The Lab Tests Every Endurance Athlete Should Run

At some point, if you're still dealing with fatigue despite doing the right things — eating well, sleeping as best you can, managing your training load — you have to stop adjusting variables and start measuring. Because there's only so far you can get by guessing when you don't actually know what's happening underneath the surface.

That's what this article is about.

When I approach lab testing for endurance athletes, I'm not asking whether a value falls within a reference range. I'm asking whether it's optimal for someone placing the kind of physiological demands on their body that endurance training creates. That's a meaningfully different bar — and it changes what you find. Regularly.

Here are the six areas I evaluate, what each test measures, and why the standard medical workup frequently misses what actually matters for performance.

Prefer to watch? The full video is below — or keep reading for the expanded breakdown

1. Iron Status: You Need More Than a Basic Iron Panel

Iron is foundational to aerobic performance. It's the functional center of hemoglobin for oxygen transport, myoglobin for oxygen storage in muscle tissue, and a required structural component of the cytochrome proteins in the mitochondrial electron transport chain. Iron insufficiency impairs both oxygen delivery and mitochondrial energy production at the same time — which is why suboptimal iron hits endurance athletes so broadly and is often so difficult to identify without the right tests.

Here's what most athletes and many physicians miss: ferritin is not included in a standard iron panel.

A basic iron panel typically measures serum iron, transferrin, and sometimes TIBC (total iron binding capacity). Those markers tell you about iron in circulation. Ferritin tells you about iron in storage — your reserve — and that's the number that matters most for endurance performance. You can have a completely normal iron panel and significantly depleted ferritin. That athlete is functionally iron deficient in every way that matters for training and recovery, but the standard test doesn't flag it because ferritin wasn't ordered.

What to order: Serum iron, transferrin saturation, TIBC, and ferritin — each specifically requested.

The threshold question: Many labs flag ferritin above 20–30 ng/mL as normal. In clinical practice, I routinely see endurance athletes with ferritin in that range who are fatigued, underperforming, and recovering poorly. For performance, I'm generally targeting ferritin above 50 ng/mL, and in many cases closer to 75–100, depending on the athlete's symptoms, training volume, and history. "Normal" and "sufficient for endurance training" are not the same target.

2. B12 Status: Why Serum B12 Alone Frequently Misses the Problem

B12 is essential for red blood cell production, neurological function, DNA synthesis, and the methylation cycle — which affects energy metabolism, neurotransmitter production, and recovery. Suboptimal B12 presents as fatigue, cognitive fog, reduced performance, and impaired recovery. The problem is that the standard test for it is a poor marker of what actually matters.

Serum B12 measures total B12 circulating in the blood. But a significant portion of circulating B12 can be bound to proteins in a biologically inactive form — unavailable to tissues. An athlete can have a serum B12 that looks perfectly normal while their cells are functionally B12 deficient. The blood test looks fine. The physiology isn't.

The better marker is methylmalonic acid (MMA). Here's why it's more meaningful: B12 is a required cofactor for the enzyme methylmalonyl-CoA mutase, which converts methylmalonic acid to succinyl-CoA in the energy metabolism pathway. When B12 is insufficient at the cellular level — regardless of serum levels — that conversion stalls, and MMA accumulates. Elevated MMA is therefore a direct marker of functional B12 deficiency in the tissues, even when serum B12 looks completely normal.

What to order: Serum B12 alongside methylmalonic acid (MMA) — urine or blood. Serum B12 tells you what's in the blood. MMA tells you what's actually happening inside the cells. Both are needed for the complete picture.

Athletes following plant-based diets or patterns that limit animal products are at significantly higher risk for functional B12 insufficiency, and this is one area where testing should be a routine part of monitoring rather than a response to symptoms alone.

3. Cortisol Rhythm: A Single Blood Draw Tells You Almost Nothing

Cortisol follows a precise daily arc under normal, healthy conditions: a sharp peak in the early morning that drives alertness and metabolic activation, followed by a gradual decline through the day that allows the nervous system to shift into parasympathetic recovery mode by evening. That rhythm is essential for both performance and recovery — and it's the rhythm itself, not just the absolute cortisol level, that breaks down under chronic training stress.

The result is the wired-but-tired pattern: flat and fatigued during the day, unable to wind down at night, sleep that's technically long enough but not restorative. Recovery becomes incomplete. The whole system gradually loses its capacity to adapt to training despite the training load remaining the same.

The problem with standard cortisol testing: A single cortisol blood draw gives you one data point at one moment. It cannot show you a rhythm. Even a morning cortisol draw only tells you whether the peak exists — it tells you nothing about what happens through the rest of the day.

What to order: A multi-point salivary cortisol test — typically four samples collected across the day, from early morning through evening — that maps the full diurnal pattern. This allows you to see whether cortisol is chronically elevated across the day, whether the morning peak is blunted and flat, or whether the rhythm is inverted (low in the morning, elevated at night). Each of those patterns represents a different physiological state and requires a different intervention. You can't distinguish between them from a single draw.

Salivary cortisol also has a practical advantage over blood cortisol: it measures free, biologically active cortisol rather than total cortisol, and it avoids the acute stress response that a blood draw itself can trigger — which can artificially inflate a single-point measurement.

4. Vitamin D: The Performance Target Is Much Higher Than the Standard Range

Vitamin D functions more like a hormone than a traditional vitamin. Receptors for it are present in virtually every tissue in the body — including muscle tissue, immune cells, and the gonads. That means vitamin D influences muscle function, immune regulation, and hormone production including testosterone. Deficiency is extremely common in athletes training indoors, training in northern latitudes, or getting outdoor training done in early morning or evening hours when UV index is insufficient for meaningful synthesis.

For endurance athletes specifically, low vitamin D is associated with elevated stress fracture risk, impaired immune function, reduced muscle strength, and lower testosterone production.

The threshold gap: Standard reference ranges consider anything above 20 ng/mL sufficient. For athletic performance, I'm targeting levels between 50 and 80 ng/mL. That's a substantially different target, and a large percentage of athletes — even those who train outdoors — fall well short of it, particularly through fall and winter months.

What to order: 25-hydroxyvitamin D (25-OH vitamin D). Simple, inexpensive, and consistently informative. If you haven't checked your vitamin D level recently, this is one of the highest-yield tests on this list for the cost.

5. Thyroid Function: TSH Alone Is an Incomplete Picture

The thyroid regulates metabolic rate, energy production, body temperature, and recovery. Suboptimal thyroid function — even subtle suboptimal function — produces fatigue, weight changes, poor recovery, cold intolerance, and cognitive slowing. In athletes, this is easy to attribute to overtraining, life stress, or inadequate sleep without ever identifying the actual driver.

The standard workup measures only TSH — thyroid stimulating hormone — which is a pituitary signal. It tells you whether the pituitary thinks the thyroid needs to produce more hormone. It does not directly measure what the thyroid is actually producing or how well the body is using it.

What gets missed with TSH-only testing: The thyroid produces primarily T4 (thyroxine), which is a relatively inactive storage form. T4 is converted to T3 (triiodothyronine) in peripheral tissues — and T3 is the active form that actually binds to cellular receptors and drives metabolic effects. That conversion process can be meaningfully impaired by chronic stress, high training loads, and micronutrient deficiencies (particularly selenium and zinc) — even when TSH and T4 are both normal. An athlete can have adequate TSH, adequate Free T4, and still have insufficient Free T3 reaching their cells. The cells aren't getting the active thyroid signal they need. You only see this if you measure all three.

What to order: TSH, Free T3, and Free T4. All three, not TSH alone. In some cases, thyroid antibodies (anti-TPO and anti-thyroglobulin) are also worth evaluating to rule out autoimmune thyroid disease, which can produce intermittent and fluctuating symptoms that look identical to training fatigue.

6. Comprehensive Micronutrient Panel: The Full-Spectrum Picture

Individual nutrient tests answer specific questions. But for athletes dealing with persistent fatigue or performance that hasn't responded to obvious interventions, a comprehensive micronutrient assessment that evaluates a broad spectrum of vitamins, minerals, amino acids, and antioxidants simultaneously can identify patterns and gaps that targeted individual tests would miss.

One panel I use and recommend is the Nutrient Zoomer from Vibrant Wellness. It's an antibody-based assessment that evaluates nutritional status across a wide range of markers — fat-soluble vitamins, water-soluble vitamins, minerals, amino acids, omega fatty acids, and antioxidant capacity — and does so by assessing functional nutrient status rather than simply circulating levels. That distinction matters: it gives you a more accurate picture of what's actually available to your cells and tissues, not just what's present in the bloodstream at the moment of the draw.

For athletes who've already addressed the obvious foundational variables and are still not where they should be, this kind of comprehensive assessment frequently reveals what's been driving the problem — and gives you a clear, prioritized roadmap for what to address rather than a list of things to try.

The Complete Testing Protocol

To pull this together, here's the full list of what I recommend for endurance athletes dealing with persistent fatigue or performance that doesn't match their training:

Iron panel with ferritin ordered separately — serum iron, transferrin saturation, TIBC, and ferritin. Do not assume ferritin was included in a basic iron test; it usually wasn't.

Serum B12 plus methylmalonic acid (MMA) — serum B12 for circulating levels, MMA to evaluate functional status at the cellular level.

Multi-point salivary cortisol — four samples across the day to map the full diurnal rhythm. A single blood draw is not an adequate substitute.

25-OH vitamin D — with a performance target of 50–80 ng/mL, not the standard "above 20" threshold.

Complete thyroid panel: TSH, Free T3, and Free T4 — all three, not TSH alone.

Comprehensive micronutrient panel — for a full-spectrum assessment of where the functional gaps are.

From Guessing to Knowing

The value of this testing isn't just information — it's the change in efficiency it produces. When you're working off symptoms alone, you're cycling through interventions based on what might be the problem. When you have data, you know what's limiting you. Fixing it becomes a direct process rather than an iterative one.

The other critical factor is interpretation. Having these numbers means relatively little if they're evaluated against general population reference ranges by someone who isn't thinking about athletic performance. These results need to be read through a performance lens — with an understanding of what optimal looks like for someone placing endurance training demands on their physiology, not just what counts as "not sick" in a sedentary adult.

That's the approach at Natural Athlete Clinic — functional lab testing with performance-oriented interpretation and a targeted protocol built around what your results actually show.

Dr. Jason Barker is a naturopathic physician with 24 years of clinical experience working with endurance athletes. He is a two-time Ironman finisher and the founder of Natural Athlete Clinic. For functional lab testing, individualized protocols, and performance-focused guidance, visit naturalathleteclinic.com.