5.11 Hormones and the Brain: The Neuroendocrine Interface

Executive summary. The endocrine system and the nervous system are not two systems. The hypothalamus sits at the junction, converting neural signals into hormonal ones, and every major hormonal axis feeds back onto the brain that controls it. This matters for athletes in a specific way: Cortisol, testosterone, oestradiol and thyroid hormone all have receptors in the hippocampus, amygdala and cortex, so disruption of any axis produces psychological symptoms before it produces obvious physical ones. Low mood, poor sleep, flat drive and impaired concentration are commonly the first signs of relative energy deficiency or overreaching, not the last. This article covers the shared architecture of an endocrine axis, the four axes most relevant to training, the central actions of each hormone, and what can and cannot be inferred from a blood test.

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Key takeaways

1. All the major axes share one architecture: Hypothalamic releasing hormone, pituitary trophic hormone, target gland hormone, negative feedback. Learn it once.
2. Hormones are released in pulses and follow daily rhythms. A single blood measurement without a timestamp is close to uninterpretable.
3. Cortisol is not a villain. It is essential, it follows a steep daily rhythm, and it is the acute stress response that enables training adaptation.
4. Low energy availability suppresses the reproductive and thyroid axes in both sexes. This is the mechanism behind relative energy deficiency in sport, and it affects the brain directly.
5. Acute post-exercise hormone spikes are poor predictors of adaptation. Chronic patterns matter far more than the size of a post-session testosterone bump.
6. Because these hormones act on the hippocampus, amygdala and prefrontal cortex, endocrine problems typically present as mood, sleep and cognition changes first.

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Beginner section: One pattern, four systems

Hormonal systems look intimidating because of the acronyms. Underneath them there is a single repeating pattern, and once you have it, all four systems in this article become the same idea with different names.

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The pattern has four steps. The hypothalamus, a small structure at the base of the brain, sends a chemical instruction to the pituitary gland just below it. The pituitary sends a different chemical instruction into the bloodstream. That instruction reaches a gland — adrenal, testis or ovary, thyroid — which releases the hormone that actually does the work. And then that final hormone travels back and tells the hypothalamus and pituitary to ease off. That last step is negative feedback, and it is what keeps the system stable.

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Now the part that gets left out of most training discussions. These hormones do not only act on muscle, bone and fat. They act on the brain, because the brain is full of receptors for them. Cortisol receptors are dense in the hippocampus, which is central to memory. Testosterone and oestradiol receptors are found throughout the hypothalamus, amygdala and cortex. Thyroid hormone sets the metabolic rate of neurons themselves.

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This is why the earliest signs that something is wrong with an athlete’s endocrine state are usually psychological. Sleep goes first, then mood, then motivation, then concentration. Strength often holds up for weeks after those changes have begun, which is exactly why relying on performance as your only monitoring tool means finding out late.

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The other thing worth internalising early is that these hormones move in rhythms. Cortisol is high in the morning and low at night, by design. Testosterone in men peaks in the early hours. So a blood test result means very little unless you know what time it was taken, and comparing your morning number to a friend’s afternoon number tells you nothing at all.

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Advanced section: The four axes, their central actions, and their failure modes

The four axes

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• hypothalamic-pituitary-adrenal (HPA) axis. Corticotropin-releasing hormone from the paraventricular nucleus drives adrenocorticotropic hormone from the anterior pituitary, which drives cortisol from the adrenal cortex (Herman et al., 2016). Cortisol acts on two receptor types: Mineralocorticoid receptors, which have high affinity and are largely occupied at basal levels, and glucocorticoid receptors, which have lower affinity and are recruited during stress. This two-receptor arrangement is why cortisol’s effects on memory and cognition are non-linear (de Kloet et al., 2005).
• HPG axis. Gonadotropin-releasing hormone is secreted in pulses, driving luteinising and follicle-stimulating hormone, which drive gonadal steroid production. Pulsatility is essential: Continuous GnRH exposure paradoxically suppresses the axis, which is the basis of certain clinical treatments. Kisspeptin neurons in the hypothalamus are the principal upstream regulator and are highly sensitive to energy status (Clarke et al., 2015).
• HPT axis. Thyrotropin-releasing hormone drives thyroid-stimulating hormone, which drives thyroxine (T4) and to a lesser extent triiodothyronine (T3). Most active T3 is produced by peripheral deiodination of T4. In prolonged energy deficit, T3 falls while TSH may remain normal — the pattern sometimes described as low-T3 syndrome (Melmed et al., 2019).
• GH / IGF-1 axis. Growth hormone release is driven by growth hormone-releasing hormone and restrained by somatostatin, and is strongly pulsatile with the largest pulses during slow-wave sleep (Van Cauter et al., 2000). Most of its anabolic action is mediated by insulin-like growth factor 1, largely hepatic. The dependence on deep sleep is the clearest link between sleep quality and tissue remodelling.

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What these hormones do inside the brain

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Cortisol has the best-characterised central actions. Acutely, moderate elevations enhance the consolidation of emotionally salient memory, which is adaptive: Events that mattered should be remembered. At high or sustained levels the picture reverses, with impaired retrieval and impaired prefrontal function (de Kloet et al., 2005). Chronic exposure in animal models is associated with dendritic atrophy in the hippocampus and dendritic growth in the amygdala — a structural shift towards threat sensitivity (McEwen, 1998). McEwen’s allostatic load framework, covered fully in Article 5.13, is the standard way of organising these findings.

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Testosterone acts both directly through androgen receptors and, after local aromatisation, through oestrogen receptors in the brain. Its behavioural effects are more context-dependent than popular accounts suggest: Administration studies find effects on risk-taking, social dominance behaviour and threat vigilance rather than a simple increase in aggression (Eisenegger et al., 2011). Clinically low testosterone is genuinely associated with low mood, poor drive and impaired recovery, which is a different claim from the idea that raising already-normal levels improves those things.

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Oestradiol is neuroprotective in several models and supports hippocampal synaptic density and verbal memory (Brinton, 2009). Its fluctuation across the menstrual cycle has measurable effects on sleep, thermoregulation, mood and, in some studies, injury risk. Female athletes are substantially under-represented in sports science research, and this is one of the areas where that gap most affects practice.

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Thyroid hormone sets neuronal metabolic rate. Both excess and deficiency present prominently as psychiatric and cognitive change: Hypothyroidism with slowed cognition, low mood and cold intolerance; hyperthyroidism with anxiety, tremor and insomnia (Melmed et al., 2019). Thyroid dysfunction is common, treatable, and frequently mistaken for overtraining or depression, which is a good reason for it to be on the differential list when an athlete deteriorates without explanation.

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Energy availability: The variable that moves everything at once

The single most important applied idea in this chapter is that these axes are not independent. Low energy availability — insufficient dietary energy relative to exercise energy expenditure — suppresses the reproductive axis, lowers T3, alters cortisol dynamics and impairs growth hormone signalling together (Loucks et al., 2011).

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This is the physiology behind relative energy deficiency in sport, which was formalised by the International Olympic Committee and explicitly broadened from the earlier female athlete triad to include male athletes. Documented consequences span menstrual dysfunction, reduced bone mineral density, impaired immune function, gastrointestinal problems, cardiovascular changes, and impaired mood and concentration (Mountjoy et al., 2018).

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The mechanism at the top of the chain is worth knowing. Kisspeptin neurons integrate metabolic signals including leptin and, when energy availability is low, reduce GnRH pulsatility (Clarke et al., 2015). This is a deliberate feature rather than a malfunction: Reproduction is suspended when energy is scarce. It is also why loss of menstrual regularity is a signal to be taken seriously rather than treated as a convenient side effect of being lean, and why a similar suppression occurs in men presenting as low libido, low drive and poor recovery.

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If any of this pattern describes you or an athlete you work with, the appropriate step is assessment by a physician and a sports dietitian. It is a medical situation and not one to manage by adjusting programme variables.

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What you can and cannot learn from a blood test

Hormone testing is widely marketed to athletes and widely misread. Four points cover most of the errors.

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1. Timing dominates. Cortisol varies several-fold across the day and testosterone substantially. A number without a collection time is not interpretable.
2. Pulsatility matters. GnRH, LH and growth hormone are secreted in pulses. A single draw may catch a peak or a trough of a normal pattern.
3. Reference ranges are population ranges. They describe where most people fall, not where you personally function best, and being inside the range does not exclude a problem while being marginally outside it does not establish one.
4. Acute post-exercise spikes predict little. The magnitude of an acute testosterone or growth hormone response to a session has repeatedly failed to predict hypertrophy or strength gain (West & Phillips, 2012). Chronic patterns are the meaningful signal.

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What is genuinely useful is trend data collected consistently — same time of day, same conditions, interpreted by a clinician alongside symptoms, training load and energy intake. What is not useful is a one-off panel bought online and interpreted against internet reference points.

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Practical section: Monitoring and protecting the axes

Almost nothing in this chapter should lead to a supplement or a hormonal intervention. Most of it leads to monitoring and to protecting sleep and energy intake.

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• Monitor the psychological markers. Sleep quality, mood, drive and concentration change earlier than performance does. A simple daily rating of each, tracked for weeks, is a better early-warning system than any blood panel available to most athletes.
• Protect energy availability first. Every axis in Figure 2 is sensitive to it. Under-eating relative to training load is the most common cause of endocrine disruption in athletes, and it is fixable.
• Treat sleep as an endocrine intervention. The largest growth hormone pulses occur in slow-wave sleep, and sleep restriction alters cortisol dynamics and reduces testosterone (Van Cauter et al., 2000; Leproult & Van Cauter, 2011). This is the highest-leverage change available and it costs nothing.
• Do not chase acute hormonal responses. Designing sessions to maximise a post-workout testosterone or growth hormone spike is not supported. Design for mechanical tension, volume and progressive overload instead.
• Take menstrual changes seriously. Loss or irregularity of menstruation in an athlete is a signal of possible energy deficiency and warrants medical assessment, not reassurance.
• Escalate to a clinician early. Unexplained fatigue, persistent low mood, cold intolerance, loss of libido or unexplained performance decline all belong in a medical consultation. Thyroid dysfunction, iron deficiency, sleep apnoea and depression are all common and treatable.

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This article is educational and is not medical advice. Hormonal interventions, including testosterone therapy and thyroid medication, are clinical decisions with real risks and require a doctor. Anabolic steroid use in particular suppresses the natural axis via the feedback loop in Figure 1, and that suppression can be prolonged.

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Sport applications

• Endurance sports with lean physique pressure. Highest risk of relative energy deficiency. Screen energy availability actively rather than waiting for symptoms.
• Weight-class and aesthetic sports. Repeated cutting cycles perturb thyroid and reproductive axes. Plan recovery phases with the same seriousness as the cut.
• Team sports with congested schedules. Sleep restriction is the dominant endocrine stressor. Travel and late fixtures are hormonal variables, not just logistical ones.
• Female athletes across the cycle. Oestradiol and progesterone fluctuations affect sleep, thermoregulation and perceived effort. Tracking symptoms is more useful than assuming a universal pattern.
• Masters athletes. Age-related changes in the GH and gonadal axes are real, and recovery capacity changes with them. This calls for adjusted frequency rather than hormonal intervention.

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Common mistakes

• Treating cortisol as purely harmful. It is essential, follows a steep daily rhythm, and the acute rise is part of how training produces adaptation.
• Interpreting a single untimed blood test. Timing and pulsatility dominate these measurements. A number without context is not information.
• Designing training to maximise hormone spikes. Acute post-exercise responses have repeatedly failed to predict long-term adaptation.
• Ignoring lost menstrual cycles in lean athletes. This is a warning sign of energy deficiency with bone and cardiovascular consequences, not a benign feature of being lean.
• Mistaking thyroid dysfunction for overtraining. The presentations overlap substantially, and one of them is diagnosable and treatable with a blood test.
• Assuming more testosterone is better within the normal range. The association between low testosterone and poor mood does not imply that raising normal levels improves performance, and exogenous use suppresses the natural axis.

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Coaching cues

• Track sleep, mood, drive and focus. They change before performance does.
• Eat enough for the training you are actually doing.
• Protect deep sleep, because that is when the growth hormone pulses happen.
• Never interpret a hormone number without knowing the time it was drawn.
• A lost menstrual cycle is a medical conversation, not a training one.
• Unexplained decline in performance deserves a blood test before a programme change.

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FAQs

Does training raise testosterone enough to matter?

Resistance training produces acute post-exercise increases in testosterone and growth hormone, but the size of those acute responses has repeatedly failed to predict subsequent gains in muscle size or strength. Chronic resting concentrations shift much less than popular accounts suggest in already-trained people. The practical implication is to programme for mechanical tension, volume and progression rather than for hormonal spikes.

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Is cortisol bad for muscle growth?

Cortisol is catabolic in isolation, but the acute rise during and after training is part of a normal adaptive response and is not something to suppress. The problem is chronic elevation combined with insufficient recovery, which is better addressed through sleep, energy intake and load management than through any attempt to blunt cortisol directly. Blunting the acute response is not a desirable goal.

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What is RED-S and how would I know if I had it?

Relative energy deficiency in sport describes impaired physiological function resulting from insufficient energy availability relative to exercise expenditure, affecting reproductive, bone, metabolic, immune, cardiovascular, gastrointestinal and psychological systems. Common signals include loss or irregularity of menstruation, low libido, recurrent illness or injury, poor sleep, low mood, cold intolerance and stalled performance despite training. It requires assessment by a physician and a sports dietitian, and it is not something to self-diagnose or self-treat.

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Why does my sleep get worse when training load is high?

Several mechanisms operate together: Elevated evening cortisol and sympathetic activity, higher core temperature, and in some athletes inadequate energy intake, which is itself associated with disrupted sleep. Because the largest growth hormone pulses occur in slow-wave sleep, this becomes self-reinforcing: High load disturbs sleep, and disturbed sleep impairs recovery. Reducing evening training intensity, increasing carbohydrate availability and protecting sleep timing all help.

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Should I get a hormone panel done?

If you have symptoms — persistent fatigue, low mood, loss of libido, menstrual changes, unexplained performance decline — then yes, through a doctor who will interpret it alongside your history rather than in isolation. As routine optimisation in an asymptomatic athlete, the value is low and the risk of over-interpreting normal variation is high.

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Do hormones explain differences between male and female athletes?

They contribute, but the picture is more complex than a single-hormone explanation and the research base is uneven, since female athletes have historically been substantially under-represented in sports science studies. Oestradiol and progesterone fluctuations across the menstrual cycle do have measurable effects on thermoregulation, sleep, perceived effort and possibly connective tissue properties, but individual variation is large and blanket cycle-based prescriptions are not well supported.

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Recommended videos

Each video below was chosen because it covers a specific part of this article in more depth than text alone allows.

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Related reading on FitXplor

• 5.10 Amino Acids and the Brain
• 1.7 Recovery Science and Adaptation
• 1.4 Exercise Physiology and Energy Systems
• 5.5 The Athlete’s Brain
• 5.6 Neurotransmitters and Neuromodulators

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References

Melmed, S., Auchus, R. J., Goldfine, A. B., Koenig, R. J., & Rosen, C. J. (Eds.). (2019). Williams Textbook of Endocrinology (14th ed.). Elsevier.

McEwen, B. S. (1998). Stress, adaptation, and disease: allostasis and allostatic load. Annals of the New York Academy of Sciences, 840, 33–44.

Herman, J. P., McKlveen, J. M., Ghosal, S., et al. (2016). Regulation of the hypothalamic-pituitary-adrenocortical stress response. Comprehensive Physiology, 6(2), 603–621.

de Kloet, E. R., Joëls, M., & Holsboer, F. (2005). Stress and the brain: from adaptation to disease. Nature Reviews Neuroscience, 6(6), 463–475.

Mountjoy, M., Sundgot-Borgen, J. K., Burke, L. M., et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine, 52(11), 687–697.

Loucks, A. B., Kiens, B., & Wright, H. H. (2011). Energy availability in athletes. Journal of Sports Sciences, 29(Suppl 1), S7–S15.

Clarke, H., Dhillo, W. S., & Jayasena, C. N. (2015). Comprehensive review on kisspeptin and its role in reproductive disorders. Endocrinology and Metabolism, 30(2), 124–141.

Van Cauter, E., Leproult, R., & Plat, L. (2000). Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels. JAMA, 284(7), 861–868.

Leproult, R., & Van Cauter, E. (2011). Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA, 305(21), 2173–2174.

West, D. W. D., & Phillips, S. M. (2012). Associations of exercise-induced hormone profiles and gains in strength and hypertrophy. European Journal of Applied Physiology, 112(7), 2693–2702.

Eisenegger, C., Haushofer, J., & Fehr, E. (2011). The role of testosterone in social interaction. Trends in Cognitive Sciences, 15(6), 263–271.

Brinton, R. D. (2009). Estrogen-induced plasticity from cells to circuits. Trends in Pharmacological Sciences, 30(4), 212–222.