Tired despite good training? This is what your blood values say

Your training plan is sound, you eat well and you go to bed on time. Yet you drag through the day and every session feels like a chore. Fatigue that does not match your effort is a signal, not a character flaw. And often the answer sits in a handful of blood values.

My take: in an athlete who stays tired, fatigue is almost never one thing. It is usually a stack of small shortfalls or an overlooked cause that no one measured on purpose.

First this: under-recovery or a deficiency?

Before looking at blood values, separate two things. Under-recovery means your load and recovery are out of balance; you fix that with rest, food and sleep, not a test. An underlying deficiency persists even when you rest well. If you do not bounce back after a real recovery week, blood testing makes sense.

A simple rule of thumb helps you choose: if the fatigue feels like ordinary tiredness that lifts after a few rest days, it is probably under-recovery. If it feels like an exhaustion that does not budge, however well you sleep and eat, then targeted blood testing is the logical next step. That distinction saves you an unnecessary test or, the other way round, weeks of muddling through while a shortfall sits below the surface.

The most common causes in athletes

This table links symptom to the marker you test first.

Iron and thyroid: the two big ones

In athletes, iron deficiency is the most common nutrition-related cause of fatigue and declining performance. Crucially, ferritin drops before your haemoglobin, so a normal haemoglobin does not rule out a shortfall. The full explanation is in iron deficiency in athletes.

An underactive thyroid gives similar symptoms: tiredness, cold and a heavy feeling. An off TSH with free T4 brings that into view. This is very much something to assess together with your GP.

What makes these two so treacherous is that they both creep in. Your fitness slips gradually, you blame a busy spell or off days, and you train harder to compensate. That makes the problem worse, because without enough iron or a properly working thyroid you cannot process the extra load. A targeted measurement breaks that spiral.

Energy balance: the cause no one measures

There is a cause of fatigue that no single blood value points to directly: structurally eating too little for your training volume. In sports medicine this is known as relative energy deficiency, and it is more common than you think, even in athletes who have no wish to lose weight. The body quietly scales back: less recovery, lower hormone production and lasting fatigue. The Voedingscentrum stresses that an active lifestyle demands more energy, and that certainly applies at high training hours. If you feel structurally tired while your blood values are largely normal, take a critical look at whether you eat enough.

Vitamin B12 and D: more often low than you think

The RIVM and the Gezondheidsraad advise extra vitamin D for many groups during the winter half-year, because there is too little sun to produce enough. A low vitamin D can contribute to feeling tired and flat. Vitamin B12 is mainly a concern for people who eat little or no animal products; the Voedingscentrum names vegetarians and vegans here explicitly.

When to take it to your GP

Fatigue that lasts for weeks, comes with weight loss, breathlessness or night sweats, belongs with your GP and not just with an online panel. A blood test helps you enter that conversation better prepared, but does not replace it. The same applies to fatigue that appears suddenly without a clear reason, or that does not match your effort while your lifestyle has not changed.

Most sport-related fatigue is, fortunately, well explained and addressable. So start systematically: first separate under-recovery from a possible deficiency, then measure the few most likely markers on purpose, and draw conclusions from the trend rather than a single number. That order prevents you from doubting your fitness for months while the cause was simple to bring into view.

To measure on purpose, build a panel with iron, thyroid and vitamins via composing your own blood test. This article belongs to blood values for endurance athletes. If you suspect too much load, read recognising overtraining through your blood values.

At Enhanced a BIG-registered doctor assesses every result, so you do not have to puzzle out what explains your fatigue yourself.