Runner's Blood Tests Explained: The Most Important Blood Markers for Endurance Athletes

Training data from wearable devices provides valuable information about fitness and recovery, but it only tells part of the story. If you experience persistent fatigue, declining performance or unusually slow recovery despite appropriate training, blood tests may help identify underlying medical or nutritional factors that wearable devices cannot detect.

Blood tests should not be performed simply because you exercise. However, when interpreted appropriately by a healthcare professional, they can provide important insights into overall health and may help explain changes in performance, energy levels or recovery. By combining blood test results with wearable health data, athletes can build a much more complete picture of their long-term wellbeing.

Endurance exercise places unique demands on the body. Over time, training may influence iron stores, vitamin levels, hormonal balance, inflammation, recovery, hydration and red blood cell production. Although most recreational runners remain healthy, persistent symptoms should not automatically be attributed to training alone.

Blood tests may help identify medical conditions — such as iron deficiency, thyroid disorders or vitamin deficiencies — that require treatment rather than changes to exercise alone. They are most valuable when interpreted in the context of your symptoms, medical history and the physiological data your wearable already collects.

Ferritin measures the body's stored iron. Iron is essential for producing haemoglobin — the protein that carries oxygen in red blood cells — which directly affects endurance capacity. Low ferritin levels may contribute to fatigue, reduced endurance, poor exercise tolerance, slower recovery and reduced training capacity.

Importantly, iron deficiency can occur even when haemoglobin remains within the normal range. This is sometimes called non-anaemic iron deficiency, and it can still significantly impair performance. Because excessive iron supplementation can also be harmful, treatment should only be started following appropriate medical assessment and laboratory confirmation — not based on symptoms alone.

A Full Blood Count evaluates haemoglobin, red blood cells, white blood cells and platelets. It may help identify anaemia, infection, inflammation or other blood disorders. For athletes experiencing unexplained fatigue, an FBC is typically one of the first investigations requested.

Results should always be interpreted alongside symptoms and medical history — a haemoglobin value at the lower end of the normal range in an athlete with significant fatigue and declining performance is a different clinical situation from the same result in someone who feels entirely well.

Vitamin B12 supports red blood cell production, nervous system function and energy metabolism. Low levels may contribute to fatigue, weakness, reduced exercise performance, tingling sensations and difficulty concentrating. Athletes following vegetarian or vegan diets have an increased risk of B12 deficiency, as B12 is found primarily in animal-derived foods.

Folate (vitamin B9) works alongside B12 and is important for red blood cell production and DNA synthesis. Both are routinely measured when investigating fatigue in athletes. Deficiency of either can contribute to a type of anaemia (megaloblastic anaemia) that impairs oxygen-carrying capacity and endurance performance.

Vitamin D plays an important role in bone health, muscle function, immune health and recovery. Low vitamin D levels are common across many countries — particularly during winter months when sun exposure is limited — and are frequently identified in both recreational and elite athletes. Correcting deficiency should be guided by healthcare professionals according to local recommendations and laboratory confirmation of levels.

The thyroid gland regulates metabolism throughout the body. Common thyroid blood tests include TSH (thyroid-stimulating hormone) and free T4. Abnormal thyroid function — whether an underactive or overactive thyroid — may contribute to fatigue, weight changes, reduced exercise capacity and difficulty recovering from training. Thyroid disorders should always be medically assessed and managed appropriately rather than attributed to overtraining.

An underactive thyroid (hypothyroidism) in particular can closely mimic the symptoms of Overtraining Syndrome — persistent fatigue, reduced performance and poor recovery — which is one reason blood tests are valuable when symptoms persist despite reducing training.

CRP is a marker of inflammation and may increase due to infection, injury, inflammatory disease or recent strenuous exercise. Because intense training can temporarily elevate CRP — particularly in the 24–48 hours after a long run or high-intensity session — results should always be interpreted within the context of recent activity and clinical symptoms rather than in isolation.

A persistently elevated CRP in a rested athlete warrants further investigation. In the context of reduced HRV and recovery scores, it may suggest ongoing systemic stress that goes beyond normal training adaptation.

Routine blood tests often include sodium, potassium, creatinine and estimated Glomerular Filtration Rate (eGFR). These help assess hydration status, kidney function and electrolyte balance. Changes may occur during illness or after prolonged endurance events, although temporary fluctuations are often physiological and resolve with rehydration and rest.

Use our eGFR Calculator to estimate kidney filtration function from your creatinine result.

No — they provide complementary rather than overlapping information. Wearables provide continuous physiological monitoring but cannot measure iron stores, vitamin deficiencies, thyroid hormones, blood glucose, inflammation markers, kidney function or cholesterol. These require laboratory testing.

Conversely, blood tests cannot show HRV, recovery score, sleep quality, daily activity, heart rate recovery or training load. Both sources of information provide unique, complementary insights into athlete health.

Healio360 is designed to bring together information that is usually reviewed separately. Your Health Intelligence dashboard combines blood test results with HRV, resting heart rate, heart rate recovery, recovery score, sleep quality, training load, medical reports and AI-generated health insights.

Instead of reviewing laboratory results or wearable data in isolation, Healio360 identifies patterns across your health history — helping you understand how physiology changes over time and making it easier to spot connections between blood markers and your training response. You can upload and review your results in the Blood Test Analysis section, and view all your wearable metrics together in Wearable Health.

Discuss blood tests with a healthcare professional if you experience persistent fatigue, declining performance, recurrent injuries, frequent illness, unexplained weight changes, breathlessness beyond expected training fatigue, or symptoms suggestive of iron deficiency or nutritional deficiency. Routine screening may also be appropriate for some competitive athletes depending on individual circumstances — your GP or sports medicine physician can advise.

Explore our calculators to estimate key markers: eGFR Calculator, HbA1c to Average Glucose Calculator, and LDL Cholesterol Calculator.

This guide is for educational purposes only and does not constitute medical or clinical advice. Always consult a qualified healthcare professional before requesting or acting on blood test results.