TRT And VO2 Max: What Endurance Athletes Should Track

Testosterone replacement therapy (TRT) doesn't just affect muscle mass and recovery. It changes hemoglobin levels, red blood cell production, and oxygen-carrying capacity, all of which feed directly into VO2 max. For endurance athletes managing diagnosed testosterone deficiency, understanding how TRT interacts with aerobic performance markers is the difference between informed treatment and decisions made without complete clinical context.

A hormone management platform can help athletes connect the dots between bloodwork and performance. But even with clinical support, athletes need to know which biomarkers matter, why they matter, and when to retest.

VO2 max measures the maximum amount of oxygen the body can use during high-intensity exercise, reported in milliliters per kilogram of body weight per minute (mL/kg/min). It is one of the strongest predictors of cardiovascular health and all-cause mortality. Testosterone therapy, when prescribed for confirmed hypogonadism, may influence this metric through its effect on erythropoiesis (red blood cell production) and hemoglobin concentration. That connection makes routine lab monitoring particularly important for any endurance athlete on TRT.

All biomarker targets, testing intervals, and treatment decisions discussed in this article should be individualized in consultation with a qualified healthcare provider. Reference ranges vary by laboratory, patient history, and clinical context.

This article covers the clinical evidence linking testosterone to VO2 max changes, the specific biomarkers endurance athletes on TRT should track, updated safety data from the TRAVERSE trial, and practical testing schedules. It does not cover biomarkers unrelated to TRT or endurance performance, and it does not apply to athletes using supraphysiologic doses outside clinical supervision.

How Does TRT Affect VO2 Max?

Testosterone increases hemoglobin and hematocrit by stimulating erythropoietin (EPO) production in the kidneys. More red blood cells mean more oxygen delivered to working muscles per heartbeat. That mechanism has a direct downstream effect on VO2 max.

A subgroup analysis of the Testosterone in Older Men with Mobility Limitations (TOM) trial, published in JCEM, measured VO2 peak in mobility-limited men aged 65 and older with low testosterone. The testosterone group gained 0.83 mL/kg/min in VO2 peak (SD 2.4), while the placebo group declined by 0.89 mL/kg/min (SD 2.5). The between-group difference reached P = 0.006.

A separate three-year trial in older men with low testosterone found that supplementation attenuated the age-related decline in VO2 peak compared to placebo, with a between-group difference of 0.91 mL/kg/min (95% CI: 0.24 to 1.59, P < 0.01). The researchers attributed the effect primarily to testosterone-driven increases in hemoglobin.

Two caveats. These trials enrolled hypogonadal older men, not competitive endurance athletes. And the VO2 max gains were modest in absolute terms. Testosterone therapy appears to slow age-related aerobic decline rather than produce the kind of VO2 max jumps that come from structured interval training. A HIIT meta-analysis of 37 studies found mean VO2 max improvements of 0.51 L/min in recreationally active adults over 6 to 13 weeks, several times larger than TRT's effect.

For endurance athletes with confirmed deficiency, the clinical takeaway is that TRT may help protect against accelerated aerobic decline. It is not a substitute for training. It is a medical treatment that, when monitored correctly, can remove a physiological deficit that training alone won't fix.

Why Is VO2 Max a Longevity Marker?

VO2 max gets attention as a fitness metric. It deserves more attention as a survival metric.

Low cardiorespiratory fitness is one of the strongest independent predictors of all-cause mortality. A JAMA Oncology analysis tracked 13,949 men from midlife fitness assessments through Medicare-linked cancer follow-up (median 6.5 years after age 65) and found that high cardiorespiratory fitness was associated with 55% lower lung cancer risk and 44% lower colorectal cancer risk compared to men with low fitness.

VO2 max peaks in the mid-20s and declines approximately 1% per year in sedentary adults after age 30. Longitudinal data on masters athletes shows that consistent training cuts that decline roughly in half, to about 0.5% per year. Testosterone also declines with age, roughly 1% per year after 40, according to Endocrine Society data. The parallel decline is not coincidental. Both testosterone and VO2 max share dependence on hemoglobin levels, mitochondrial function, and lean body mass.

For men with diagnosed testosterone deficiency, the aerobic decline can accelerate beyond what aging alone would predict. That acceleration is part of the clinical rationale for TRT in symptomatic patients: restoring testosterone to the physiologic range may help preserve the cardiovascular fitness that protects against chronic disease.

VO2 max alone doesn't determine endurance performance, though. Two athletes with identical VO2 max scores can finish a race minutes apart. Lactate threshold (the fastest sustainable pace before fatigue chemistry takes over) and movement economy (how much oxygen each stride costs) are equally important. VO2 max sets the ceiling. Threshold and economy determine how much of that ceiling gets used.

What Biomarkers Should Endurance Athletes on TRT Monitor?

TRT changes blood chemistry. Some of those changes may support performance markers. Others create risks that call for active monitoring. The biomarkers below represent a commonly recommended panel for endurance athletes on testosterone therapy, based on current clinical guidelines. Individual testing protocols should be determined in consultation with the prescribing provider.

Hematocrit and hemoglobin

This is the highest-priority monitoring target for athletes on TRT. Testosterone drives red blood cell production, which is partly why it can support VO2 max. But overproduction can push hematocrit into potentially dangerous territory.

Multiple clinical guidelines flag elevated hematocrit above guideline-defined thresholds as a signal that may warrant dose reassessment or therapeutic phlebotomy (Bhasin et al., 2018). A 2025 retrospective study found that a notable proportion of men on TRT experienced clinically meaningful hematocrit elevations (Neidhart et al., 2025). Elevated hematocrit thickens the blood, increases clotting risk, and can lead to stroke or pulmonary embolism.

Endurance athletes face a compounding factor: training itself raises hematocrit through plasma volume shifts and altitude exposure. An athlete on TRT who also trains at elevation or uses heat acclimatization protocols may need more frequent hematocrit checks, not fewer.

Total and free testosterone

Monitoring testosterone levels confirms that therapy is maintaining the intended physiologic range. The Endocrine Society recommends two separate early-morning blood draws, one to three weeks apart, to establish baseline and verify deficiency. On TRT, follow-up labs confirm the dose is producing levels within the provider-determined target range for total testosterone.

Free testosterone and sex hormone-binding globulin (SHBG) provide additional context. Total testosterone can appear within reference range while free testosterone (the biologically active fraction) remains low due to elevated SHBG. This distinction matters for athletes, because free testosterone correlates more closely with symptoms and physical performance markers than total testosterone alone.

Lipid profile and cardiovascular markers

A lipid panel measures total cholesterol, HDL, LDL, VLDL, and triglycerides. Testosterone therapy can shift lipid values in both directions. Some patients see HDL decrease modestly on TRT, which may increase cardiovascular risk if left unchecked.

Even active athletes aren't immune to lipid problems. High training volumes don't guarantee healthy lipid profiles. Fueling strategies heavy in carbohydrates (common among endurance athletes) can push triglyceride levels and metabolic markers into unhealthy ranges even when aerobic fitness is strong. Quarterly lipid panels catch these shifts before they become clinical problems.

C-reactive protein and inflammation

CRP is a liver-produced protein that rises with systemic inflammation. For endurance athletes, an acute CRP spike after a hard race or heavy training block is normal. A chronically elevated CRP at rest is not.

Chronic low-grade inflammation accelerates aging, impairs recovery, and increases the risk of cardiovascular disease, cancer, and metabolic disorders. Monitoring CRP alongside TRT helps distinguish training-induced inflammation from a systemic problem that requires medical attention. A resting CRP within the provider-determined reference range is the goal. Persistent elevation, especially during recovery periods, warrants further investigation.

Cortisol

Cortisol and testosterone exist in a seesaw relationship. Chronically elevated cortisol suppresses testosterone production and accelerates muscle breakdown, both of which undercut endurance performance. Athletes who overtrain without adequate recovery often drive cortisol up while simultaneously depleting testosterone.

Tracking morning cortisol alongside testosterone gives clinicians a clearer picture of the hormonal environment. Persistently elevated cortisol in an athlete on TRT may indicate overtraining, inadequate sleep, or an underlying condition that the testosterone dose alone won't fix. The root causes of imbalance often extend beyond a single lab value.

Heart rate variability

HRV measures the variation in time between heartbeats. It reflects how well the autonomic nervous system handles stress, including the physical stress of training.

Higher HRV correlates with better recovery capacity, lower resting inflammation, and improved cardiovascular health. Athletes on TRT benefit from daily HRV tracking (via a wearable device) because it provides a real-time signal that lab tests, drawn weeks apart, can't capture. A downward HRV trend over days or weeks can indicate overtraining, inadequate recovery, or a medication issue before other symptoms appear.

What Did the TRAVERSE Trial Change About TRT Safety?

The TRAVERSE trial, published in the New England Journal of Medicine (Lincoff et al., 2023), was the largest randomized controlled trial of testosterone therapy and cardiovascular outcomes to date. It enrolled 5,246 men aged 45 to 80 with hypogonadism and preexisting or high cardiovascular risk.

The primary finding: testosterone gel did not increase major adverse cardiac events (MACE) compared to placebo (7.0% vs. 7.3%; hazard ratio 0.96, 95% CI 0.78 to 1.17, P < 0.001 for noninferiority). That result led the FDA to update labeling, removing its previous cardiovascular black-box warning from testosterone products in February 2025.

The TRAVERSE data was not entirely reassuring, though. The testosterone group showed higher rates of atrial fibrillation, acute kidney injury, and pulmonary embolism. The FDA's updated labeling added a new warning about blood pressure increases based on the trial data.

For endurance athletes, the practical implication is that cardiovascular monitoring on TRT is more targeted than before, not less necessary. Blood pressure, hematocrit, and lipid monitoring remain important. The removed black-box warning reflects better data on MACE specifically. It doesn't mean TRT carries zero cardiovascular risk.

How Often Should Endurance Athletes Get Bloodwork?

The generally recommended testing cadence for athletes on TRT is quarterly. Routine bloodwork typically includes testosterone (total and free), hematocrit, hemoglobin, estradiol, PSA, a lipid panel, CRP, and a basic metabolic panel. Providers may adjust this panel based on individual risk factors and treatment history.

Several factors influence how often to retest. The lifecycle of red blood cells is approximately three months, so hematocrit and hemoglobin shifts don't stabilize faster than that. CRP tests may be repeated a few weeks apart if a single reading comes back elevated, to rule out transient causes like a recent race or illness. HRV should be tracked daily, since the overall trend matters more than any single reading.

Seasonal variation also affects results. Vitamin D fluctuates with sun exposure, training volume shifts across race seasons, and dietary patterns change. A quarterly cadence catches these fluctuations before they compound into clinical problems.

The goal of regular testing isn't to chase abnormal results. It's to confirm that the current protocol (TRT dose, training load, nutrition, recovery) is working as intended. Small adjustments based on quarterly labs can prevent the kind of large corrections that disrupt training cycles.

Working With a Provider to Set Individualized Targets

Biomarker reference ranges published in clinical guidelines are population-level starting points, not personalized targets. A value that falls within a standard reference range can still be clinically relevant if it represents a meaningful change from an individual's baseline or if it coincides with symptoms.

A common testosterone question is whether a lab value inside the reference range means everything is fine. It doesn't, necessarily. Context matters: training history, symptom burden, and how a given value compares to previous draws are all part of the clinical picture.

Providers experienced in managing athletes on TRT will set individualized targets for each biomarker based on the patient's history, goals, comorbidities, and response to therapy. The most productive approach is shared decision-making, where the athlete brings symptom logs and training data, and the provider interprets lab results within that context.

Disclaimer: This blog post is intended for informational purposes only and should not be considered medical advice. Always consult a healthcare professional before making changes to your health routine.

FAQs

Does TRT directly improve VO2 max in endurance athletes? 

Clinical trials in hypogonadal older men show that testosterone may attenuate the age-related decline in VO2 peak, with a between-group difference of about 0.9 mL/kg/min over three years (Storer et al., JCEM 2018). The effect is linked to hemoglobin increases, not a direct aerobic training adaptation. No large trials have studied competitive endurance athletes specifically.

What is the most important biomarker to monitor on TRT? 

Hematocrit. Testosterone stimulates red blood cell production, and unchecked elevation above guideline-defined thresholds increases the risk of stroke, pulmonary embolism, and deep vein thrombosis. A 2025 retrospective study found that a notable proportion of men on TRT experienced clinically meaningful hematocrit elevations. Regular monitoring allows providers to adjust dosing before complications develop.

How much does VO2 max decline with age? 

VO2 max drops approximately 1% per year after age 30 in sedentary adults. In masters athletes who maintain consistent training, the decline slows to roughly 0.5% per year. Testosterone deficiency may accelerate that decline beyond what aging alone would predict.

Is TRT approved for improving athletic performance? 

No. The FDA approves testosterone only for treating hypogonadism caused by a specific medical condition. It is not approved for age-related decline alone, athletic performance, weight loss, or longevity. Testosterone is a Schedule III controlled substance, and a valid prescription requires lab-confirmed deficiency plus clinical symptoms.

Can an athlete have a high VO2 max and still underperform? 

Yes. VO2 max sets the aerobic ceiling, but lactate threshold and movement economy determine how much of that ceiling gets used in a race. An athlete with a strong VO2 max and poor economy can lose to someone with a lower ceiling but better efficiency at every distance from the 5K up.

How often should athletes on TRT get blood work? 

Quarterly is generally recommended. The panel typically includes total and free testosterone, hematocrit, hemoglobin, estradiol, PSA, a lipid panel, CRP, and a basic metabolic panel. HRV should be tracked daily via a wearable device. More frequent testing may be warranted after dose changes or during heavy training blocks. Providers may adjust the panel based on individual needs.

What did the TRAVERSE trial conclude about TRT and heart risk? 

The 5,246-patient trial found that testosterone therapy did not increase major adverse cardiac events compared to placebo (HR 0.96). The FDA removed its cardiovascular black-box warning in February 2025 based on this data, but added a blood pressure warning. Higher rates of atrial fibrillation, acute kidney injury, and pulmonary embolism were observed in the testosterone group. Patients should discuss their cardiovascular risk profile with their provider.

References

• J Clin Endocrinol Metab. 2016;101(6):2562-2569

• J Clin Endocrinol Metab. 2018;103(8):2861-2869

• PLOS ONE. 2013;8(9):e73182

• JAMA Oncol. 2015;1(2):231-237

• J Clin Endocrinol Metab. 2018;103(5):1715-1744

• N Engl J Med. 2023;389(2):107-117

• FDA Drug Safety Communication. February 28, 2025

• Endocrine Society Clinical Practice Guideline. 2018