isolation.
- Hormonal responses to specialized exercise or stimulation tests appear more promising than resting measurements, but these protocols remain largely confined to research and specialist settings.
- Blood tests are most valuable for differential diagnosis, helping identify alternative causes of fatigue and performance decline such as iron deficiency, anaemia, thyroid disorders, infection, inflammation, RED-S, depression, or burnout.
- EROS study: 117 parameters in 51 athletes; no single biomarker was diagnostic, but combined clinical + hormonal scores achieved high accuracy in the sample (requires further validation).
- A normal blood panel does not rule out overtraining syndrome, and an abnormal laboratory result does not prove it.
- Laboratory findings should always be interpreted alongside the athlete’s overall clinical picture rather than used as standalone evidence for or against OTS.
Introduction: Can Blood Tests Detect Overtraining
Every now and then, an athlete comes to my clinic because they feel persistently fatigued, their performance has stopped improving, or their training no longer seems to produce the expected results. Some also describe low motivation, loss of enjoyment, anxiety, or depressive symptoms. One of the biggest clinical challenges is that these symptoms often overlap. It is not always immediately clear whether an athlete is developing overtraining syndrome (OTS), experiencing burnout, struggling with depression, or dealing with a combination of several factors.
Many of these athletes are highly conscientious individuals who approach their training, studies, careers, and other responsibilities with the same relentless commitment. From a clinical perspective, it is often the cumulative burden of these demands—not just the training itself—that deserves careful attention.
A question I hear repeatedly is whether there is a blood test that can confirm or rule out overtraining. Overtraining syndrome sits at the severe end of a physiological breakdown resulting from a sustained imbalance between training load and recovery. Athletes, coaches, and sports medicine clinicians often turn to blood tests when performance suddenly declines, hoping that a panel of laboratory values will confirm the diagnosis and point toward a solution.
Unfortunately, the answer is not that simple. While some laboratory tests can provide useful supporting information and help identify alternative explanations for persistent fatigue and declining performance—such as iron deficiency or thyroid disorders—no single blood test can diagnose overtraining syndrome. Understanding what blood tests can and cannot tell us is one of the most important clinical distinctions in the evaluation of an underperforming athlete.
How to Detect Overtraining? The Core Diagnostic Problem
OTS is defined by a long-term performance decrement that occurs after a persisting imbalance between training-related and nontraining-related load and recovery [1]. Because of the lack of a gold standard diagnostic test, OTS remains a diagnosis of exclusion [2].
The joint consensus statement of the European College of Sport Science (ECSS) and the American College of Sports Medicine (ACSM), published in Medicine and Science in Sports and Exercise, established that athletes move through a continuum from functional overreaching — a short-term decrement from which performance improves after recovery — to non-functional overreaching (recovers in weeks to months), to full overtraining syndrome (may require months to years of recovery) [1]. The distinction between non-functional overreaching and OTS is very difficult and will depend on the clinical outcome and exclusion diagnosis [1]. Survey studies of elite runners cited in the same consensus statement reported that 60% of female and 64% of male athletes indicated having experienced at least one previous episode consistent with overtraining syndrome [1].
A keyword in the recognition of OTS might be ‘prolonged maladaptation’ not only of the athlete, but also of several biological, neurochemical, and hormonal regulation mechanisms [1].
The central problem is that overtraining syndrome does not have a clear diagnostic criterion that can definitively confirm the condition. There is no single test, no specific laboratory value, and no blood panel that can prove that an athlete is overtrained.
In practice, the diagnosis is clinical and qualitative. It depends on the physician’s overall assessment: the athlete’s history, training load, recovery, symptoms, performance trend, life stress, sleep, nutrition, and the exclusion of other medical explanations. In many cases, the most important diagnostic tool is not the blood test but the history.
This is also why blood tests can be both useful and misleading. They can help rule out other causes of fatigue and declining performance, such as thyroid disease, iron deficiency, anaemia, inflammation, or other medical conditions. But if those tests are normal, that does not automatically prove overtraining syndrome. OTS is a multifactorial condition that can affect many systems at the same time, but no single abnormality in one system is enough to confirm it.
The clinical picture is further complicated by overlap with burnout, depression, anxiety, and general life stress. An athlete may be under-recovered from training, psychologically exhausted, medically unwell, or experiencing several of these at once. This is why reducing the diagnosis to one biomarker is rarely helpful. The real clinical task is to understand the whole pattern.
Why Resting Blood Tests for Overtraining Routinely Fail
The most important practical point for any athlete or coach ordering blood work: measurements at rest of selected blood markers such as urea, uric acid, ammonia, enzymes (creatine kinase activity) or hormones including the ratio between (free) serum testosterone and cortisol, may serve to reveal circumstances which, for the long term, impair the exercise performance, but they are not reliable for diagnosing established OTS [3].
This finding holds up across decades of research. A review in the Journal of Sports Science & Medicine concluded that to date, no single reliable objective marker of impending overtraining has been identified [4]. The EROS study — the most comprehensive investigation of overtraining blood markers to date — evaluated 117 parameters in 51 participants and concluded that individually, none of the markers accurately distinguished OTS-affected from healthy athletes [2].
Why do resting blood tests fail to diagnose overtraining syndrome? The answer lies partly in physiology. Resting hormonal and biochemical markers are inconsistent and generally insufficient for diagnosing OTS [5]. Rather than producing a consistent hormonal “fingerprint” at rest, the dysfunction appears to reveal itself in how the body responds to physiological stress. This is one of the main reasons why a standard resting blood test panel cannot diagnose overtraining on its own.
From a clinical perspective, there is another practical limitation. In theory, repeated measurements of hormones such as testosterone and cortisol could provide additional context if an athlete’s true baseline values were already known. In reality, however, this is rarely the case. Most athletes do not have hormone measurements taken during a healthy, well-recovered period, and by the time they present with persistent fatigue and declining performance, those baseline values are no longer available. Hormone concentrations also vary considerably between individuals, making isolated measurements difficult to interpret. A testosterone or cortisol value may fall within the laboratory reference range yet still differ substantially from that athlete’s usual physiological state—or it may appear abnormal without confirming overtraining syndrome.
Even if reliable baseline measurements were available, they would still provide only supportive information rather than a definitive diagnosis. In my view, they may add useful context, but they cannot replace a careful clinical assessment based on the athlete’s history, symptoms, performance changes, and the exclusion of other medical conditions.
Overtraining Blood Markers That Show the Most Promise
Hormonal Response to Stimulation
The strongest evidence points to stimulated hormonal responses rather than resting levels. Potential biomarkers of OTS include higher creatine kinase (CK), lower postexercise lactate, and blunted exercise-stimulated prolactin, growth hormone (GH), cortisol and adrenocorticotropic hormone (ACTH) responses [5].
In the two-bout exercise protocol — two consecutive incremental exercise tests to exhaustion with four hours of rest in between — ACTH and prolactin reactions to the second exercise bout were much higher in non-functionally overreached athletes compared with OTS athletes and showed the highest sensitivity for making the distinction [6].
Furthermore, maximal blood lactate concentration was lower in OTS compared with non-functional overreaching; however, the authors note that more data should be collected before this test can be used as the gold standard [6]. This protocol is not a standard clinical blood draw — it requires a specialized sports medicine setting — but it currently represents one of the most studied physiological approaches for distinguishing non-functional overreaching from overtraining syndrome in research settings.
Although these findings are scientifically interesting, they have limited applicability in routine clinical practice. The protocol requires specialized exercise testing combined with repeated hormonal measurements and is generally confined to research settings or specialized sports medicine and endocrinology services. In my clinical experience, athletes presenting with suspected overtraining are rarely evaluated using these protocols. Even when such testing is available, the results are best viewed as supportive information rather than a definitive diagnostic tool. The diagnosis still depends primarily on the overall clinical picture, including the athlete’s history, symptoms, performance changes, and exclusion of alternative medical conditions.
The Testosterone-to-Cortisol Ratio as an Overtraining Blood Marker
The testosterone-to-cortisol ratio (T/C ratio) has long been used as an indication of the anabolic/catabolic balance. This ratio decreases in relation to the intensity and duration of physical exercise, as well as during periods of intense training or repetitive competition, and can be reversed by regenerative measures [7]. However, it seems more likely that the testosterone/cortisol ratio indicates the actual physiological strain in training, rather than overtraining syndrome [7].
In a prospective study of endurance athletes who developed overtraining characteristics during a high-volume training phase, the overtrained subjects had a significant decline in testosterone (6.8±1.0 to 4.4±1.0 ng/ml), while the testosterone/cortisol ratio showed a significant decline (0.83±0.26 to 0.36±0.08) [8]. Other studies have failed to observe a significant change in this ratio during progressive increases in training loads in well-trained athletes [4]. Its value for monitoring training stress remains uncertain, and it should not be treated as a diagnostic blood test for overtraining syndrome.
From a clinical perspective, there are also important practical limitations. Although testosterone can be measured relatively easily, it is usually investigated to evaluate conditions such as hypogonadism rather than suspected overtraining. Cortisol is influenced by numerous physiological factors, including circadian rhythm, recent exercise, psychological stress, illness, and sleep, making isolated measurements particularly difficult to interpret. Even testosterone concentrations show considerable biological variability and are affected by factors such as the time of day and day-to-day variation.
In theory, serial measurements compared with an athlete’s own healthy baseline could provide more meaningful information than a single test result. In reality, however, these baseline measurements are rarely available because athletes typically seek medical attention only after symptoms have already developed. By that stage, there is usually no reliable pre-illness baseline against which current hormone concentrations can be compared.
Ultimately, however, even serial hormonal measurements do not change the fundamental nature of the diagnosis. Overtraining syndrome remains a clinical diagnosis based on the overall picture rather than on any single laboratory result. In my experience, isolated hormonal measurements are often more likely to create uncertainty than clarity when interpreted without appropriate clinical context. Even when longitudinal data are available, hormonal changes should be regarded as supportive findings rather than evidence for or against overtraining syndrome. They may add useful context, but the diagnosis still depends primarily on the athlete’s history, symptoms, performance changes, and the exclusion of alternative medical conditions.
Creatine Kinase and Glutamine
Higher levels of serum creatine kinase (CK) have been reported in athletes with overtraining syndrome compared with healthy athletes [9]. However, CK is also a normal response to heavy training and can be elevated for many reasons unrelated to overtraining. In my clinical experience, CK is generally more useful as a marker of muscle damage than as a marker of overtraining syndrome. As with other laboratory markers, it may provide supportive information, but its value in confirming or excluding OTS is limited.
Regarding glutamine: plasma glutamine concentration was decreased in overtrained athletes and after long-term exercise such as marathon racing, and was increased after short-term, high intensity exercise [10]. Abnormally low levels of plasma glutamine are commonly reported in overtrained athletes, although not all studies have found a fall during periods of increased training and overtraining [4]. Diminished glutamine to glutamate ratio has also been evidenced in plasma in resting state in OTS-affected athletes [9]. Plasma glutamine shows promise as a supplementary overtraining blood marker but lacks the sensitivity and specificity required for standalone diagnostic use.
What the EROS Study Found About Blood Tests and Overtraining
The Endocrine and Metabolic Responses on Overtraining Syndrome (EROS) study, which concurrently compared 117 parameters in 51 participants, identified more than 45 potential biomarkers of OTS [2]. None worked individually. The EROS research group ultimately proposed three diagnostic scores — the EROS-CLINICAL, EROS-SIMPLIFIED, and EROS-COMPLETE — combining 11 clinical parameters, 4 basal hormones (GH, prolactin, total testosterone, and the testosterone-to-estradiol ratio), and 5 hormonal responses to an insulin tolerance test (cortisol, ACTH, GH, prolactin) [2]. In this sample of athletes, the proposed diagnostic tools were found to be effective for diagnosing OTS with 100% accuracy, without the need to exclude confounding disorders — though these tools require further validation in larger and more diverse populations before they can be considered broadly applicable [2].
The EROS findings are particularly interesting because they represent one of the most systematic attempts to move beyond the traditional “diagnosis of exclusion” model. While OTS diagnosis still relies heavily on clinical assessment and ruling out other causes, studies like EROS suggest that in the future a more objective, laboratory-supported approach may become feasible. However, at present these combined scores remain research tools rather than ready-to-use clinical tests, and larger prospective studies are needed to confirm their utility across different sports, age groups, and training backgrounds.
What Blood Tests Can Actually Detect When Overtraining Is Suspected
Even if blood tests cannot confirm OTS, they play a critical role in the clinical evaluation: ruling out other causes of underperformance. Relevant screening ofteb includes a comprehensive metabolic panel, complete blood count, erythrocyte sedimentation rate, C-reactive protein, iron studies, creatine kinase, and thyroid-stimulating hormone [11].
The conditions that most commonly mimic OTS and require exclusion include:
- Iron deficiency and anaemia — one of the most common performance-limiting conditions in athletes, particularly female athletes, that presents with fatigue and performance decline indistinguishable from early overtraining. Ferritin levels are a key first-line marker.
- Thyroid dysfunction — hypothyroidism produces fatigue, reduced capacity, and mood disturbance remarkably similar to OTS
- Post-viral fatigue and glandular fever — frequently follows intense training phases and can look identical to overtraining
- Clinical depression — the psychological overlap with OTS is substantial and requires careful clinical assessment
- Relative Energy Deficiency in Sport (RED-S) — research has identified that many of the negative outcomes of training overload may be primarily associated with misdiagnosed under-fueling, low energy availability, and/or low carbohydrate availability [12]
In clinical practice, blood tests are usually more useful for differential diagnosis than for confirming overtraining syndrome itself. When I assess an underperforming athlete with persistent fatigue, I commonly look for alternative explanations such as anaemia, iron deficiency, thyroid dysfunction, inflammation, recent or ongoing infection, depression, burnout, and other causes of reduced performance.
Basic laboratory testing may therefore play an important role in excluding treatable conditions. However, once those possibilities have been considered, the assessment remains primarily clinical. The diagnosis depends much more on the athlete’s history, symptom pattern, recovery, performance trend, and overall context than on any single blood result.
Conclusion: Can Blood Tests Detect Overtraining?
Blood tests are valuable in the assessment of an underperforming athlete, but not because they can diagnose overtraining syndrome. Their main role is to help identify or exclude other medical explanations for persistent fatigue and declining performance, such as iron deficiency, anaemia, thyroid dysfunction, inflammation, infection, or other conditions that may mimic overtraining.
The key clinical point is that OTS is not a diagnosis that can be reduced to a single number. Resting blood markers, hormone ratios, CK, lactate responses, or even more complex testing protocols may provide supportive information in selected settings, but they do not replace the broader clinical assessment. The diagnosis still depends on the athlete’s history, symptom pattern, performance changes, recovery trajectory, psychological state, life stress, and the exclusion of alternative causes.
In practice, this means that a normal blood panel does not rule out overtraining syndrome, and an abnormal result does not prove it. Blood tests can clarify the differential diagnosis, but they cannot carry the diagnosis alone. For athletes and clinicians, the most useful approach is to treat laboratory results as one piece of the puzzle — not as the puzzle itself. Promising research such as the EROS study suggests that more objective tools may emerge in the future, but at present OTS diagnosis remains fundamentally clinical.
References
[1] https://pubmed.ncbi.nlm.nih.gov/23247672/
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC7193300/
[3] https://pubmed.ncbi.nlm.nih.gov/11817995/
[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC3963240/
[5] https://pmc.ncbi.nlm.nih.gov/articles/PMC6590962/
[6] https://pubmed.ncbi.nlm.nih.gov/18703548/
[7] https://pubmed.ncbi.nlm.nih.gov/8584849/
[8] https://pmc.ncbi.nlm.nih.gov/articles/PMC7098450/
[9] https://pubmed.ncbi.nlm.nih.gov/34496702/
[10] https://pubmed.ncbi.nlm.nih.gov/8775515/
