Most committed athletes aren't short on effort. If you train 8–15 hours a week and feel flat for weeks at a time, part of the answer may be in your blood — and a standard check is not built to find it.

Clinically "normal" is not the same as ready to train

Lab reference ranges are set from the general population and are designed to flag disease. They are not designed to tell you whether your body can absorb a heavy training block. Being "in range" means no medical problem was found. It doesn't mean your iron, energy and hormones are where an athlete needs them (Lee et al., 2017).

Three things a general check often misses

  • Ferritin (iron stores): many labs only flag ferritin below roughly 15–30 µg/L. In athletes, ferritin below about 35 µg/L is commonly treated as iron depletion, and endurance athletes often aim to stay well above that. You can have normal haemoglobin and still have low iron stores that leave you tired (Peeling et al., 2008; Sim et al., 2019).
  • Inflammation (hs-CRP): a hard session or race raises CRP for a day or two. A persistently raised level, alongside poor sleep, high resting heart rate or falling performance, is a sign to look at load and fuelling — not to read in isolation (Lee et al., 2017).
  • Hormones and energy: eating too little for your training can lower thyroid hormone (T3), sex hormones such as testosterone and oestrogen, and disrupt menstrual cycles. These markers are most useful read together, not one at a time (Mountjoy et al., 2023).

How we use blood work

  • Test in context: results shift with hard training, hydration, muscle damage and illness. We read them against your training load, sleep and, for women, cycle phase.
  • Don't supplement blind: taking iron, zinc or other products without a baseline can cause harm. Iron in particular should only be taken when a deficiency is confirmed, ideally with your doctor's input (Sim et al., 2019).
  • Track trends: one blood draw is a snapshot. Repeat tests under the same conditions show whether you're adapting or running down.

Quick checklist

  • "Normal" rules out disease; it doesn't confirm you're ready for heavy training.
  • Ferritin under about 35 µg/L is worth discussing if you're an endurance athlete.
  • Test in the morning, rested, ideally fasted, and not within 24–48 hours of a very hard session or race.
  • Build training on a confirmed baseline, and retest under the same conditions.

Thresholds vary between labs and individuals. Any abnormal result should be reviewed with your doctor.