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The lowered testosterone responses seen in periods of overtraining could be due to an exposure to regular elevations of exercise-induced cortisol from the daily training sessions. The blunted cortisol alterations seen with overtraining may be a protective mechanism for the body, when it is repeatedly exposed to increased cortisol levels. In conclusion, the up-to-date data on the effect of exercise on serum testosterone concentrations in men have significant inter-individual and inter-study variability. Multiple studies found a direct link between post-exercise serum testosterone changes and muscular hypertrophic adaptation/increase in lean body mass . Whether this temporary surge in post-training serum testosterone concentrations has any impact on the extent of muscle anabolism and hypertrophy is widely debated. Based on the current literature, no conclusions can be drawn on the effect of exercise on basal serum testosterone concentrations in older men.
In this study, 10 athletes with documented reductions in performance performed this test and then followed to monitor for performance recovery. While a short ~ 30-minute rise in testosterone is often observed after exercise, T levels go back to baseline quickly and are not generally impacted by long-term training. The testosterone-to-cortisol ratio (the most common overtraining biomarker in coaching) moved in the opposite direction from what the model would predict.14 Specifically, testosterone levels increased and cortisol levels decreased. A systematic review of 22 resistance training overtraining studies found that 10 of them reported zero performance decline under deliberately imposed overload.
As males age, the amount of total and free forms of testosterone in the circulation change as does SHBG (see Figure 2) leading to a gradual overall reduction in the hormone forms in the blood; see subsequent section for discussion on the phenomena of andropause in males. For example, Table 2 presents the reference values reported by the Mayo Clinic (14), as well as from the innovative study by Travison et al. which attempted to develop harmonized reference values of testosterone for wide clinical use (15). Testosterone production is controlled by the hypothalamic -pituitary-gonadal (HPG) regulatory axis which involves the hormones gonadotrophin-releasing hormone (GnRH), luteinizing hormones (LH), and follicle-stimulating hormones (FSH). Testosterone production is regulated by the hypothalamic-pituitary-gonadal (HPG) axis which involves the hypothalamic hormone gonadotrophin-releasing hormone (GnRH), and the pituitary hormones luteinizing hormones (LH), and follicle-stimulating hormones (FSH) (see Figure 1).
12  Daily training frequency did not produce apparent overtraining, though the finding that only the volume arm got bigger does warrant a separate discussion about what drives hypertrophy. While this is a small case series, the findings are hard to reconcile with standard overtraining concerns regarding lifting heavy.10 In another study, three competitive strength athletes attempted a one-rep max squat every single day for 30 consecutive days, plus additional volume at submaximal loads. Think of how challenging it would be to get someone to exercise –which is one of the best treatments for both conditions – if they have been told their body is falling apart. Regardless of its accuracy,  some individuals will respond to negative information by performing worse or engaging in negative behaviors like skipping a training session. Similarly, many influencers on social media will suggest overtraining means "I trained a lot and feel bad." In sports medicine, it refers to a diagnosis of exclusion with specific criteria.
It should be noted, for some clinicians and medical professional groups, hypogonadism is characterized by not just low testosterone but includes at least one clinical sign or symptom (9). Furthermore, the clinical definition of what constituents "low testosterone" and the diagnostic threshold for hypogonadism diagnosis varies too. These authors did an exhaustive examination of the available research literature as well as the extensive database from the International Association of Athletics Federation (IAAF) on athletes who have competed over many years at elite levels in track and field (i.e., athletics). A depiction of the typical changes observed in total and free testosterone as well as sex hormone-binding globulin over the course of a male lifespan.
Additionally, testosterone exhibits positive effects on erythropoiesis and hemoglobin concentrations (16). Many aspects of the above influences affect the male physiology advantageous for sporting performance. The pubertal effects also include enlargement of the sebaceous glands, penis enlargement, increased libido, increased frequency of erections, increased muscle mass development, deepening of the voice, increased height, bone maturation, loss of scalp hair, and growth of facial, chest, leg, and axillary hair. During this time a multitude of physiological changes occur; e.g., body odor develops, oiliness of the skin and hair increase, acne develops, accelerated growth spurts occur, and pubic, early facial, and axillary hair grow. The free and albumin-bound forms of testosterone constitute what is referred to as bioavailable testosterone (i.e., able to interact with androgenic receptors at target tissues).
Aubry et al. compared athletes acutely fatigued without decrement of performance and FOR athletes, and catecholamines were found to be reduced in FOR compared to the non-FOR fatigued subjects. It was also observed that studies with previously affected athletes tend to show more pronounced altered responses than studies that induces FOR/NFOR, perhaps due to the above-mentioned possible difference between FOR (when overload is performed, a FOR/NFOR state is induced, not OTS) and OTS (most studies with previously affected subjects tended to include OTS, not FOR/NFOR athletes). Additionally, once basal levels are altered, a diagnosis of OTS/NFOR/FOR is unlikely, as these conditions should not be diagnosed in the presence of endocrine alterations 1, 4, 28. TRH stimulation test for thyroid function and LHRH stimulation test for ovary function are able to show depletion of these glands prior to basal levels as well. In fact, severe adrenal insufficiency and final stage of diabetes are required to present basal hypocortisolism and fasting hyperglycemia with inappropriate hypoinsulinemia, respectively.
A 2024 systematic review found that nocebo effects in sport and exercise were roughly twice the magnitude of placebo effects on performance across 20 studies.3 Language about fatigue states produces real physiological effects. The flawed stress-recovery-adaptation model is not the only source of confusion — the word overtraining itself is doing at least four different jobs simultaneously. Overtraining Syndrome (OTS) is defined as a persistent decrease in performance resulting from exercise that lasts for months to potentially years.
The effect of exercise on basal serum testosterone concentrations in obese individuals has been evaluated in multiple studies. Moreover, the previously mentioned study by Hansen et al. showed unchanged resting testosterone concentrations during unilateral biceps curl exercise alone or in combination with bilateral knee extensions and leg press. Thus, although low basal testosterone concentration is likely due to HPA suppression and hypogonadotropic hypogonadism during chronic exercise, additional contributing factors affecting the serum testosterone concentrations in the absence of LH suppression are yet to be determined.

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