BLOOD TESTING FOR CYCLISTS: WHICH MARKERS ACTUALLY MATTER AND HOW TO READ THEM

You are four weeks into a training block. The structure is right. The intervals are hitting their numbers. Recovery days feel like recovery days. And yet something is not connecting. The Tuesday session that should feel hard-but-doable feels like you are dragging an anchor. The watts are there on paper but the sensation is wrong — like the engine is misfiring on a fuel it cannot quite access.

You check the training load. It is fine. You check your sleep. Seven and a half hours, consistent. You check your diet. Ticking every box.

So you do what most cyclists do. You push harder. You add an extra interval. You tell yourself it is a bad week. And you grind through another month before the wheels properly come off.

Here is what nobody tells you. For a significant percentage of endurance athletes, the answer is not in the training plan. It is in the blood. And most of them will never find it — not because the tests do not exist, but because the reference ranges their GP uses were designed for people who sit at a desk all day, not people who ride five days a week.

The difference between clinical normal and athletic optimal is where months get wasted. Sometimes years.

Why Clinical Reference Ranges Lie to Athletes

Your GP works with reference ranges built from population data. That population is mostly sedentary, mostly untrained, and mostly not doing six hours on a bike every weekend. The ranges are designed to catch disease. They are spectacularly bad at catching suboptimal performance.

Here is a concrete example. The clinical low threshold for ferritin — the protein that stores iron — sits at around 12-15 ng/mL in most labs. Below that, your GP flags iron deficiency. Above it, you get told everything is fine.

Richard Burden's research at the University of Bath tells a different story. Performance in endurance athletes starts to degrade when ferritin drops below 50 ng/mL. That is three to four times higher than the clinical threshold. A cyclist sitting at 25 ng/mL is technically normal by their GP's standards and functionally compromised on the bike.

This is not a niche problem. Nicky Keay, whose work on Relative Energy Deficiency in Sport (RED-S) has reshaped how sports medicine thinks about endocrine function in athletes, has been saying for years that we need athlete-specific reference ranges. The clinical ranges were never built for people who burn 3,000-4,000 calories on a Saturday morning.

The good news. Once you know which markers to track and what the athletic ranges actually are, this becomes one of the most fixable performance limiters in the sport. No extra training. No new equipment. Just information that already exists in your blood, waiting to be read properly.

The Six Markers That Actually Matter

You do not need to test everything. A comprehensive athletic blood panel covers six categories that, between them, explain 90% of the unexplained fatigue, stalled progress, and phantom overtraining that cyclists bring to coaches and GPs every year.

1. Iron and Ferritin: The Oxygen Pipeline

Iron is the mineral that makes haemoglobin work. Haemoglobin carries oxygen from your lungs to your muscles. Without adequate iron, your oxygen delivery system runs at a deficit that no amount of VO2max training can overcome. You are literally building an engine that cannot get enough fuel.

Ferritin is the storage form of iron — the reserve tank. When ferritin drops, your body is spending iron faster than it is replacing it. And cyclists burn through iron at a remarkable rate.

Why cyclists are particularly vulnerable:

• Foot-strike haemolysis. Every pedal stroke, particularly when riding hard, causes micro-damage to red blood cells. Not as dramatic as running, but cumulative over 8-15 hours a week.
• Sweat losses. A litre of sweat contains roughly 0.3-0.4 mg of iron. On a three-hour summer ride, that adds up.
• GI blood loss. Hard efforts, particularly in the heat, divert blood away from the gut. The resulting low-grade GI irritation can cause microscopic bleeding you never notice.
• Inadequate dietary intake. Cyclists watching their weight often cut red meat — the best dietary source of haem iron.

What to test: Serum ferritin, serum iron, transferrin saturation, full blood count (for haemoglobin and red cell indices).

Athletic optimal ranges:

• Ferritin: 50-150 ng/mL (clinical normal starts at 12-15)
• Haemoglobin: 140-170 g/L for men, 120-150 g/L for women (note: endurance athletes may sit slightly lower due to plasma volume expansion — this is not the same as iron deficiency)
• Transferrin saturation: 20-45%

Red flags: Ferritin below 30 ng/mL with fatigue. Ferritin below 50 ng/mL with a downward trend across two panels. Haemoglobin dropping while training load is stable. Any of these warrant action, not another training block.

Richard Burden's work is clear on this: do not wait for clinical anaemia to start addressing iron status. By the time your GP calls it a problem, you have been underperforming for months.

2. Vitamin D: The Marker Almost Everyone Gets Wrong

Vitamin D is not really a vitamin. It is a hormone precursor that influences bone density, immune function, muscle repair, inflammation, and mood. And if you live anywhere north of 35 degrees latitude — which covers most of the UK, Ireland, Northern Europe, and northern United States — you are almost certainly deficient by the end of winter.

The clinical sufficiency threshold for vitamin D is 50 nmol/L (20 ng/mL). Above that, most GPs will tell you you are fine.

The athletic optimal range is 75-125 nmol/L (30-50 ng/mL). The research from Nicky Keay and others working in sports endocrinology consistently shows that athletes below 75 nmol/L have higher rates of stress fractures, slower recovery from muscle damage, and impaired immune function — particularly the upper respiratory tract infections that ruin training blocks every February and March.

Why cyclists miss this:

You ride outside. You assume you are getting sun. But vitamin D synthesis requires UVB radiation at the right angle, which in the UK and Ireland is essentially absent from October through March. Even in summer, sunscreen (which you should be wearing) blocks UVB. And the amount of skin exposed while wearing cycling kit is limited.

What to test: 25-hydroxyvitamin D (25(OH)D). This is the standard test. Ignore anyone offering 1,25-dihydroxyvitamin D as a screening test — it does not reflect stored levels.

Athletic optimal range: 75-125 nmol/L (30-50 ng/mL).

Supplementation: If you are below 75 nmol/L, 2,000-4,000 IU of vitamin D3 daily through winter (October-April) is the standard recommendation for athletes. Take it with a fat-containing meal — vitamin D is fat-soluble and absorbs poorly without dietary fat. Retest at 8-12 weeks to confirm you are moving in the right direction.

This is the cheapest performance-adjacent intervention in cycling. A year's supply of quality vitamin D3 costs less than a set of brake pads.

3. Testosterone and Hormonal Markers: Especially for Masters Cyclists

If you are over 40, your testosterone is declining. That is not speculation — it is physiology. Roughly 1-2% per year after age 30, with the decline accelerating slightly after 50. The question is whether your decline is normal, faster than expected, or clinically significant.

Here is where it gets really interesting. Total testosterone is the number most GPs report. But total testosterone includes a large fraction bound to sex hormone-binding globulin (SHBG), which is biologically unavailable. What matters for muscle repair, recovery speed, red blood cell production, and bone density is free testosterone — the unbound fraction that actually reaches the tissue.

SHBG increases with age. It increases with endurance training. It increases with caloric restriction. Three things that describe most masters cyclists perfectly. Two riders with identical total testosterone can have dramatically different free testosterone, and the one training 10 hours a week on a caloric deficit may be functionally depleted while looking clinically normal.

What to test: Total testosterone, free testosterone (or calculated free T from total T + SHBG + albumin), SHBG, and — if your GP will include it — the cortisol-to-testosterone ratio.

Athletic context: The cortisol-to-testosterone ratio matters more than either number in isolation. When cortisol stays chronically elevated (from overtraining, underfuelling, or life stress) and testosterone drops, your capacity to absorb training shrinks. This is the hormonal signature of overreaching, and it presents as fatigue, mood changes, and stalled fitness that look exactly like overtraining syndrome.

What a low result means: Persistently low free testosterone with symptoms — fatigue that rest does not fix, loss of motivation, reduced recovery between sessions, loss of muscle despite training — warrants a proper endocrine workup. This is a conversation with a sports-literate GP or endocrinologist, not a self-diagnosis from a home blood kit.

What a low result does not mean: It does not mean you need TRT. Training structure, sleep quality, caloric adequacy, body fat levels, and stress management address the majority of borderline results. The training itself is hormonal medicine — high-intensity intervals acutely raise testosterone and growth hormone. Zone 2 work reduces chronic cortisol. Strength training maintains anabolic signalling. Fix the inputs before reaching for pharmaceutical solutions.

4. Thyroid Function: The Overlooked Mimic

Thyroid dysfunction affects up to 10% of endurance athletes. It mimics overtraining so precisely that many athletes spend months adjusting their training before anyone thinks to check a simple blood test.

The thyroid gland regulates metabolic rate. When it underperforms — subclinical hypothyroidism — everything slows down. Recovery takes longer. Weight loss stalls despite a caloric deficit. Fatigue becomes a background constant that rest weeks do not shift. Heart rate at threshold feels laboured. Motivation fades.

Every single one of those symptoms gets attributed to overtraining first. And every single one of them will persist until the actual cause is identified.

What to test: TSH (thyroid-stimulating hormone), free T3, and free T4. TSH alone misses subclinical cases where the thyroid is struggling but compensating. The full panel takes 30 seconds longer to order and catches problems that TSH alone will not.

Athletic optimal ranges:

• TSH: 0.5-2.5 mIU/L (clinical normal extends to 4.0-4.5, but values above 2.5 in an athlete warrant monitoring)
• Free T4: 12-22 pmol/L
• Free T3: 3.5-6.5 pmol/L

Red flags: TSH above 3.0 with fatigue symptoms. Free T3 at the bottom of the range with persistent tiredness. Any combination of unexplained weight gain, cold intolerance, and prolonged recovery that does not respond to reduced training load.

Nicky Keay's research on RED-S has highlighted that thyroid suppression is often a downstream consequence of chronic energy deficiency — the body downregulates metabolism to conserve energy when caloric intake is insufficient for training load. In those cases, eating more fixes the thyroid. No medication required.

5. Full Blood Count: The Baseline You Cannot Skip

A full blood count (FBC) is the most basic and most informative panel you can run. It measures haemoglobin, red blood cells (count, size, and shape), white blood cells, and platelets. It costs almost nothing and tells you an enormous amount about your body's current state.

What to look for:

• Haemoglobin. Your oxygen carrier. If haemoglobin is dropping while training load is stable, something is wrong — iron deficiency, overtraining, or chronic inflammation are the usual suspects.
• Mean corpuscular volume (MCV). The size of your red blood cells. High MCV suggests B12 or folate deficiency. Low MCV suggests iron deficiency. Either way, the training plan is not the problem.
• White blood cell count. Chronically elevated white cells suggest ongoing inflammation or infection. Chronically suppressed white cells can indicate overtraining syndrome. Both are red flags in an athlete who is not acutely ill.
• Haematocrit. The proportion of your blood that is red blood cells. Endurance training increases plasma volume, which dilutes haematocrit — so-called sports anaemia. This is a positive adaptation, not a problem, but it needs to be distinguished from actual anaemia.

The trend is everything. A single FBC snapshot is useful. Three snapshots across a season are transformative. You are looking for movement, not absolutes. Haemoglobin drifting down by 5-10 g/L across two panels while training load increases is a signal. A static number in isolation is just a number.

6. CRP: Your Inflammation Smoke Alarm

High-sensitivity C-reactive protein (hs-CRP) measures systemic inflammation. It is not cycling-specific, but it is performance-specific, because chronic low-grade inflammation is one of the most reliable predictors of poor recovery, increased injury risk, and stalled fitness.

Context matters: CRP rises acutely after hard training. That is normal. A CRP reading taken 48 hours after a race or a brutal interval session will be elevated and tells you nothing useful. CRP measured during a recovery week — when acute training stress has subsided — is a genuine baseline.

Athletic context:

• CRP below 1.0 mg/L: Low systemic inflammation. This is where you want to be.
• CRP 1.0-3.0 mg/L: Moderate. Worth monitoring. May reflect accumulated training stress, poor sleep, or dietary inflammation.
• CRP above 3.0 mg/L during a rest week: This is a signal. Something is driving inflammation that training will compound, not fix. Gut health, chronic low-grade infection, poor sleep, ultra-processed diet, or excessive training volume are the common culprits.

Let me be really clear about this. Training on top of chronic inflammation is like doing intervals with a slow puncture. You can push through it. You will not get faster. And you will wear out the tyre.

When to Test: Timing Is Half the Battle

Testing at the wrong time is worse than not testing at all, because the numbers will mislead you. Every marker on this list is influenced by acute training stress, hydration status, time of day, and recent nutrition.

The protocol that produces reliable results:

1. Fasted morning draw. First thing in the morning, before food or coffee. Cortisol peaks in the early morning, so consistency here matters for hormonal markers. Ferritin is also affected by recent food intake.

2. During a recovery week. Not after your hardest block. Not during race week. A recovery or deload week gives the closest approximation of your actual baseline. Hard training temporarily suppresses testosterone, elevates CRP, and redistributes iron — all of which create false signals.

3. Same lab each time. Different labs use different assays, different machines, and slightly different reference ranges. Comparing a result from Lab A in January with Lab B in June introduces noise that obscures the trend. Pick one lab. Stick with it.

4. Same time of year. Vitamin D in July will look very different from vitamin D in February. Comparing like-for-like across years requires seasonal consistency.

5. Minimum twice a year. End of winter base (January-February) and mid-season (June-July) gives you two data points that span the key training phases. Three times a year — adding an end-of-season test in October — is better if you are tracking something specific.

How to Read Your Results Without Losing Your Mind

You get the email. You open the PDF. You see a wall of numbers, some flagged, some not. Here is how to make sense of it without spiralling.

Step one: Ignore the flags for a moment. The lab flags values outside their reference range, which — as we have established — are built for sedentary populations. A ferritin of 25 ng/mL will not get flagged. That does not mean it is fine for you.

Step two: Compare to athletic optimal ranges, not clinical ranges. Use the ranges in this guide. If a marker sits inside the clinical range but outside the athletic range, it deserves attention. Not panic. Attention.

Step three: Look at the trend, not the snapshot. One low vitamin D result is a data point. Two consecutive low results six months apart are a pattern. Three results showing a downward trend are a directive. This is why you test consistently and keep your results filed where you can access them.

Step four: Cluster, do not isolate. Low ferritin plus low haemoglobin plus low MCV is a clear iron deficiency picture. Low ferritin alone might be early-stage depletion or a lab artefact. Markers that move together tell a story. Markers in isolation can mislead.

Step five: Context before conclusions. Were you at the end of a hard block? Were you fighting off a cold? Did you have a terrible week of sleep? Every result needs to be read against the backdrop of what was happening in your training and life at the time of the draw. This is why a sports-literate practitioner adds so much value — they ask the questions that a set of reference ranges cannot.

The Athletic Optimal Ranges Cheat Sheet

Here is every marker covered in this guide, with both the clinical range your GP uses and the athletic optimal range that actually matters for performance.

| Marker | Clinical Normal | Athletic Optimal | Why the Gap Matters | |--------|----------------|-----------------|-------------------| | Ferritin | 12-300 ng/mL | 50-150 ng/mL | Performance drops well before clinical deficiency | | Haemoglobin (M) | 130-170 g/L | 140-170 g/L | Low-normal often reflects functional iron deficit | | Haemoglobin (F) | 115-155 g/L | 120-150 g/L | Same as above, with higher prevalence of actual deficiency | | Vitamin D | >50 nmol/L | 75-125 nmol/L | Immune, bone, and recovery benefits kick in above 75 | | TSH | 0.4-4.5 mIU/L | 0.5-2.5 mIU/L | Above 2.5 in a fatigued athlete warrants investigation | | Free T4 | 9-25 pmol/L | 12-22 pmol/L | Sitting at the bottom of clinical range is not optimal | | Total Testosterone (M) | 8.6-29 nmol/L | 15-25 nmol/L | Below 15 with symptoms warrants endocrine review | | CRP | <5.0 mg/L | <1.0 mg/L | Below 1.0 is low inflammation, above 3.0 is a red flag |

Print this. Take it to your blood draw. Compare your results against these columns, not the ones on the lab report.

What Your GP Will Not Tell You (Because They Were Not Trained To)

This is not a criticism of GPs. They are brilliant at what they are trained to do — diagnose and treat disease. They are not trained to optimise athletic performance. Different job. Different lens.

Here is what that means in practice:

They will not proactively test ferritin in men. Iron deficiency screening in primary care is heavily weighted towards women of reproductive age. Male cyclists requesting ferritin need to ask specifically. Some GPs will push back. Push back harder.

They will not test free testosterone. Most GP panels include total testosterone only. Free testosterone or SHBG require a specific request. In the UK, your GP can order these through NHS pathways, but they may not understand why you want them. Print your last result, show the trend, explain the symptoms.

They will not flag ferritin at 25 ng/mL. It is inside the clinical range. They have 200 patients to see this week. Flagging a value that their system marks as normal is not a battle they are going to fight on your behalf. That is your job.

They will interpret thyroid through a disease lens. A TSH of 3.5 is clinically normal. In a fatigued endurance athlete with unexplained weight gain and prolonged recovery, it might be clinically normal but athletically significant. Nicky Keay has published extensively on this distinction — thyroid suppression as a marker of energy deficiency in athletes, not a primary thyroid disorder. The treatment is calories, not levothyroxine.

The practical solution: find a sports-literate GP, a sports medicine clinic, or a specialist like those in British Association of Sport and Exercise Medicine (BASEM) or the British Association of Sport and Exercise Sciences (BASES) who understands athletic physiology. Alternatively, use a private blood testing service, learn to read the numbers yourself (this guide is a solid start), and bring the results to your GP when something warrants medical attention.

The Minimum Viable Blood Panel

If you are going to test one thing, test ferritin. If you are going to test five things, test these:

1. Ferritin — the single most common nutritional deficiency in endurance athletes
2. Full blood count — haemoglobin, MCV, white cells, the structural basics
3. Vitamin D (25-OH) — particularly if you train through winter in the UK or Ireland
4. TSH + free T4 — rules out the thyroid mimic that catches so many athletes
5. CRP (high-sensitivity) — your inflammation baseline

If you are over 40 and male, add total testosterone, free testosterone (or SHBG + albumin for the calculated free T), and cortisol. If you are female, add the same hormonal panel and discuss cycle-phase timing with whoever is ordering the blood work — oestrogen and progesterone fluctuations influence when in your cycle results are most meaningful.

If budget is tight, rotate markers. Ferritin and FBC every six months. Vitamin D once a year in February. Thyroid and hormones annually. You do not need everything every time. You need the right things at the right time.

The Red Flags That Mean Stop Training and See Someone

Most blood work results call for monitoring, not alarm. But some combinations demand immediate attention:

• Haemoglobin below 120 g/L (men) or 110 g/L (women) with ferritin below 20 ng/mL. This is frank iron deficiency anaemia. Stop training hard. See your GP. Get a proper workup including iron studies, reticulocyte count, and potentially GI investigation if the deficiency is unexplained.

• TSH above 4.0 mIU/L with fatigue, weight gain, and cold intolerance. This needs medical assessment. Hypothyroidism is treatable but requires prescription medication, not training adjustments.

• Total testosterone below 8 nmol/L (men) on two separate morning fasted draws. This is below the clinical threshold and warrants endocrine referral regardless of symptoms.

• CRP above 10 mg/L during a rest period without an obvious acute infection. Something is driving significant inflammation. Find out what.

• Haemoglobin rising sharply without altitude training or supplementation. This is unusual and warrants investigation — it can indicate dehydration (benign) or something more serious.

None of these are training problems. All of them will masquerade as training problems until someone looks at the blood work.

Making It Actionable: Your Testing Protocol

Here is the protocol I recommend to every cyclist in the community.

January-February (end of base phase): Full panel — ferritin, FBC, vitamin D, thyroid (TSH, free T3, free T4), CRP, and hormones if you are over 40. This gives you the winter baseline. Vitamin D will likely be at its lowest. Ferritin may have drifted if you ramped up training volume through autumn without adjusting iron intake.

June-July (mid-season): Repeat ferritin, FBC, CRP. Add anything that was borderline on the first panel. This catches mid-season depletion — the gradual iron drain that accompanies high training volumes and racing.

October (optional, end of season): Vitamin D recheck before winter supplementation begins. Hormonal markers if you had a particularly heavy season. This is the "reset and plan" panel.

Every panel: Fasted. Morning. Recovery week. Same lab. File every result where you can find it in a year.

The Bigger Picture

Blood testing is not a magic bullet. It will not make you faster on its own. What it will do is eliminate invisible bottlenecks — the iron deficiency that steals 3-5% of your power, the vitamin D deficit that lands you in bed with every cold that circulates through the office, the thyroid suppression that makes every interval feel harder than it should.

These are fixable problems. Every single one of them. And fixing them often produces improvements that months of additional training cannot match, because you cannot out-train a nutritional deficiency or a hormonal imbalance. The training only works when the underlying chemistry supports it.

Two blood draws a year. A conversation with a sports-literate practitioner. The willingness to read your numbers through an athletic lens rather than a clinical one.

That is all it takes.

And if you want to discuss your own blood work results with other cyclists who are tracking the same markers, comparing notes, and holding each other accountable to actually following through — the Roadman Cycling community is where that conversation happens every week.