My mate from the testosterone article (the one who was convinced his testosterone was cooked) finally got blood work done.
His testosterone was fine. His ferritin was 19.
He'd been training five or six days a week for two years. Running three mornings, lifting three evenings. He ate well, or so he thought, but he'd never considered that his training volume was quietly depleting his iron stores through a combination of foot-strike haemolysis, increased GI losses, and sweat.
His performance had been declining for months. He'd assumed it was overtraining, or age, or just a bad phase. It wasn't. It was a blood test away from an explanation.
I'm not an athlete. I go to the gym four times a week, I run occasionally, and I'd describe myself as "recreationally active," which is a polite way of saying I'm fit enough to not embarrass myself but nowhere near competitive. This article isn't written for elite athletes with sports medicine teams and quarterly testing protocols. It's for the person who trains seriously (four to six sessions a week, structured programming, genuine commitment) but has never thought of blood testing as part of their performance toolkit.
Because here's the thing that surprised me when I started researching this: the harder you train, the more your blood markers matter. Training doesn't just demand energy and recovery. It demands iron, it stresses your immune system, it suppresses hormones, it generates inflammation, and it depletes micronutrients at rates that sedentary people don't experience. If you're optimising your training, your nutrition, and your sleep but never checking whether your body's internal environment can actually support that load, you're driving a car without checking the dashboard.
A note before we get into it
General information only. I'm not a sports scientist or exercise physiologist. Athletes with specific performance goals, medical conditions, or who are using performance-enhancing substances should work with a sports medicine physician.
If you're experiencing excessive fatigue, recurrent illness, persistent injury, or menstrual irregularity alongside heavy training, see a clinician. These may indicate overtraining syndrome or relative energy deficiency in sport (RED-S), both of which are serious.
Why athletes need different testing
A sedentary person's blood work and an athlete's blood work can look very different. What counts as "normal" shifts based on training load.
Iron depletes faster. Athletes lose iron through multiple mechanisms: foot-strike haemolysis (red blood cell destruction from repetitive impact, particularly in runners), increased gastrointestinal blood loss during intense exercise, iron lost in sweat, and the hepcidin response. Exercise triggers an inflammatory protein that temporarily blocks iron absorption for several hours post-training.
Inflammation is part of the process, but too much is a problem. Training creates controlled tissue damage that triggers inflammation as part of adaptation. CRP and other inflammatory markers can be chronically mildly elevated in heavily training athletes. But persistently elevated inflammation beyond what training explains may indicate overtraining, illness, or inadequate recovery.
Hormones respond to training load. Testosterone can be suppressed by overtraining, under-fuelling, and insufficient sleep. Cortisol rises with training stress. The testosterone-to-cortisol ratio is sometimes used as a rough indicator of recovery balance, though its clinical utility is debated.
Micronutrient demands increase. Magnesium, zinc, B vitamins, and vitamin D are all utilised at higher rates during intense training. Deficiencies that might be subclinical in a sedentary person can become performance-limiting in an athlete.
Reference ranges may not capture "optimal." Standard reference ranges are derived from general populations. An athlete with a ferritin of 20 is "in range" but almost certainly performance-limited. Many sports medicine physicians use higher thresholds for athletes, with ferritin above 50 µg/L at minimum and ideally above 70–100 µg/L for endurance athletes.
The athlete blood panel
Iron Studies (Ferritin)
The most important test for athletes
Iron is the foundation of oxygen delivery and energy production. Low ferritin in an athlete means reduced haemoglobin synthesis capacity, impaired oxygen transport, reduced aerobic capacity, and slower recovery. Every endurance metric suffers.
Athlete-specific thresholds
Many sports medicine physicians recommend ferritin above 50 µg/L for active individuals and above 70–100 µg/L for endurance athletes. The standard lab lower limit of 15 µg/L is far too low for performance contexts.
Who's most at risk
Female athletes (menstruation plus training losses), distance runners (foot-strike haemolysis), vegetarian and vegan athletes, and anyone in heavy training blocks.
Test it with Bloody Good:
Product: Iron Studies (Including Ferritin)
Read more: Iron and ferritin article
Full Blood Count (FBC)
Your oxygen-carrying capacity
Haemoglobin is your oxygen-carrying capacity. Low haemoglobin means your VO2max ceiling drops regardless of your fitness. FBC also provides haematocrit (relevant for hydration status and endurance) and white blood cell count (immune function under training stress).
Watch out for sports anaemia
Some endurance athletes develop "sports anaemia," a dilutional pseudoanaemia caused by expanded plasma volume from training. This looks like low haemoglobin but isn't true anaemia. Your clinician can distinguish between this and genuine iron-deficiency anaemia by looking at ferritin and red blood cell indices alongside haemoglobin.
Test it with Bloody Good:
Product: Full Blood Count (FBC)
Vitamin D
Recovery, bones, and immune function
Vitamin D plays roles in muscle function, bone health, immune regulation, and recovery from training. Deficiency has been associated with increased injury risk, impaired muscle recovery, and reduced immune function. Athletes training indoors (swimmers, gym-based athletes) or in southern Australia during winter are at particular risk.
Testosterone and Cortisol
The recovery balance
Testosterone supports muscle protein synthesis, recovery, and motivation. Cortisol is the primary stress hormone and rises with training load, sleep deprivation, and psychological stress.
In athletes, the relationship between the two is informative. Sustained high cortisol with suppressed testosterone may indicate overtraining, under-recovery, or under-fuelling. This pattern (sometimes called an unfavourable testosterone-to-cortisol ratio) is associated with impaired adaptation, mood disturbance, and performance decline.
A few caveats
Single blood cortisol has limitations (covered in the cortisol article). Testosterone varies with time of day, sleep quality, and acute training. Trends over time are more informative than single readings.
Test it with Bloody Good:
Product: Testosterone Free/Total + SHBG
Product: Cortisol Blood Test
Read more: Testosterone article, Cortisol article
CRP (Inflammation)
Training inflammation vs something worse
CRP tracks systemic inflammation. In athletes, it helps distinguish between normal training-induced inflammation and problematic chronic inflammation from overtraining, illness, or injury.
Timing matters
CRP can be acutely elevated for 24–72 hours after a hard session. Test during a relative rest period (at least 48 hours after intense training) for a more representative baseline.
Test it with Bloody Good:
Product: High-Sensitivity CRP
HbA1c and Fasting Glucose
Metabolic baseline
Blood sugar regulation affects energy availability, recovery, and body composition. While athletes are generally metabolically healthy, insulin resistance can develop alongside weight gain, ageing, or excessive carbohydrate intake. Worth monitoring as a metabolic baseline.
Test it with Bloody Good:
Product: HbA1c
Magnesium
Cramps, sleep, and recovery
Magnesium is involved in muscle contraction, energy production, electrolyte balance, and sleep quality. Athletes lose magnesium through sweat and have higher requirements due to increased metabolic demand. Deficiency can present as muscle cramps, poor sleep, fatigue, and impaired recovery.
Testing limitations
Serum magnesium only reflects circulating levels (about 1% of total body magnesium). It's an imperfect marker but the most accessible one. Low serum magnesium is clinically significant. Normal serum magnesium doesn't fully rule out tissue deficiency.
Test it with Bloody Good:
Product: Magnesium Blood Test
Liver Function (ALT/AST)
Exercise vs liver pathology
AST and ALT can be elevated after intense training, particularly resistance training and high-impact exercise, because these enzymes are present in muscle tissue as well as the liver. An athlete with mildly elevated AST after a heavy training week may have exercise-induced elevation, not liver pathology. Context matters here.
Test it with Bloody Good:
Read more: Liver function article
Thyroid Function
The hidden contributor
Thyroid dysfunction can cause fatigue, weight changes, and performance decline that mimics overtraining. It's particularly relevant for female athletes and anyone with a family history of thyroid disease. TSH screening ensures thyroid isn't a hidden contributor to declining performance.
The issue nobody discusses enough: RED-S and the female athlete
Relative Energy Deficiency in Sport (RED-S), formerly known as the Female Athlete Triad, is a serious condition caused by insufficient energy intake relative to training expenditure. It affects both women and men, though it's more commonly identified in women.
RED-S impairs menstrual function (amenorrhoea or irregular periods), bone health (stress fractures, low bone density), immune function, cardiovascular health, metabolic rate, and psychological wellbeing. It's driven by under-fuelling, which may be intentional (disordered eating) or unintentional (simply not eating enough to match training demands).
How blood tests help identify RED-S
Low or suppressed oestrogen (in women) or testosterone (in men). Low ferritin. Low vitamin D. Low T3 (the body downregulates thyroid function to conserve energy). Low IGF-1, a growth factor that reflects nutritional status. Elevated cortisol. Menstrual irregularity confirmed by hormonal panel.
RED-S is under-recognised, especially in recreational athletes and in male athletes. If you're training hard, losing your period (women), experiencing recurrent stress fractures, getting sick frequently, or losing performance despite increased training, RED-S should be on the differential. Talk to a sports medicine physician or a GP with sports medicine interest.
When to test: pre-season, mid-season, recovery
Pre-season / baseline: Full panel. Iron studies, FBC, vitamin D, testosterone, cortisol, thyroid, CRP, HbA1c, magnesium, liver function. This establishes your baseline before training load increases.
Mid-season / heavy training block (3–4 months in): Targeted retest. Ferritin, FBC, CRP, vitamin D, testosterone/cortisol if overtraining is a concern. This catches depletion before it becomes performance-limiting.
Off-season / recovery: Full panel again. Compare against your pre-season baseline. This is where you see whether the season depleted you and what needs rebuilding before the next cycle.
After illness or injury: Retest iron, FBC, and CRP to assess recovery status before resuming full training.
How to prepare: the athlete-specific pitfalls
Rest for 48 hours before testing. Intense exercise can acutely elevate CRP, AST, ALT, cortisol, and white blood cells while temporarily suppressing testosterone. Testing after a rest day (or two) gives a more representative baseline.
Test in the morning, fasted. Testosterone peaks in the morning. Fasting gives cleaner iron and glucose results.
Don't test during acute illness. Illness elevates inflammatory markers and suppresses ferritin accuracy (ferritin rises as an acute-phase reactant).
Stop iron supplements 24–48 hours before. Otherwise serum iron will be artificially elevated.
Mention all supplements. Pre-workouts, protein powders, creatine, BCAAs, iron, vitamin D, magnesium. Tell your clinician everything. Some supplements affect blood markers directly.
Tests to consider through Bloody Good
The athlete performance panel
The Performance Check covers 50+ biomarkers designed for active people: iron studies, FBC, testosterone, thyroid, cholesterol, liver function, vitamin D, HbA1c, and more. This is the purpose-built option for athletes who want a targeted health check without assembling individual tests.
Building your own athlete panel
Iron Studies (Including Ferritin) — the single most important athlete test
Full Blood Count (FBC) — haemoglobin and oxygen-carrying capacity
Vitamin D (25-OH) — muscle function and recovery
Testosterone Free/Total + SHBG — hormonal recovery status
Cortisol Blood Test — stress load assessment
High-Sensitivity CRP — inflammation baseline
Magnesium Blood Test — electrolyte and recovery
Thyroid Function Test (TFT) — metabolic rate
HbA1c — metabolic health
For the broadest coverage
The Bloody Good Test covers 100 biomarkers including everything above plus liver, kidney, cholesterol, B12, and more. For athletes who want a complete annual health baseline alongside performance markers.
What to do with your results
If ferritin is below 50 µg/L: Supplement and address the cause. For athletes, this is performance-limiting territory. Oral iron with vitamin C, taken on rest days (absorption is better when hepcidin is lower). Retest at 3 months. If refractory to oral supplementation, discuss iron infusion with your GP.
If testosterone is suppressed alongside elevated cortisol: Evaluate your training load, sleep, nutrition, and stress. This pattern often indicates under-recovery or under-fuelling, not a primary hormonal problem. Reduce training volume, improve sleep, ensure adequate caloric intake, and retest in 4–6 weeks.
If CRP is persistently elevated beyond training explanation: Investigate for underlying illness, injury, or immune dysfunction. Consider a rest period and retest.
If vitamin D is below 50 nmol/L: Supplement. This is directly relevant to bone health, injury risk, and immune function in athletes.
If multiple markers are off (low iron + low vitamin D + suppressed hormones + elevated inflammation): This pattern suggests systemic under-fuelling or overtraining. Step back from training. See a sports medicine physician or GP. This may be RED-S or overtraining syndrome, both of which require clinical management, not just supplementation.
If everything looks good: You've confirmed your body is supporting your training load. Keep testing seasonally to maintain the feedback loop.
Explore more biomarkers
Browse the Bloody Good Biomarker Directory
General information only. This article is not medical advice and is not a substitute for care from a qualified health professional. If you have concerning symptoms or urgent health issues, seek medical attention promptly.