Overtraining syndrome: a diagnosis of exclusion

A 28-year-old runner is preparing for his first marathon. Over the past three months, he has increased his mileage week after week, added interval workouts and started sleeping less because of a new job. His times get worse instead of better. He starts workouts but cannot finish them, feels heavy legs, wakes up tired and is irritable. He takes ten days off, comes back and runs even worse. His coach mentions "overtraining", and he asks the doctor whether there is a test that confirms it. The case is fictional, and the question is common in sports medicine clinics.

The sources' answer is direct: the confirming test does not exist yet. The consensus of the European College of Sport Science with the American College of Sports Medicine describes overtraining syndrome as a diagnosis of exclusion, made after ruling out diseases that explain the drop in performance and often recognized only in retrospect (Meeusen, 2013). The clinical work is to organize that exclusion without falling into two errors: missing a treatable disease or ordering too many tests.

Key points

• Functional overreaching, non-functional overreaching and overtraining syndrome form a continuum, separated by recovery time and not by a test (Meeusen, 2013).
• The diagnosis requires ruling out anemia, infections, endocrine diseases, eating disorders, injuries and other causes of fatigue (Meeusen, 2013).
• In a series of athletes with fatigue or repeated infections, a medical cause was found in 68 out of every 100 (Kreher, 2012).
• Relative energy deficiency in sport overlaps with overtraining, and the IOC consensus asks that it be ruled out first (Mountjoy, 2023).
• Subjective measures of well-being respond better to training load than many blood markers (Saw, 2016).
• Treatment is rest or relative rest, with a gradual return and no validated protocol (Kreher, 2025).

Summary

• From training that adapts to training that makes you ill
• The symptoms: many, nonspecific and variable
• Diagnosis of exclusion: what to rule out first
• The tests: to exclude, not to confirm
• Relative energy deficiency: the differential that overlaps the most
• Monitoring load to prevent
• Treatment and return to training
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

From training that adapts to training that makes you ill

Every planned workout produces fatigue. After a block of high load, performance drops for a few days and, with adequate rest, returns above the previous level. The 2013 consensus calls this phase functional overreaching: a short, expected drop followed by improvement (Meeusen, 2013). It is the basis of almost every periodization program.

When the load stays high and recovery stays short, the picture can progress to non-functional overreaching. The drop in performance lasts weeks to months, comes with physical and mood symptoms, and recovery is complete, but the athlete loses the season or part of it. Overtraining syndrome sits at the end of the continuum: a prolonged maladaptation, with a drop in performance that persists for months and symptoms across several systems (Meeusen, 2013).

What separates the three phases is the time to recovery, not the intensity of symptoms on the day of the visit. That is why the consensus admits that, in practice, the difference between non-functional overreaching and overtraining syndrome often becomes clear only later, when one knows how long the athlete took to return (Meeusen, 2013). This look back has a cost: retrospective bias makes the picture seem obvious once the outcome is known, which does little to help whoever decides in the first month.

Phase	Typical duration of the drop in performance	Outcome	Source
Functional overreaching	Days to a few weeks	Improved performance after rest	Meeusen, 2013
Non-functional overreaching	Weeks to months	Complete recovery, with loss of part of the season	Meeusen, 2013
Overtraining syndrome	Months, sometimes longer	Slow and uncertain recovery; may end the career	Meeusen, 2013
Practical reference	Improvement in less than 14 to 21 days of rest suggests overreaching	Persistence beyond that raises suspicion of the syndrome	Kreher, 2012

The two-to-three-week timeframe in the last line is a clinical reference proposed by Kreher and Schwartz, not a validated criterion (Kreher, 2012). It serves to guide the conversation with the athlete and the reassessment plan.

How often it happens

The numbers vary with the definition, the sport and the method, and almost all come from questionnaires. The 2013 consensus gathers some of them. In elite swimmers, about 10 in every 100 had non-functional overreaching or overtraining syndrome in each training cycle, ranging from 7 to 21 in every 100 across studies. Over their careers, at least one episode was reported by 60 in every 100 elite female runners and 64 in every 100 elite male runners, against 33 in every 100 adults who ran but were not elite (Meeusen, 2013).

Young athletes also appear in the series. The same consensus cites 35 in every 100 young swimmers, 37 in every 100 Swedish junior athletes and 29 in every 100 British youth-category swimmers with at least one reported episode (Meeusen, 2013). These data mix non-functional overreaching and the syndrome, and the true syndrome, lasting months, is much less common. The practical message is that a drop in performance from excessive load is frequent, and the severe form is rare.

Endurance sports dominate the series, but the picture appears in team and strength sports. The consensus lists possible triggers beyond volume and intensity: monotonous training, too many competitions, psychological stress, poor sleep, altitude and heat (Meeusen, 2013). Armstrong and colleagues propose viewing the syndrome as a complex systems phenomenon, in which several small stressors add up until the capacity to adapt runs out (Armstrong, 2021). The model explains why the same training plan makes one athlete ill and not another.

The symptoms: many, non-specific and variable

The central symptom is the drop in performance that persists despite rest. Kreher and Schwartz describe typical clues in the history: the athlete starts training but cannot complete it, loses the "final sprint" in races and needs more effort to hold paces that were once easy (Kreher, 2012). The complaint usually comes with fatigue, heavy legs, persistent muscle pain, repeated respiratory infections, poor sleep, loss of appetite, irritability, lack of motivation and depressed mood.

The 2013 consensus notes that symptoms vary greatly between athletes and between sports, and that none of them is exclusive to the syndrome (Meeusen, 2013). Older descriptions separated a "sympathetic" form, with agitation and high resting heart rate, from a "parasympathetic" form, with apathy and low heart rate. The division still appears in training texts, but studies show inconsistent results, and the consensus advises against using it for diagnosis.

Mood, sleep and mental health

Mood changes are among the most consistent. Questionnaires such as the Profile of Mood States and the RESTQ-Sport capture the worsening of mood and perceived recovery and are cited by the consensus as monitoring tools (Meeusen, 2013). The overlap with depression is large, and the athlete with very depressed mood deserves a mental health assessment of their own. When there is intense sadness, hopelessness or thoughts of death, the direct question is part of the consultation, and the post on suicide risk shows what the clinical assessment captures.

Sleep is also part of the picture. Lastella and colleagues reviewed sleep studies in athletes under high load and found, in one of them, sleep efficiency of 95 in every 100 minutes in bed in swimmers without overreaching, against 82 in every 100 in swimmers with overreaching (Lastella, 2018). The authors conclude that sleep is a possible indicator, but that studies are lacking to use it as a marker.

A diagnosis of exclusion: what to rule out first

Overtraining syndrome can only be diagnosed after excluding other causes of reduced performance and fatigue (Meeusen, 2013). The list is long because many diseases common in young people present this way. The 2013 consensus provides a list-style script, and Kreher and Schwartz adapt it for the consultation (Kreher, 2012).

Group	What to look for	Source
Blood and iron	Anemia and low ferritin	Meeusen, 2013
Infections	Mononucleosis (Epstein-Barr virus), other viral infections, Lyme disease in risk areas	Meeusen, 2013
Endocrine diseases	Diabetes, thyroid disease, adrenal disease	Meeusen, 2013
Nutrition and energy	Eating disorders and low energy availability	Mountjoy, 2023
Muscle and inflammation	High CK, raised ESR and CRP, orthopedic injury	Meeusen, 2013
Heart and lung	Heart disease, adult-onset asthma, allergies	Meeusen, 2013
Kidneys and liver	Altered creatinine and liver enzymes	Meeusen, 2013

The frequency of medical causes justifies the workup. Kreher and Schwartz cite a series of athletes at regional level or above, evaluated for fatigue or repeated infections, in which a cause was found in 68 out of every 100 (Kreher, 2012). Thinking of overtraining first and disease second reverses the order and delays treatment of simple conditions, such as iron deficiency anemia or hypothyroidism.

The reasoning is that of any differential diagnosis: compare two similar diseases side by side and look for the clue that separates them. Mononucleosis, for example, usually comes with sore throat, enlarged lymph nodes and fever; anemia, with pallor and shortness of breath on exertion; hypothyroidism, with cold intolerance and weight gain. The absence of these clues lowers the probability of each disease, but does not replace the examination when it remains plausible.

Training and life history

Before any test, the training history often says more. The consensus advises asking about increases in volume and intensity, monotony, number of competitions, travel with time zone changes, heat and altitude, and about psychological and social factors, such as study, work, relationships and pressure for results (Meeusen, 2013). Listening to the patient's schedule to the end helps find the stressor the person did not connect to training, such as the job change in this case.

Tests: to rule out, not to confirm

Blood tests are there to rule out diseases, not to prove the syndrome. Kreher and Schwartz suggest a basic panel: complete blood count, metabolic panel with potassium, magnesium and glucose, ESR, CRP, iron studies, CK and TSH. In athletes with atopy or respiratory complaints, pulmonary function is added to the list (Kreher, 2012). The 2013 consensus proposes a similar set, with urea, creatinine, liver enzymes, sodium and ferritin (Meeusen, 2013).

Each test ordered must answer a clinical question. The pretest probability guides this choice: a test that does not change management adds cost and the chance of a meaningless borderline result. In athletes this weighs more, because intense training changes several values, and the consensus recommends interpreting results against the athlete's own baseline values (Meeusen, 2013).

Why no hormone confirms it

The search for a hormonal marker is old. The systematic review by Cadegiani and Kater gathered 38 studies and concluded that baseline hormones, such as cortisol, testosterone, growth hormone and thyroid hormones, are usually normal in the syndrome. The most consistent changes appeared in responses to stimulation tests, with a diminished response of ACTH and growth hormone (Cadegiani, 2017).

The 2013 consensus reaches a similar conclusion. The ratio between testosterone and cortisol reflects recent training load, but does not diagnose the syndrome. Resting heart rate and heart rate variability gave inconsistent results across studies (Meeusen, 2013). The most specific proposal of the consensus is a protocol with two maximal exercise tests separated by four hours, measuring the response of ACTH, prolactin and growth hormone. The protocol requires a specialized laboratory and remained restricted to research.

The EROS studies

A Brazilian group tested this approach in athletes with the syndrome. In the EROS-DIAGNOSIS study, Cadegiani and colleagues compared athletes with overtraining syndrome, healthy athletes and sedentary people, with hormonal stimulation tests, body composition and questionnaires, and proposed scores with 100 out of 100 accuracy in the sample studied (Cadegiani, 2020). In the EROS-PREDICTORS study, with 39 participants, patterns of eating, sleep and social life were associated with clinical and biochemical changes (Cadegiani, 2020).

The results are interesting, but they call for caution. The samples were small, only men, and the perfect accuracy occurred in the same sample in which the scores were created. The scoping review by Carrard and colleagues, with 39 studies, concluded that the EROS scores need to be validated in larger samples and in women before clinical use, and that the diagnosis still depends on exclusion (Carrard, 2022).

Marker	What the evidence shows	Source
Baseline hormones	Generally normal; they differentiate poorly	Cadegiani, 2017
Testosterone to cortisol ratio	Reflects recent load; it diagnoses poorly	Meeusen, 2013
Responses to stimulation tests	Reduced ACTH and GH responses; use in research	Cadegiani, 2017
Heart rate and variability	Inconsistent results across studies	Meeusen, 2013
EROS scores	High accuracy in a small male sample; validation is lacking	Carrard, 2022
Well-being questionnaires	They respond to load better than many objective markers	Saw, 2016

Relative energy deficiency: the differential that overlaps the most

Relative energy deficiency in sport, known by the acronym REDs, appears when energy intake falls below what the body needs to train and maintain basic functions. The 2023 International Olympic Committee consensus describes a large overlap between REDs and overtraining syndrome: both involve the hypothalamic-pituitary-adrenal axis, both reduce performance and both lack a single marker (Mountjoy, 2023).

The strongest data in the consensus come from the review of studies on overtraining: in 18 of 21 of them, there were signs of low energy availability in the athletes studied. The document recommends ruling out low energy availability before diagnosing overtraining syndrome (Mountjoy, 2023). In practice, this means asking about diet, weight, menstrual cycle in female athletes, libido in male athletes, stress fractures and risky eating behaviors.

The EROS-PREDICTORS study reinforces the link, because carbohydrate and protein intake were associated with hormonal and metabolic behavior (Cadegiani, 2020). Separating the two conditions is not always possible, and both hypotheses can coexist in the same athlete. The clinical point is that correcting the lack of energy is treatment, while labeling everything as overtraining can hide an eating disorder. The post about relative energy deficiency in sport discusses this differential from the REDs side.

Monitoring load to prevent

Because the diagnosis is difficult, prevention gains weight. The systematic review by Saw and colleagues, with 56 studies, compared subjective measures, such as mood, stress and recovery questionnaires, with objective measures, such as hormones, inflammatory markers and heart rate. The subjective measures responded better to acute and chronic changes in training load (Saw, 2016).

The 2013 consensus recommends keeping training records with the rating of perceived exertion for each session, varying the stimuli to reduce monotony, planning rest days, adjusting the load in weeks with competitions and travel, and taking care of sleep and nutrition (Meeusen, 2013). Kreher advocates education of athletes, coaches and parents as the most useful strategy, because those who live with the athlete notice the changes before the tests do (Kreher, 2016).

Baseline values

An idea repeated in the sources is to compare the athlete with himself. Exercise tests, questionnaires and exams have more value when there is a baseline value obtained during the period of good form (Meeusen, 2013). Teams that take this measurement at the start of the season have something to compare when performance drops.

Treatment and return to training

The central treatment is rest or relative rest. Kreher and Schwartz suggest, after the initial phase, a return with short, light sessions, from 5 to 10 minutes a day, gradually expanded to about an hour, increasing volume first and only then intensity (Kreher, 2012). The authors recommend considering follow-up with a psychologist, especially when mood is greatly affected or the pressure for results is high.

The case series by Kreher and colleagues, published in 2025, shows how variable this path is. Three endurance athletes with the syndrome had different trajectories: two returned to competition and one was still recovering 22 months later. The authors acknowledge that there is no definitive return protocol and that the plan must be individual (Kreher, 2025).

Waiting with a plan

Waiting is part of the diagnosis and the treatment, and time as a test only works with a plan. A planned reassessment, with a date, criteria for improvement and warning signs, turns uncertainty into a roadmap. If the athlete does not improve within the expected time, or fever, weight loss, bleeding or new symptoms appear, the exclusion list is reviewed again.

The decision to stop or reduce training affects schedule, scholarship, contract and identity. Shared decision-making with the athlete, and with the coach if the athlete wishes, increases the chance that the plan will be followed.

The two risks: missing the disease and creating the label

The first risk is missing the disease: calling anemia, mononucleosis or an eating disorder overtraining. The second is excess. An athlete who is tired after a hard block, with functional overreaching, may receive hormone tests, MRIs and weeks away without need. The post on overdiagnosis shows how the wrong label also causes harm, and quaternary prevention describes how one borderline test pulls in another until it becomes a cascade.

The middle path is what the sources propose: a detailed training history, a physical examination, a panel of tests with clear questions, exclusion of low energy availability, guided rest and a scheduled reassessment. In the runner's case, the ten-day break was too short to separate the phases, and the job change added a stressor that the training spreadsheet did not show.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. The drop in an athlete's performance is a good exercise in diagnosis of exclusion: each clue may point to anemia, infection, thyroid disease, low energy availability or excessive load. The post on heuristics shows when the shortcut "a tired athlete trained too much" helps and when it gets in the way.

The limits on using the game are described on the NEXO Economy page. If you research the teaching of clinical reasoning, the Researcher page explains how to get in touch. Professionals who write or review cases find information on Curator and Creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is there a blood test that confirms overtraining syndrome?

There is not yet. Tests serve to exclude other diseases, and baseline hormones are usually normal (Cadegiani, 2017).

What is the difference between overreaching and overtraining?

Recovery time. Overreaching resolves in days or weeks; overtraining syndrome lasts months (Meeusen, 2013).

How much rest is needed?

It depends on the phase. Improvement in less than two to three weeks suggests overreaching; the syndrome can take months, and the return is gradual (Kreher, 2012).

Can eating too little cause the same picture?

It can. Relative energy deficiency in sport overlaps with the syndrome, and the IOC consensus calls for excluding it before the diagnosis of overtraining (Mountjoy, 2023).

Do heart rate variability watches and apps detect overtraining?

Studies with heart rate and variability have given inconsistent results, and simple well-being questionnaires respond better to load (Saw, 2016).

Take-home messages

• Functional overreaching, non-functional overreaching and overtraining syndrome form a continuum separated by recovery time.
• The syndrome is a diagnosis of exclusion, often recognized only in retrospect.
• Anemia, infections, endocrine diseases, injuries and eating disorders come first on the list.
• Low energy availability must be excluded before speaking of overtraining.
• Tests are used to rule out; baseline hormones are usually normal.
• Wellness questionnaires and load records help more in prevention than blood markers.
• Treatment is guided rest, with a gradual return and planned reassessment.

Play the case of the day on NEXO.

References

1. Meeusen R, Duclos M, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Med Sci Sports Exerc. 2013;45(1):186-205. PMID 23247672. DOI 10.1249/MSS.0b013e318279a10a.
2. Kreher JB, Schwartz JB. Overtraining syndrome: a practical guide. Sports Health. 2012;4(2):128-138. PMID 23016079. DOI 10.1177/1941738111434406.
3. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073-1097. PMID 37752011. DOI 10.1136/bjsports-2023-106994.
4. Carrard J, Rigort AC, Appenzeller-Herzog C, et al. Diagnosing Overtraining Syndrome: A Scoping Review. Sports Health. 2022;14(5):665-673. PMID 34496702. DOI 10.1177/19417381211044739.
5. Cadegiani FA, Kater CE. Hormonal aspects of overtraining syndrome: a systematic review. BMC Sports Sci Med Rehabil. 2017;9:14. PMID 28785411. DOI 10.1186/s13102-017-0079-8.
6. Saw AE, Main LC, Gastin PB. Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review. Br J Sports Med. 2016;50(5):281-291. PMID 26423706. DOI 10.1136/bjsports-2015-094758.
7. Cadegiani FA, da Silva PHL, Abrao TCP, Kater CE. Diagnosis of Overtraining Syndrome: Results of the Endocrine and Metabolic Responses on Overtraining Syndrome Study: EROS-DIAGNOSIS. J Sports Med (Hindawi Publ Corp). 2020;2020:3937819. PMID 32373644. DOI 10.1155/2020/3937819.
8. Cadegiani FA, Kater CE. Eating, Sleep, and Social Patterns as Independent Predictors of Clinical, Metabolic, and Biochemical Behaviors Among Elite Male Athletes: The EROS-PREDICTORS Study. Front Endocrinol (Lausanne). 2020;11:414. PMID 32670198. DOI 10.3389/fendo.2020.00414.
9. Armstrong LE, Bergeron MF, Lee EC, Mershon JE, Armstrong EM. Overtraining Syndrome as a Complex Systems Phenomenon. Front Netw Physiol. 2021;1:794392. PMID 36925581. DOI 10.3389/fnetp.2021.794392.
10. Lastella M, Vincent GE, Duffield R, et al. Can Sleep Be Used as an Indicator of Overreaching and Overtraining in Athletes? Front Physiol. 2018;9:436. PMID 29740346. DOI 10.3389/fphys.2018.00436.
11. Kreher JB, Stepanek J, Amrine R, Reddy S. Overtraining Syndrome (OTS) in Three Endurance Athletes and Roads to Recovery. BMJ Case Rep. 2025. PMID 40623787. DOI 10.1136/bcr-2025-265066.
12. Kreher JB. Diagnosis and prevention of overtraining syndrome: an opinion on education strategies. Open Access J Sports Med. 2016;7:115-122. PMID 27660501. DOI 10.2147/OAJSM.S91657.