Low TSH in Athletes: Understanding Lower-Normal Values in Endurance Sports
Table of Contents
Key Takeaways: Low TSH in Athletes
- Low TSH in athletes is usually most useful as a context-dependent finding, not as a diagnosis by itself.
- Endurance athletes may show TSH, free T4, and free T3 values toward the lower end of the reference range, but this does not automatically mean thyroid disease.
- Reduced energy availability and leptin-related hypothalamic signalling are plausible explanations, but the current evidence is mainly associative.
- Athlete status should not be used to dismiss genuinely abnormal thyroid results. Suspected hyperthyroidism, hypothyroidism, or pituitary disease still requires standard medical evaluation.
- Thyroid results are most useful when interpreted together with symptoms, training load, energy intake, weight trajectory, recovery status, and serial measurements over time.
Introduction: Why Low TSH in Athletes Is Misread
Thyroid panels in athletes can sometimes look unfamiliar at first glance. Clinicians are often used to seeing a compensatory pattern: if one thyroid marker moves down, another tends to move up. In primary hypothyroidism, for example, TSH is typically elevated while free T4 may be low. In hyperthyroid patterns, TSH is usually suppressed while thyroid hormone levels are higher.
What can be less familiar, even in clinical practice, is the athlete-specific pattern where TSH, free T4, and sometimes free T3 may all sit toward the lower end of the reference range at the same time. In my clinical experience, this is a pattern I encounter from time to time in trained individuals. It should not automatically be interpreted as central hypothyroidism, especially when the values remain within the laboratory reference range and the broader clinical picture does not otherwise suggest pituitary or thyroid disease.
The key point is that lower-normal thyroid values in athletes are physiologically interesting, but not always clinically pathological. This article explains the mechanisms proposed to underlie lower-normal TSH in athletes, what observational research says about its magnitude and sport-specificity, where the diagnostic risk lies, and how to interpret thyroid panels intelligently in trained populations.
How Common Is Low TSH in Athletes?
The most comprehensive data on low TSH in athletes comes from a 2024 study at the Italian National Olympic Committee, which evaluated 1,342 Olympic athletes (mean age 25.6 ± 5.1 years) across power, skill, endurance, and mixed sports disciplines [1]. All athletes included in the analysis had TSH within the standard reference range (0.27–4.2 uUI/mL), and those with known thyroid disease, thyroid hormone supplementation, or TSH outside the reference range were excluded.
Endurance athletes presented the lowest TSH (1.93 ± 0.7 uUI/mL in endurance, 2.00 ± 0.8 in skill, 2.02 ± 0.8 in power, and 2.18 ± 0.8 in mixed; p pooled <0.0001; pairwise: E vs. M, p <0.0001; P vs. M, p = 0.002; S vs. M, p = 0.009; remaining not significant), fT3 (4.9 ± 0.7 pmol/L in endurance vs. 5.1 ± 0.7 pmol/L in other disciplines; p pooled = 0.007), and fT4 (15.5 ± 2.2 pmol/L in endurance vs. 16.2–16.5 pmol/L in other disciplines; p pooled < 0.0001) in comparison to the remaining sporting disciplines [1]. These differences were more pronounced in male athletes; endurance males had the lowest TSH (p=0.007), fT3 (p=0.001), and fT4 levels (p<0.0001) [1].
This is a cross-sectional observational study; the data show that lower-normal values are commonly observed in endurance athletes, not that endurance training causes them. The Di Gioia dataset does, however, provide a more sport-specific distribution than standard population reference ranges. An endurance athlete with a TSH of 1.0 may be sitting well below the midpoint of the population reference range while falling close to the mean for their sporting category — a pattern that explains why low TSH in athletes is so frequently misinterpreted on standard lab reports.
In practice, this lower-end thyroid pattern is not unique to athletes. I tend to think of it as part of a broader physiological response to reduced energy availability. It may appear in endurance athletes during periods of heavy training or inadequate caloric intake, but a similar pattern can also be seen outside sport in people who have lost a significant amount of weight, are undernourished for other reasons, or are in a prolonged low-energy state.
This is also why the pattern makes clinical sense in conditions such as anorexia nervosa, where the body may reduce energy expenditure as part of a wider adaptive response to undernutrition. That does not mean every athlete with lower-normal TSH has an eating disorder, and it does not mean the finding is automatically pathological. The more useful interpretation is that lower-normal TSH, free T4, and free T3 should be read in the context of energy availability, weight trajectory, symptoms, training load, and overall clinical status.
Proposed Mechanisms Behind Low TSH in Athletes
The evidence base varies substantially across the proposed mechanisms, and all should be understood as hypotheses or partially supported models rather than established causal chains.
Mechanism 1: Leptin-Related Hypothalamic Effects
One proposed mechanism involves leptin, energy homeostasis, and hypothalamic regulation. The authors of the Olympic athlete study suggest that long-term training promotes a catabolic state with hormonal feedback from depleted adipose tissue, inducing decreased leptin levels; these in turn may suppress hypothalamic function and reduce central stimulation of TSH, resulting in reduced values of TSH, fT3, and fT4 — described as a possible energy-saving mechanism in exercising individuals [1]. The authors explicitly note this model is proposed rather than causally established in their retrospective cross-sectional data [1].
Baylor and Hackney observed thyroid and leptin changes in a 20-week exercise training study of collegiate female athletes [2]. In the (–) responder rowers (n=10), fT3 concentration decreased (P<0.05) from baseline during an intense training period — at 5 weeks by –28.2 (6.2)% and at 10 weeks by –24.9 (7.9)% — then returned towards baseline levels (20 weeks compared to baseline, P>0.05). Similar changes (P<0.05), at comparable times, were noted for leptin and TSH concentrations in the (–) responder rowers [2]. The mechanism is unclear; the authors speculate that “the decrease in concentrations of TSH and fT3 could be attributable to a lower hypothalamic-pituitary signaling action, and this is related to the decreased leptin concentrations, and could represent a possible means of energy conservation in these exercising women” [2]. Importantly, the non-responder rowers (n=7) and control subjects displayed no significant hormone changes over the 20 weeks [2] — this was not a universal response.
In my view, this pattern is not uniquely athletic, but more broadly related to energy availability. The leptin-related explanation makes the most physiological sense to me: when energy intake, fat mass, or overall energy availability falls, the body may begin to conserve energy, and the thyroid axis can shift in the same direction.
I recognise this from my own athletic background as well. During periods of significant weight cutting, weight loss can sometimes slow despite continued dieting. Athletes often describe this as “energy-saving mode.” It is not a precise medical diagnosis, but it captures an important clinical idea: the body is not just a passive calorie calculator. When energy availability remains low, hormonal systems may shift toward conservation rather than further expenditure.
That is why I interpret lower-normal TSH, free T4, and free T3 in context. The key question is often not only “Is the thyroid abnormal?” but also “Is this happening alongside heavy training, weight loss, under-fuelling, or poor recovery?”
Mechanism 2: T4-to-T3 Conversion in Skeletal Muscle — Animal Evidence
A second proposed mechanism involves the type 2 deiodinase enzyme (D2), which catalyzes local conversion of thyroxine (T4) into triiodothyronine (T3) within skeletal muscle. In animal models (rats and mice), acute treadmill exercise increased skeletal muscle D2 expression through a β-adrenergic receptor-dependent mechanism, and the accelerated conversion of T4 to T3 within myocytes mediated part of the PGC-1α induction by exercise and its downstream effects on mitochondrial function [3]. These findings come from rodent studies; whether a similar mechanism contributes to lower circulating thyroid hormone levels in human athletes has not been established.
Mechanism 3: TSH Patterns in Physically Active Adults
A large cross-sectional NHANES analysis of 5,877 American adults found that with increasing physical activity, TSH, FT4, and TT4 levels showed an overall decreasing trend (p=0.002; p<0.001; p<0.0001), while FT3 and TT3 levels gradually increased (p<0.0001; p<0.0001) [4]. Within the range of PAM <5,000 MET-minutes/week, the relationship between physical activity and TSH showed an L-shaped curve [4]. This is a cross-sectional general population study, not an athlete cohort; causal interpretation and direct extrapolation to competitive athletes require caution [4].
Interpreting Low TSH in Athletes: Practical Approach
Clinical practice guidelines are not definitive in regard to what classifies a patient as having hypothyroidism in the athletic context [5]. The Di Gioia Olympic athlete data provides a useful sport-specific distribution: endurance athletes had a mean TSH of 1.93 ± 0.7 uUI/mL, shifted distinctly left compared to mixed-sport athletes (2.18 ± 0.8 uUI/mL; E vs. M, p <0.0001) [1]. A value that appears low-normal in a population context may be entirely typical within the endurance athlete distribution.
In practice, evaluating low TSH in athletes involves:
- Measuring TSH, free T3, and free T4 together — not TSH in isolation
- Considering training phase: post-peak training load versus off-season
- Evaluating nutritional status and caloric intake
- Looking for trends over serial measurements rather than single-point values
- Recognising that subclinical primary hypothyroidism is characterised by an elevated TSH with normal free T4 — opposite to the pattern discussed here
Patterns worth noting in clinical context: Low TSH in athletes + Low fT3 + Normal or low fT4 → Pattern associated with central or hypothalamic changes; evaluate energy availability; consider RED-S in the clinical context
One important caveat is that this discussion applies only when genuine thyroid disease is not suspected. If the clinical picture raises concern for hyperthyroidism or hypothyroidism, those conditions should be evaluated on their own merits, and too much emphasis should not be placed on athlete-specific thyroid patterns simply because the patient happens to train regularly.
From a clinical perspective, I see this phenomenon as more of a physiological curiosity than a finding with major standalone clinical implications. It may reflect reduced energy availability or broader energy-conservation mechanisms, but it does not usually drive medical decision-making on its own. Low energy availability, RED-S, or under-fuelling are not diagnosed from thyroid hormone levels, and thyroid results are typically only one small piece of a much larger clinical picture.
In my experience, this may partly explain why many clinicians are unfamiliar with the pattern. It is scientifically and physiologically interesting, but it rarely changes management. At most, it may provide additional context that supports an assessment already based on symptoms, nutritional status, training history, body-weight changes, and overall clinical evaluation.
When Low TSH in Athletes Warrants Clinical Attention
Low TSH in athletes with no symptoms and stable performance is generally not a primary clinical concern based on the current evidence [1][5]. However, the same values in an athlete with fatigue, declining performance, or signs of energy deficiency raise a different clinical question.
The RED-S literature describes how low energy availability can affect multiple hormonal axes simultaneously. In that context, low TSH in athletes may coexist with changes in testosterone (HPG axis), cortisol (HPA axis), and markers of overreaching such as elevated resting heart rate. When multiple axes show concurrent changes, low TSH in athletes is no longer a single isolated observation. Whether and to what extent restoring energy availability normalises these values is a clinical question that goes beyond the data in the current reference set [6].
Athlete status alone is not usually a reason to measure TSH or thyroid hormones. In practice, these tests are more commonly ordered as part of a broader medical assessment, a periodic health check, or an evaluation for symptoms where thyroid disease is part of the differential diagnosis. Being an athlete may affect how the results are interpreted, but it does not by itself make thyroid testing necessary.
The same principle applies when TSH is genuinely low. I would not treat an abnormal TSH differently simply because the patient is an athlete. If TSH is suppressed, the next question is whether free T4 and/or free T3 are high or high-normal, and whether the clinical picture suggests hyperthyroidism. If true hyperthyroidism is suspected, it should be evaluated and managed according to the same clinical principles used in the general population.
In other words, the athlete-specific lower-normal thyroid pattern is useful context, not an exemption from standard medical reasoning. It can help prevent overinterpretation of borderline findings, but it should not be used to dismiss genuinely abnormal thyroid results.
Conclusion: Low TSH in Athletes
Low TSH in athletes is best understood as a context-dependent finding, not a diagnosis by itself. In endurance athletes, TSH, free T4, and free T3 may sit toward the lower end of the reference range, and observational data suggest this pattern is more common in endurance disciplines than in some other sports. One plausible explanation is reduced energy availability and leptin-related hypothalamic signalling, but the current evidence should still be interpreted as associative rather than proof of a single causal pathway.
Clinically, the most important point is not to overinterpret the finding in either direction. A lower-normal thyroid profile in a well-performing athlete may simply provide useful physiological context, especially if training load, nutrition, and recovery explain the broader picture. At the same time, athlete status should not be used to dismiss genuinely abnormal thyroid results. If the clinical picture suggests hyperthyroidism, hypothyroidism, pituitary disease, or another medical condition, the evaluation should follow standard clinical principles.
In my view, this makes low TSH in athletes a useful but limited observation. It may help explain why some trained or under-fuelled individuals show a slightly different thyroid pattern, but it does not diagnose RED-S, overtraining, or thyroid disease on its own. The practical value lies in interpreting TSH together with free T4, free T3, symptoms, body-weight changes, energy intake, training history, and recovery status — not in treating one laboratory number as the whole story.
Bibliography
[1] https://doi.org/10.3390/biomedicines12071530
[2] https://doi.org/10.1007/s00421-002-0737-7
[3] https://doi.org/10.1113/JP272440
[4] https://doi.org/10.1186/s12889-024-18768-4
