Testosterone, IGF-1, and Hormonal Balance: The Hidden Factor in Endurance Performance

Endurance athletes obsess over splits, wattage, VO₂ max, and carbohydrate timing — yet one of the biggest performance limiters never shows up on a watch or power meter: hormones.

Testosterone, IGF-1, cortisol, estrogen, and growth hormone quietly orchestrate recovery, adaptation, mood, and stamina. When they fall out of balance — which happens surprisingly often in endurance sports — athletes pay the price with stalled progress, chronic fatigue, and higher injury risk. Here’s what the research actually shows and why serious endurance athletes can no longer afford to ignore their hormonal health.

The Paradox of Endurance Training: Fitter on the Outside, Suppressed on the Inside

Intense, prolonged training is a double-edged sword.

•  ≈16–40% of male endurance athletes have clinically low testosterone despite being lean and highly trained.

•  Up to 30% of elite female track and field athletes report menstrual irregularities; 4–20% experience full amenorrhea.

•  Both sexes frequently show suppressed IGF-1, elevated cortisol, and signs of Relative Energy Deficiency in Sport (RED-S).

These aren’t rare edge cases — they’re common enough that leading sports-medicine researchers now consider hormonal disruption part of the endurance territory.

Male Endurance Athletes: The Low-Testosterone Problem Nobody Talks About

Testosterone isn’t just for sprinters and weightlifters. In endurance athletes it supports:

•  Red blood cell production and oxygen delivery

•  Maintenance of lean mass (critical for running economy and cycling power-to-weight)

•  Motivation, mood, and recovery capacity

Yet distance runners, cyclists, and triathletes routinely present with resting testosterone levels 20–40% below age-matched sedentary men. One study of elite distance runners found that the subgroup with the lowest testosterone also sustained significantly more bone-stress injuries. Anthony C. Hackney, PhD (University of North Carolina) summarized it bluntly:

“Endurance athletes may paradoxically face testosterone levels lower than non-athletes, despite training at elite levels.”

Female Endurance Athletes: The Female Athlete Triad → RED-S Evolution

Low energy availability quickly disrupts the hypothalamic-pituitary-ovarian axis, leading to oligomenorrhea or amenorrhea. The downstream effects are serious:

•  Reduced bone mineral density (sometimes irreversible)

•  Impaired glycogen replenishment

•  Higher risk of stress fractures

Elite female runners with menstrual dysfunction routinely show “high turnover” bone metabolism — lots of breakdown and formation markers, but net poorer bone quality.

Where Men and Women Overlap

Both sexes suffer when energy intake fails to match expenditure:

•  Elevated cortisol → catabolism, poor sleep, immune suppression

•  Suppressed IGF-1 → slower muscle repair and adaptation

•  Blunted reproductive hormones → fatigue, low mood, libido crashes

A 2023 study of Irish male endurance athletes found 77% were in low energy availability (<30 kcal/kg LBM/day) with corresponding drops in IGF-1 and increases in bone-turnover markers — virtually identical to classic RED-S presentations in women.

Key Hormones Every Endurance Athlete Should Understand

1. Testosterone (Total + Free) & SHBG

Low levels → poorer recovery, reduced hematocrit, higher injury risk, low motivation.

2. IGF-1

The primary mediator of post-exercise muscle repair and metabolic adaptation. Chronic low IGF-1 is a red flag for overtraining or under-fueling.

3. Cortisol (morning + evening) & DHEA-S

Chronic elevation destroys gains. A flattened diurnal curve is one of the earliest signs of non-functional overreaching.

4. Female-Specific: Estradiol, Progesterone, LH/FSH

Tracking across the cycle (or lack thereof) reveals energy-availability problems long before bone density scans do.

5. Thyroid Panel (TSH, Free T4, Free T3, Reverse T3)

Endurance training + low energy availability frequently drives “low-T3 syndrome,” tanking metabolism and recovery.

Hormone Replacement Therapy (HRT) in Athletes: Helpful Tool or Forbidden Fruit?

When true clinical deficiency exists (not just “low-normal” from hard training), HRT can restore energy, mood, and training response. Testosterone therapy in hypogonadal men and estrogen/progesterone restoration in amenorrheic women have both been shown to improve bone health and performance markers.

However:

•  Risks exist (cardiovascular, hematocrit, lipid changes)

•  Most competitive governing bodies require Therapeutic Use Exemptions (TUEs) and strict documentation

•  Indiscriminate use for “optimization” is ineffective and dangerous

Shalender Bhasin, MD (Brigham and Women’s Hospital), puts it clearly: “Testosterone therapy should be reserved for those with clinically confirmed deficiency, as improper use carries significant risks.”

Practical Takeaways for Endurance Athletes and Coaches

1.  Get baseline and periodic blood/saliva testing — especially during high-volume blocks.

2.  Treat low energy availability first (increase calories, strategic refueling, occasional lower-volume weeks). Hormones usually rebound without medication.

3.  Prioritize sleep — it’s the single most potent “hormone optimizer” available.

4.  Monitor objective markers (morning heart rate, HRV, mood, libido, menstrual status) as early-warning signals.

5.  Work with a sports endocrinologist or physician familiar with RED-S if numbers stay suppressed despite adequate fueling.

Final Thought

Endurance athletes already live by data. Pace, power, heart-rate variability — we track it all. Hormones are simply the next layer of that data. Ignoring them doesn’t make them irrelevant; it just leaves you guessing why the training that worked last season suddenly isn’t working now.

In 2025 and beyond, the most successful endurance athletes won’t just be the ones who train the hardest — they’ll be the ones who train the smartest, and that includes listening to the chemical messengers that ultimately decide whether you adapt or break down.

Don’t guess. Test. Adjust. Thrive.

Contact us for your lab testing