Testosterone, IGF-1, and HRT: How Hormones Impact Endurance Training

Endurance athletes count everything: pace, VO₂ max, sleep hours, even grams of carbs consumed before a long run. 

But what about hormones? 

These chemical messengers don't show up on a smartwatch, yet they quietly dictate how well athletes recover, adapt, and perform. 

Hormones like testosterone, estrogen, cortisol, IGF-1, and growth hormone act as the body's internal coaches. They regulate muscle repair, mood, and stamina. 

If they're out of sync, athletes risk fatigue, overtraining syndrome, or even stress fractures.

Consider this: 16.5% of male endurance athletes show testosterone levels low enough to qualify as deficient, despite being otherwise healthy and fit, that’s nearly one in six.  

Female athletes are similarly affected: a study of elite British track and field athletes found about 30% reported menstrual irregularities, and 4% were amenorrheic, even among athletes performing at the highest levels. 

So, do hormones affect athletic performance? The short answer is yes. 

The better question is: how can athletes and their clinicians track and manage these shifts before performance suffers? 

This is where personalized lab testing and biomarker tracking, like the advanced hormone panels at Access Medical Labs, become tools for modern endurance sports athletes. 

 

Key Takeaways 

• Hormones drive endurance performance. Testosterone, estrogen, cortisol, IGF-1, and growth hormone regulate energy, muscle repair, mood, and recovery. 

• Training can suppress hormone levels. Long-term endurance training often lowers testosterone, disrupts estrogen cycles, and elevates cortisol, all of which impact performance. 

• Male and female athletes face different risks. Men are prone to low testosterone, while women may experience menstrual cycle disruptions and RED-S (Relative Energy Deficiency in Sport). 

• Hormone Replacement Therapy (HRT) is not a shortcut. It can help restore deficiencies, but it carries medical risks and regulatory restrictions for athletes. 

• Lab testing reveals hidden imbalances, enabling personalized training. Advanced hormone panels enable athletes and clinicians to make more informed decisions about training, nutrition, and recovery. 

Here's what you'll find in this article:

• How Hormonal Imbalance Affects Endurance Athletes 
• The Hidden Role of Testosterone in Endurance Sports 
• The Role of IGF-1 in Recovery and Muscle Health 
• Hormone Replacement Therapy (HRT) in Athletes 
• The Cortisol Question: Stress, Recovery, and Overtraining 
• Why Endurance Athletes Should Track Hormones, Not Just Miles 

How Hormonal Imbalance Affects Endurance Athletes

When training stress outweighs recovery, hormones falter. 

Prolonged training stress elevates cortisol, which can blunt testosterone, interfere with sleep, and reduce glycogen replenishment. 

This is why athletes can be training more and adapting less, systemic inflammation blocks the gains. 

Dr. Ara Suppiah, MD, renowned Sports Medicine Physician and trusted consultant for elite athletes, explained, “The more inflamed you are, the less you’re able to absorb training and recovery.” 

Female Athletes: Menstrual Cycles, Bone Health, and RED-S 

Among female endurance athletes, hormonal imbalance often shows up through menstrual disruption, including oligomenorrhea (infrequent periods) or amenorrhea (absence of menstruation).  

These correlate with significant risks to bone mineral density (BMD).  

For example, runners and other endurance-trained women with menstrual dysfunction frequently have lower BMD compared to their eumenorrheic peers.  

“Complex high turnover” bone metabolism has been observed in elite female long-distance runners who report menstrual disorders: increased markers of bone absorption (like NTx) together with bone formation markers, yet poorer bone quality overall.  

Low energy availability (LEA), which can stem from training volume, travel, or inadequate fueling, is a common driver of these effects and increases the likelihood of functional hypothalamic amenorrhea.  

Male Athletes: Testosterone Suppression & Energy Deficit

In male endurance athletes, hormonal disruption also happens, though it often gets less attention.  

Studies point to chronically lowered resting testosterone levels relative to age-matched sedentary men in those who train rigorously.  

For example, long-term endurance runners with low energy availability (LEA) showed about a 30% reduction in testosterone compared to less heavily trained peers.  

One cross-sectional study of 24 elite distance runners found that roughly 40% had low testosterone and also had significantly more bone stress injuries than athletes with normal testosterone.  

Additionally, highly trained male endurance athletes frequently present with reduced metabolic rate, suppressed IGF-1, and other hormonal shifts during periods of high training volume coupled with insufficient fueling.  

Where Women And Men Athletes Overlap 

Both male and female endurance athletes can experience similar hormonal disruptions, including low energy availability, increased cortisol, and a suppressed reproductive axis, but the presentation differs.  

Women are more likely to show menstrual irregularities and compromised bone density (especially after prolonged amenorrhea), while men more often exhibit low testosterone, reduced libido, or decreased muscle recovery. 

Research on Relative Energy Deficiency in Sport (RED-S) emphasizes that both sexes experience health and performance consequences when energy intake doesn’t match energy demand.  

For example, a recent study of Irish male endurance athletes showed 76.9% with energy availability below ~30 kcal/kg lean body mass/day, along with endocrine disruptions like low IGF-1 and elevated markers of bone turnover.  

Interventions tend to differ: for women, restoring menstrual regularity via nutrition, weight, and reducing stress; for men, careful adjustment of training load and monitoring testosterone and metabolic hormones.  

Understanding both sexes’ patterns helps clinicians personalize tests, monitor biomarkers early, and steer recovery and performance strategies. 

Why biomarker testing matters for athletes

The Hidden Role of Testosterone in Endurance Sports

Testosterone is often associated with strength athletes, but it plays a quiet, powerful role in endurance sports. 

Adequate testosterone supports the production of red blood cells, oxygen transport, and lean muscle mass. These are all critical for long-distance athletes. 

Yet paradoxically, endurance training itself can lower testosterone. 

A 2016 review found that male endurance athletes often present with testosterone levels lower than those of sedentary men, a condition linked to decreased energy and mood disturbances. 

Anthony C. Hackney, PhD, University of North Carolina, Chapel Hill, shared, "Endurance athletes may paradoxically face testosterone levels lower than non-athletes, despite training at elite levels." 

Low testosterone saps performance and impairs recovery, reduces motivation, and increases injury risk.  

That's why clinicians working with athletes should monitor androgen levels regularly, especially during high-volume training cycles.  The main androgen is testosterone, but the category also includes dihydrotestosterone (DHT), androstenedione, and dehydroepiandrosterone (DHEA), all of which play roles in muscle development, energy, and overall athletic function. 

Why biomarker testing matters for athletes

The Role of IGF-1 in Recovery and Muscle Health

IGF-1 (Insulin-like Growth Factor 1) is a major player in post-training adaptation. 

It regulates protein synthesis, helps repair microtears in muscle, and influences bone density, making it critical for athletes who stress the same tissues day after day. 

Healthy IGF-1 levels allow athletes to bounce back faster, adapt to training loads, and maintain metabolic flexibility. 

But when IGF-1 dips, it is often due to overtraining or under-fueling. That's when athletes may experience persistent soreness, fatigue, or stalled progress. 

As Victor Owino, PhD, explains, "IGF-1 plays a direct role in skeletal muscle hypertrophy and regeneration, and inadequate levels impair recovery from training stress." 

Why biomarker testing matters for athletes

Hormone Replacement Therapy (HRT) in Athletes

When athletes present with clinically confirmed hormone deficiencies, HRT may be considered. 

Restoring testosterone or estrogen can improve energy, stabilize mood, and reduce fatigue. 

However, HRT isn't a shortcut. The risks are real: cardiovascular strain, lipid changes, and compliance issues with sports regulators. 

As Shalender Bhasin, MD, Brigham and Women's Hospital, emphasizes: 

"Testosterone therapy should be reserved for those with clinically confirmed deficiency, as improper use carries significant risks." 

For competitive athletes, regulatory restrictions also come into play. 

Governing bodies may prohibit or closely monitor HRT use, making biomarker testing the first line of decision-making. 

For practitioners, the decision around HRT is about treating deficiencies safely and ethically. 

Why biomarker testing matters for athletes

The Cortisol Question: Stress, Recovery, and Overtraining

Cortisol is the body's primary stress hormone. 

During exercise, it mobilizes energy and supports endurance. 

But when cortisol stays elevated, due to insufficient recovery, poor sleep, or under-fueling, it can backfire, leading to chronic fatigue and impaired recovery. 

One habit consistently tied to better hormonal resilience: high-quality sleep. 

Dr. Ara Suppiah, MD agrees, stating, “As a physician I can prescribe anti-inflammatories, but the best anti-inflammatory is sleep.” 

Romain Meeusen, PhD, of Vrije Universiteit Brussel, explained, "Prolonged elevations in cortisol blunt anabolic processes, suppress immunity, and increase risk of overtraining syndrome." 

Tracking cortisol rhythms gives clinicians insight into whether an athlete is adapting or heading toward burnout. 

Why biomarker testing matters for athletes

Endurance athletes should start tracking hormones, not just miles

For endurance athletes, hormones act as the body's scoreboard. Hormones reveal whether training stress is building fitness or breaking it down. 

Testosterone, IGF-1, cortisol, estrogen, and growth hormone all interact to shape recovery, stamina, and health. 

As Dr. Ara Suppiah explains, if the underlying triggers aren’t addressed, “the allostatic load keeps driving dysfunction,” leaving athletes stuck in a cycle of fatigue and poor adaptation. 

Without testing, these shifts are invisible. With personalized hormone panels from Access Medical Labs, clinicians can detect imbalances early, adjust recovery protocols, and support athletes in sustaining both performance and long-term health. 

Endurance athletes already trust data for pace and distance. Adding biomarker tracking ensures they're not guessing about the most important performance metrics of all, their own physiology. 

Disclaimer: Content on the Access Labs blog is for informational purposes only and reflects the views of individual contributors, not necessarily those of Access Medical Labs. We do not endorse specific treatments, products, or protocols. This content is not a substitute for professional medical advice. Always consult a qualified healthcare provider regarding any medical concerns.

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