Athlete Biomarkers: Pre-Season Bloodwork Guide 2026

TL;DR: A pre-season panel of 12 markers shows how training and load appear in the blood. Ferritin, the cortisol-testosterone ratio and CRP are looked at particularly often. How values are interpreted is for a doctor to decide in the context of symptoms and other findings.

Why a blood test before the season?

You’ve been training for months. Volume is climbing, races are approaching — but performance is stuck somewhere. Fatigue after training lasts longer than expected. Resting heart rate is higher than last autumn. HRV is dropping.

Possible associations are overtraining, iron deficiency or low vitamin D. All three look similar from the outside. A blood test can help tell them apart.

A pre-season panel describes three things. First, it shows values that can relate to training progress, such as iron stores or vitamin D; what follows from them is for a doctor to decide. Second, it gives a baseline: if values deviate mid-season, the personal normal is known. Third, overreaching can show up in the lab before performance declines.

Timing in the example: 6–8 weeks before the first race or the start of the main training phase. Labs usually ask for a fasted morning draw. Acute training load can distort CK, white blood cell count and ferritin; this is why the time since the last intensive session matters.

The Pre-Season Athlete Panel: 12 Markers at a Glance

Here is the complete panel with the lab reference. Laboratory norms apply to “healthy average people”; in athletes, values can shift. The reference range of your lab remains what counts for interpretation.

Example: A ferritin of 22 ng/ml is, for a woman, above the lower limit of 15 ng/ml shown in the table. In Lab2go you see the value over time next to your lab’s reference range; what it means for one’s own sport is something to discuss with the doctor.

Iron and Ferritin: The Most Common Bottleneck in Endurance Sport

Ferritin is a frequently examined single marker for endurance athletes — and often misinterpreted. The laboratory “normal” threshold of 15 ng/ml means iron stores are not empty. Whether that is enough for the load in sport is discussed in sports medicine.

Why iron demand can be higher in athletes:

• Haemolysis from mechanical stress (foot-strike in running destroys red blood cells)
• Sweat iron losses (especially during long endurance sessions)
• Inflammation-triggered hepcidin release after hard training blocks iron absorption for 3–6 hours
• Increased red blood cell turnover from altitude training or intensive phases

Consequences of low ferritin:

Studies describe a lower VO2max at low ferritin — even without overt anaemia (latent iron deficiency). How strongly this plays out in an individual case varies.

Athletic anaemia vs. dilutional anaemia:

Endurance athletes often show lower haemoglobin due to increased plasma volume — this is physiological (dilutional anaemia) and not a problem. True anaemia is diagnosed by a doctor and has to be told apart from it. Whether iron is supplemented is for a doctor to decide. Read more in the ferritin and iron deficiency guide and the iron supplementation guide.

Hormones: Testosterone, Cortisol and Thyroid

Testosterone and Cortisol: The Ratio Matters

Testosterone and cortisol in isolation tell only half the story. Sports medicine therefore looks at the cortisol-testosterone ratio (C/T ratio). It reflects the balance between catabolic (cortisol) and anabolic (testosterone) processes.

Calculation: Cortisol (μg/dl) ÷ Testosterone (nmol/L)

Fixed cut-offs for the C/T ratio are inconsistent and are not given here; what counts is the course at the same lab and interpretation by a doctor. In overtraining, testosterone is described as dropping (hypothalamus reduces GnRH signalling) while cortisol can remain elevated — the ratio then rises.

Thyroid: Free T3 Is the Most Sensitive Marker

TSH is often misleadingly normal in athletes. The hypothalamic-pituitary axis responds to energy deficit by suppressing free T3 — while TSH stays normal. Free T3 is the active thyroid hormone driving energy metabolism in every cell.

In RED-S (Relative Energy Deficiency in Sport), a drop in free T3 is described — sometimes at the lower edge of the reference range, yet still reported as “normal.” The result: impaired fat oxidation, slower muscle recovery, reduced resting metabolic rate.

Read more about thyroid markers in the thyroid values guide.

Muscle Markers: CK, LDH and Myoglobin

CK (Creatine Kinase)

CK is the most direct measure of muscle cell damage. It rises acutely after intensive training and falls back within 72 hours. For interpretation:

• Acute post-training CK rise: normal and an expression of muscle adaptation
• Clearly elevated values: described as overloading; the muscles then need time to recover
• Persistently elevated CK (without recent hard training): described as a signal of chronic muscle stress
• Very high values: can point to rhabdomyolysis and call for prompt medical evaluation

Timing: CK depends on the time since the last intensive session. A draw shortly after a hard interval produces an acute spike, not a chronic picture.

LDH (Lactate Dehydrogenase)

LDH rises with cell damage in both muscle and erythrocytes. In endurance athletes, elevated LDH is described as an indicator of mechanical haemolysis — red blood cells destroyed by foot-strike impact during running. Persistently elevated LDH is read alongside haemoglobin and ferritin.

Inflammation Markers: CRP and White Blood Cells

CRP (C-Reactive Protein)

CRP measures systemic inflammation. After hard training, CRP rises acutely — this is a normal training effect. A CRP that stays elevated between sessions is more informative.

Interpretation: For hs-CRP, the 2003 statement by the AHA and CDC names risk categories for cardiovascular risk (below 1.0 mg/L low, 1.0–3.0 mg/L intermediate, above 3.0 mg/L high). In athletes, acute influences such as training and infections make interpretation harder.

Your hs-CRP is 2.8 mg/L three weeks running — despite normal training? Possible associations are sleep, nutrition or a subclinical infection; the doctor interprets this in light of symptoms and other findings.

White Blood Cells

In overtraining, white blood cell count is described as dropping — the immune system is suppressed. Values below the reference range in athletes with high training volume count as a possible warning sign. More frequent infections are often mistakenly attributed to the training itself.

RED-S and Overtraining Syndrome: When Biomarkers Warn

RED-S (Relative Energy Deficiency in Sport)

RED-S is the updated, sex-neutral definition of the old “Female Athlete Triad.” It affects men just as much as women — wherever caloric intake and training energy demand consistently fail to match.

Laboratory signatures of RED-S:

• Ferritin declining (despite normal diet)
• Free T3 reduced
• Testosterone (men) reduced
• IGF-1 suppressed
• Cortisol elevated, insulin and fasting glucose very low

RED-S is considered reversible; assessment and treatment belong in medical and sports-medicine hands.

Overtraining Syndrome (OTS)

OTS is the extreme form of chronic overload. Diagnosis is clinical — there is no single blood test for it. The lab provides clues:

Marker combination in OTS:

• C/T ratio elevated
• Testosterone reduced (men)
• Haemoglobin declining across several measurement points
• CK chronically elevated
• White blood cells below the reference range
• HRV persistently below personal baseline trend

No single marker alone makes the diagnosis. Several of these patterns together count as a stronger signal; the diagnosis is made by a doctor. Read more about HRV as an early warning system in the HRV guide and resting heart rate as a fitness marker.

HRV and Resting Heart Rate: Daily Trend Markers Between Blood Tests

Blood tests are snapshots. HRV (heart rate variability) and resting heart rate deliver data daily. Combined with lab values, they create the full picture.

HRV trends that predict lab findings:

• Declining HRV baseline over 2+ weeks + elevated resting heart rate: blood test will likely show elevated C/T ratio or declining ferritin
• Stable HRV despite increasing training volume: adaptation is working well, lab values probably stable
• Sharp HRV drop after one week with no training change: infection markers (CRP, white blood cells) come into question

Wearable data does not replace the lab — but it gives clues about phases in which lab values can be especially informative. More on this in the wearable data quality guide.

Example Sequence: Pre-Season Documentation

Here is an example sequence for triathletes, runners or CrossFit athletes who want to document their first data-driven season.

Week 0 (Pre-Season Blood Draw):

Looked at: complete blood count, ferritin, vitamin D (25-OH), TSH, free T3, testosterone (men), morning cortisol, CK, hs-CRP, whole-blood magnesium, LDH, urea.

Timing: labs usually ask for a fasted morning draw; for CK, white blood cells and ferritin, the time since the last hard session matters. The results can be documented in lab2go.

Weeks 1–4 (Influencing factors described in studies):

• Low ferritin: iron preparations differ in form and tolerability; whether to supplement is for a doctor to decide. The blood draw protocol describes the measurement conditions.
• Low vitamin D: studies have examined giving vitamin D; the dose is set by a doctor.
• Elevated CRP: studies describe associations with sleep, nutrition and training volume.
• Elevated C/T ratio: sports medicine describes an association with high training volume and missing rest days.

Control measurement:

A later measurement can include ferritin, vitamin D, CRP and the C/T ratio (testosterone + cortisol). How often to measure is decided by the doctor. In lab2go you see trend curves — not just the current value.

Full details on systematic biomarker baseline tracking here.

Summary: Blood as a Training Partner

If you keep a training log, you can also keep a blood log. Ferritin, testosterone, cortisol, CK and CRP describe what is happening in the body — independently of what the Garmin screen says.

A pre-season panel costs around 80–120 euros as a self-pay service. In lab2go you can document your values and their trend over time, including a mid-season control measurement.

This article does not replace medical diagnosis. For persistently very high CK, severely reduced testosterone, or symptoms such as sustained extreme fatigue, cardiac arrhythmias or joint pain, a medical evaluation is always indicated.