What are the essential biomarkers runners should track? The numbers that flag fatigue, overtraining, and low testosterone

For male runners, the essential biomarker list starts with ferritin and iron studies, vitamin D, glucose, and a hormone panel that includes total testosterone, free testosterone, LH, and FSH, because a low testosterone value only matters when symptoms persist and the pituitary signals are checked too. Magnesium and potassium are usually checked when clinically indicated, such as with heavy sweat loss, GI loss, diuretic use, arrhythmia symptoms, heat illness, or abnormal Comprehensive Metabolic Panel findings. High sensitivity C reactive protein can add context about systemic inflammation, but it is optional rather than a core runner biomarker. Pair those labs with resting heart rate, VO2 max, and lactate threshold, and the panel can raise suspicion for iron deficiency, vitamin D deficiency, metabolic issues, inflammation, or androgen suppression, while underfueling is assessed mainly from history, symptoms, weight change, diet, and training context.

Key takeaways

• Endurance athletes are at increased risk of low iron status because of higher iron needs and inadequate dietary intake,^[1] and in distance runners with low iron status, intravenous iron raised hemoglobin mass, VO2 max, and run time to exhaustion.^[2]
• The Endocrine Society defines vitamin D deficiency as 25 hydroxyvitamin D below 20 ng/mL,^[3] and in athletes vitamin D supports calcium absorption, bone health, and muscle function, including protection against stress fractures.^[4]
• Low testosterone is not diagnosed from one low lab alone. Men need persistent symptoms, a repeat fasting morning result, and LH plus FSH to distinguish primary from secondary hypogonadism.^[5]
• A 2017 case report described a male ultramarathoner aged 52 whose spinal fracture involved osteopenia that the authors attributed most likely to vitamin D and testosterone deficiency, a reminder that hormones matter for runners’ bones, not just for lifters.^[6]
• Lactate thresholds correlate strongly with endurance running performance, and some of them estimate the maximal lactate steady state, which makes them useful for setting training intensities.^[7]

Why these biomarkers matter for male runners

Biomarkers are measurable signs of how your body is handling training stress, recovery, nutrition, and hormonal signaling. For runners, falling performance can reflect low iron status, which impairs exercise performance,^[1] or a combination of excessive training load and inadequate recovery.^[8] According to the 2013 joint consensus statement of the European College of Sport Science and the American College of Sports Medicine, infection, negative energy balance, inadequate carbohydrate or protein intake, iron deficiency, and magnesium deficiency are among the conditions to rule out when an athlete’s performance declines.^[8]

Running performance depends on oxygen delivery, energy availability, tissue repair, and endocrine stability. Iron is needed for oxygen transport and energy metabolism.^[1] Ferritin is used to track iron stores, and it rises when iron is replaced.^[2] Vitamin D supports bone and muscle function.^[4] Glucose tells you whether fuel handling is supporting training. High sensitivity C reactive protein can provide optional context about systemic inflammation, but it is not a standalone diagnostic test. Testosterone, LH, and FSH together help show whether the reproductive hormone axis is intact or suppressed and where a problem starts.^[5]

The athlete iron literature describes negative effects of iron deficiency on exercise performance in endurance athletes,^[1] while the joint overtraining consensus describes fatigue, performance decline, and mood disturbance as the main symptoms of overtraining syndrome.^[8]

How the most useful runner biomarkers work

Iron and ferritin

Ferritin is the protein that stores iron. Iron is required to make hemoglobin and myoglobin, which move oxygen from the lungs into blood and then into working muscle. According to a 2014 review in Applied Physiology, Nutrition, and Metabolism, endurance athletes are at increased risk of suboptimal iron status because of higher iron needs and inadequate dietary intake, and exercise related inflammation raises hepcidin, a hormone that traps iron and limits its availability.^[1]

According to a 2014 trial in 27 highly trained distance runners published in Medicine and Science in Sports and Exercise, both intravenous and oral iron raised ferritin over 6 weeks, and runners who started with low iron status and received intravenous iron gained about 4.9 percent in hemoglobin mass, 3.3 percent in VO2 max, and 9.3 percent in run time to exhaustion.^[2] Hemoglobin concentration, the value reported on a standard blood count, did not change meaningfully in any group, and the authors judged it less sensitive than total hemoglobin mass. That is why ferritin is checked directly instead of being inferred from a normal hemoglobin.^[2]

Vitamin D, magnesium, and potassium

25 hydroxyvitamin D is the standard blood marker for vitamin D status. According to the Endocrine Society’s 2011 vitamin D guideline, it is the initial test in people at risk of deficiency, with deficiency defined as less than 20 ng/mL and insufficiency as 21 to 29 ng/mL.^[3] A 2025 review in Nutrients describes vitamin D as important in athletes for calcium absorption, bone health including stress fracture prevention, and muscle function, which matters for runners who absorb thousands of foot strikes per session.^[4]

Magnesium and potassium are intracellular electrolytes involved in muscle contraction, nerve signaling, and fluid balance. The same review notes that magnesium is needed for energy production in muscle and for activating vitamin D, and that athletes may need more of it than sedentary people.^[4] Even so, magnesium and potassium are usually checked when clinically indicated rather than as universal runner biomarkers. Situations that can justify testing include heavy sweat loss, GI loss, diuretic use, arrhythmia symptoms, heat illness, or abnormal Comprehensive Metabolic Panel findings. When they are low or shifting, runners may notice cramping, twitching, unusual fatigue, or a session that suddenly feels harder in heat or high sweat conditions.

Inflammation and glucose

High sensitivity C reactive protein, often shortened to hs CRP, is a blood marker of systemic inflammation that can serve as an optional context marker rather than a core runner biomarker. Hard or long exercise raises inflammatory markers temporarily as part of muscle repair, so a value drawn soon after a race is difficult to interpret, while a value that stays elevated outside the expected recovery window can prompt a closer look at training load, illness, and sleep. The joint overtraining consensus reviewed hormonal, biochemical, and immune markers and concluded that none of them meets the criteria for general use, so hs CRP cannot diagnose overtraining by itself.^[8]

Glucose is the sugar circulating in your blood and supplying immediate fuel. For runners, both chronic underfueling and poor metabolic control can show up as bonking, shakiness, irritability, and a steep drop in pace late in a long run. When fasting glucose is abnormal, or insulin is high in a man with excess body fat, the issue may be more metabolic than purely training related.

Testosterone, LH, and FSH

Total testosterone is the amount circulating in blood. Free testosterone is the fraction available to tissues. According to the 2018 Endocrine Society guideline on testosterone therapy, hypogonadism should be diagnosed only in men with symptoms and consistently low testosterone, confirmed by repeating a fasting morning draw, and free testosterone should be measured by equilibrium dialysis or estimated with an accurate formula when total testosterone is near the lower limit of normal or SHBG is altered.^[5] The same guideline recommends evaluating the cause once deficiency is confirmed, including LH and FSH, because they show whether the problem is primary or secondary.^[5]

High LH with low testosterone suggests primary hypogonadism. Low or normal LH with low testosterone suggests secondary or functional hypogonadism, a pattern that can appear in men with chronic energy deficit, obesity, metabolic disease, sleep loss, or heavy endurance load. A 2017 case report in Medicina described a male master ultramarathoner aged 52 with a thoracic spine fracture caused by a benign vertebral hemangioma combined with osteopenia, which the authors attributed most likely to vitamin D and testosterone deficiency.^[6] A single case cannot show how common this is, but it illustrates why bone health and hormones belong in the same conversation for older endurance runners.

VO2 max, lactate threshold, and resting heart rate

VO2 max is the maximum rate your body can take in and use oxygen during exercise. Lactate thresholds are points on the blood lactate curve during a graded exercise test, such as the first rise above baseline or the maximal lactate steady state, the highest intensity at which lactate production and clearance stay in balance. According to a 2009 review in Sports Medicine that identified 25 lactate threshold concepts, most of the 32 studies comparing thresholds with endurance results reported strong correlations, particularly for running events, and some thresholds can estimate the maximal lactate steady state, which supports their use in setting training intensities.^[7]

Resting heart rate is the easiest field measure to track at home. A rise that persists alongside fatigue, sleep disruption, or falling performance is a reason to look more closely, but the joint overtraining consensus found that no single marker meets the criteria to diagnose overtraining on its own.^[8] Wearables are useful for trends, but lab measured VO2 max and lactate testing remain more accurate than watch estimates.

What abnormal runner biomarkers can point to

Iron deficiency with or without anemia. Endurance athletes are at increased risk because of higher iron needs and inadequate dietary intake.^[1] High mileage runners, men restricting calories, vegetarians, and men with gastrointestinal blood loss have additional reasons to check. In the distance runner trial, hemoglobin concentration stayed flat even as iron stores and hemoglobin mass rose, so a normal blood count does not rule out low iron stores.^[2]

Bone stress injury risk. The Endocrine Society defines vitamin D deficiency as 25 hydroxyvitamin D below 20 ng/mL.^[3] Because vitamin D is needed for calcium absorption, low status is linked to stress fractures in athletes.^[4] In runners, low vitamin D can coexist with low testosterone, as in the 2017 ultramarathoner case.^[6] Repeated bone tenderness, slow healing, or a stress reaction in a runner with these findings deserves evaluation.

Nonfunctional overreaching or overtraining syndrome. Short term overreaching can be productive and leads to better performance after recovery. When performance keeps falling despite rest and fatigue and mood changes persist, the pattern looks more like nonfunctional overreaching or overtraining syndrome. The joint consensus statement emphasizes that telling the two apart is difficult and depends on the clinical course and on excluding other causes.^[8] One abnormal hs CRP result after a marathon or one rough week of sleep does not establish a diagnosis; timing, symptoms, and repeat testing matter.

Functional hypogonadism. In men, low testosterone with low or normal LH suggests the brain is not signaling strongly enough even though the testes may still be capable of responding. That can happen with obesity, insulin resistance, type 2 diabetes, some medications, chronic stress, poor sleep, or persistent low energy availability. High LH with low testosterone points instead to primary hypogonadism, which is a different problem and requires a different treatment path.^[5] A single low testosterone draw after illness is not a diagnosis, which is why the result is repeated on another fasting morning sample.

Symptoms and signals men actually notice

Men usually notice the pattern before they know the lab.

• Your easy pace is suddenly 30 to 60 seconds per mile slower at the same perceived effort for 10 to 14 days.
• Your morning resting heart rate sits 5 to 10 beats per minute above your normal baseline for several mornings in a row.
• You bonk around 60 to 90 minutes into long runs that used to feel routine, even when your route and breakfast are similar.
• Normal soreness now lasts beyond 72 hours, or your legs feel heavy after sessions that used to feel easy.
• You are more winded on hills, more irritable after runs, or need extra recovery between repeats at familiar paces.
• You notice more cramps, eyelid twitching, calf tightness, or an unusually “flat” feeling after hot or sweaty runs.
• Your libido drops, morning erections become less frequent, or erections feel less reliable during heavy training blocks.
• You keep picking up bone pain, shin tenderness, or repeat soft tissue strains instead of bouncing back between workouts.

Myth vs fact

Myth: My watch data are enough

Fact: Wearables are useful for trends, but they cannot measure ferritin, vitamin D, testosterone, LH, or FSH, and they estimate VO2 max rather than directly testing it. Lactate thresholds are derived from blood lactate measured during graded exercise tests,^[7] and testosterone should be measured with an accurate, reliable assay on fasting morning samples.^[5]

Myth: One low testosterone result diagnoses hypogonadism

Fact: Male hypogonadism requires both persistent symptoms and repeat biochemical evidence on a morning draw. LH and FSH must be checked alongside testosterone to classify primary vs secondary disease, because treatment depends on that distinction.^[5]

Myth: A high CRP after a race always means something is wrong

Fact: Acute inflammation after hard exercise is expected and is part of repair. What matters is whether inflammation stays elevated at rest, repeats outside heavy training, or matches a bigger pattern of fatigue, illness, sleep disruption, and falling performance. No single biochemical or immune marker, hs CRP included, is accepted as a test for overtraining.^[8]

Myth: More iron always improves performance

Fact: Iron helps when iron status is low. It is not a free speed supplement, and excess iron can be harmful. The 2014 athlete iron review recommends screening iron status first and then raising intake through diet or supplements where it is low,^[1] and in the distance runner trial the performance gains appeared in runners who started with low iron status.^[2]

Myth: Low testosterone in runners just means you need TRT

Fact: Not necessarily. Men with low or normal LH and low testosterone often have a secondary or functional pattern, which is different from primary testicular failure, and the first step is identifying what is suppressing the axis. Testosterone therapy suppresses LH, FSH, and sperm production, and the Endocrine Society recommends against starting it in men planning fertility in the near term.^[5] Treatments that stimulate the body’s own production, such as fertility preserving treatment such as enclomiphene, are an alternative some men discuss with a clinician.

What to do if your numbers are off

A baseline lab panel once a year is a reasonable approach for male runners when training and health are stable. Rechecks for iron, vitamin D, or other deficiencies should follow the specific abnormality being treated, and hormone monitoring during treatment follows its own schedule.

1. Step 1: Test at the right time. Do not draw labs the morning after a race, a long run, or a night of heavy drinking. For hormone testing, use a morning draw between 07:00 and 11:00. A practical runner panel often includes CBC, ferritin, iron studies, vitamin D, Comprehensive Metabolic Panel, and fasting glucose, with hs CRP as an optional context marker and magnesium or potassium checked when clinically indicated; when symptoms suggest it, total and free testosterone measured with accurate assays, LH, FSH, and estradiol.
2. Step 2: Match the abnormal number to the real world problem. Low ferritin plus breathless hill repeats points one way. Low vitamin D plus shin pain points another. Low testosterone with low or normal LH suggests secondary or functional hypogonadism. High LH with low testosterone suggests primary hypogonadism. Those are not interchangeable findings, and they should not get the same treatment.
3. Step 3: Retest after you change something. If you increase iron intake, fix sleep, cut training load, or start treatment, repeat the relevant markers rather than assuming you solved it. For men who start testosterone therapy, the Endocrine Society recommends a standardized monitoring plan during the first year that includes symptoms, side effects, serum testosterone, and hematocrit, and follow up hormone labs are part of any hormone treatment plan.^[5]

Veedma offers testing that includes total and free testosterone measured by LC-MS/MS with LH and FSH, or a review of labs you already have. Licensed providers interpret the results and, when appropriate, prescribe enclomiphene or the enclomiphene plus tadalafil tablet. Enclomiphene is not an FDA-approved drug; it is prescribed after a provider confirms the diagnosis and is compounded by a licensed U.S. pharmacy.

Bottom line

The essential biomarkers runners should track are the ones that change decisions: iron status, vitamin D, glucose, and a properly interpreted male hormone panel with total testosterone, free testosterone, LH, and FSH, backed by resting heart rate, VO2 max, and lactate threshold, with magnesium and potassium checked when clinically indicated and hs CRP used as optional context. For men, that mix covers several common, testable reasons training stops working.

References

1. Hinton PS. Iron and the endurance athlete. Appl Physiol Nutr Metab. 2014;39:1012-8. PMID: 25017111
2. Garvican LA, Saunders PU, Cardoso T, et al. Intravenous iron supplementation in distance runners with low or suboptimal ferritin. Med Sci Sports Exerc. 2014;46:376-85. PMID: 23872938
3. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96:1911-30. PMID: 21646368
4. Dominguez LJ, Veronese N, Ragusa FS, et al. The Importance of Vitamin D and Magnesium in Athletes. Nutrients. 2025;17. PMID: 40431395
5. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103:1715-1744. PMID: 29562364
6. Knechtle B, Nikolaidis PT, Lutz B, et al. Pathologic fracture of the thoracic spine in a male master ultra-marathoner due to the combination of a vertebral hemangioma and osteopenia. Medicina (Kaunas). 2017;53:131-137. PMID: 28416169
7. Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they?. Sports Med. 2009;39:469-90. PMID: 19453206
8. Meeusen R, Duclos M, Foster C, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Med Sci Sports Exerc. 2013;45:186-205. PMID: 23247672