Runners burn through iron faster than they replace it. If your pace has quietly stopped improving and you can't work out why, this is usually where to look first.

Why Runners Test Differently

Distance running drains iron three ways at once: foot strike damages red cells, you lose it in sweat, and absorption drops after hard sessions. Low iron shows up as heavy legs and a pace that won't budge long before a standard blood count calls it anaemia. That's the reason ferritin matters more here than haemoglobin.

Training also shifts electrolytes and pushes muscle enzymes into the blood. Both normal in moderation. Both worth measuring when performance drops for no reason, or when recovery between sessions just stops working.

Who Should Consider These Tests

• Your pace has dropped and your training hasn't changed

• Legs feel heavy, recovery has slowed right down

• You run more than four times a week

• You're training for a half or full

• You're vegetarian and training hard

• You've cramped badly or felt faint on a long run

What Each Test Tells You

This page lists 8 options. Here's what each one actually measures, so you can pick instead of guessing.

Blood tests

• Electrolyte Panel: Sodium, potassium and the other salts that run your nerves and heart rhythm. They shift fast with heavy sweating, vomiting or water pills.

• Serum Iron Test: Iron circulating right now, as opposed to what's in storage. Read it next to ferritin, never on its own.

• Complete Blood Picture (CBP): Counts your red cells, white cells and platelets. It's the cheapest way to catch anaemia or an infection you didn't know you had, and it's the single most booked test we do.

• Creatine Kinase (CK) Test: Leaks out when muscle breaks down. High after a brutal session is normal. High when you've rested for two days is not.

• Thyroid Function Tests: Checks the hormones that set how fast your body runs. Worth doing if you're tired, gaining or losing weight without trying, or your hair's thinning.

• Ferritin Test: This is your iron savings account, and it empties before your blood count changes. That's why you can feel exhausted while a routine CBP looks perfectly fine.

• Vitamin D (25 - Hydroxy Vitamin D, Serum): Your body makes this from sunlight, and most people who work indoors don't get enough. Low levels show up as aching bones, weak muscles and flat mood.

Scans

• Electrocardiogram (ECG): A few minutes lying still while it records your heart's rhythm. Quick, cheap, and the standard starting point for anything cardiac.

How to Prepare

• No fasting: Complete Blood Picture (CBP), Electrolyte Panel, Vitamin D (25 - Hydroxy Vitamin D, Serum)

• No hard training for 48 hours: Creatine Kinase (CK) Test

• Nothing to do beforehand: Electrocardiogram (ECG)

• Morning sample is better: Ferritin Test

• Morning, after an overnight fast: Serum Iron Test

• No fasting. Take your thyroid tablet after the sample, not before: Thyroid Function Tests

Tell the collection team everything you're taking, prescription or not. Plenty of supplements move these numbers, and without that context your report reads wrong.

How Often to Repeat

Twice a year for regular runners. Ideally once in base training and once at peak load. Sooner if your pace falls off a cliff.

Understanding Your Results

Worth knowing: an underactive thyroid produces fatigue, heavy legs and poor recovery that look exactly like overtraining. The two need opposite responses, which is why thyroid sits on this page next to the iron markers.

These tests point at a diagnosis. They don't make one. Read your report with a doctor who knows your history and what you're currently taking.