Fatigue has the longest list of causes of any symptom, which is why it is the one most often waved away. Every item on that list produces the same feeling, and from the inside you cannot tell them apart. That is the case for measuring rather than guessing, and a small number of cheap tests separate most of them.
It is also why so many people arrive here having already been tested and told they are fine. A standard panel is built to detect disease, and most of what makes people tired is not disease. So a clear result and a real problem are not in conflict: the panel ruled out the dangerous things, and it never set out to explain the tiredness.
One question does more work than any test on this page: what kind of tired is it? Tiredness that sleep does not touch, tiredness that arrives an hour after lunch, and tiredness that comes with breathlessness on the stairs are three different problems pointing in three different directions.
Last updated August 14, 2026
Your iron stores, and the single test here most likely to turn something up. Stores empty first, while a blood count still reads normal, so you can be short of iron for a long time before anything is flagged [1]. Two randomized trials gave iron to women who were tired but not anemic, and both found it helped [2] [3].
Ferritin has a second job that gets in the way. As well as storing iron, it rises whenever there is inflammation anywhere in your body, so a ferritin that reads normal can be a low one propped up by something else [4]. hs-CRP measures that inflammation, which is what tells you whether the ferritin number can be trusted.
A second reading on your iron, taken a different way. Inflammation does not push this number up the way it pushes ferritin up, so it cannot hide a shortage: a saturation below 20% says too little iron is reaching the bone marrow where red blood cells are made, even when ferritin has not dropped far [1].
An underactive thyroid is the classic medical cause of tiredness that sleep does not fix, and it usually arrives with company: feeling cold, slowing down, dry skin, weight gain. An overactive one also causes exhaustion, through poor sleep and a body running hot. One cheap test covers both directions.
Read alongside TSH to separate an underactive thyroid from a borderline one. TSH on its own tells you the pituitary is asking for more hormone; free T4 tells you whether you are getting it.
Being low on it causes fatigue and brain fog before it causes anything more obvious, and it is common in people eating little or no animal food, in anyone over sixty, and in long-term users of metformin or acid-reducing medication [5]. It is also one of the few deficiencies where correcting it early matters, because the nerve damage from a long-standing one does not fully reverse.
Deficiency is common, and it is a reasonable thing to know about yourself. Keep the expectation modest: the evidence that correcting a low level fixes fatigue is much weaker than the evidence that low levels are widespread. If yours is low it is still worth correcting, and 2,000 IU of D3 is the usual daily dose.
Catches outright anemia, which ferritin alone will not show you, and gives the wider picture: white cells, platelets, and the size of your red cells, which points at whether a shortage is iron or B12.
A complete read of the thyroid, paired with the most common non-thyroid causes of the very same symptoms. When you are tired, foggy, or cold, this panel separates a thyroid problem from being low on iron or B vitamins, which feels identical.
This is the panel built for exactly this question, and it is the best value on the page. It covers the thyroid completely, including the antibodies that say whether a thyroid problem is autoimmune, and it covers iron twice over with ferritin and iron with TIBC, plus B12 and folate. It does not include vitamin D, which is a paid add-on to it, and it does not include hs-CRP or a blood count.
Want a different mix? Browse the full library, or tell us what you have noticed if you are not sure where to start.
Almost every article about fatigue treats it as one condition. It is at least five, and sorting which one you have narrows the field faster than any panel.
You can have more than one of these at once, and plenty of people do. The point of sorting them is not to arrive at a single answer. It is to stop spending money on the wrong question.
Low iron is the most common thing this page turns up, and most advice stops right there: take a supplement, job done. A low ferritin is a finding, not a diagnosis. Iron does not leave on its own, so the question that matters is where it went.
If you menstruate, heavy or long periods are the likely answer, and worth mentioning to a doctor rather than assuming they are just how yours are. If you do not menstruate, and particularly if you are a man or past menopause, unexplained iron deficiency is something a doctor should look into rather than something to correct with a supplement and move on from. The reason is blunt: the list of things that drain iron without producing symptoms of their own includes bleeding in the gut.
There is one cause worth knowing by name because it is missed so often. In a controlled screening study, celiac disease turned up in about one in 44 anemic patients, against one in 498 who were not anemic, and that held even in the groups whose low iron already has an easy explanation, premenopausal women and people over 65 [6]. Screening starts with a blood test for tTG antibodies, which your immune system makes when gluten damages the gut lining. There are two versions of that antibody, IgA and IgG, and it is worth having both measured. The IgA one is the standard test, but roughly one person in fifty with celiac cannot produce much IgA at all, so their result comes back clear when it should not. The IgG version catches them. A positive goes to a gastroenterologist, because this test screens and a biopsy diagnoses.
Only worth ordering if your iron came back low, or if you have gut symptoms alongside the tiredness. This is not a test for everyone who is tired.
The version covering both antibodies, for the reason above. We earn a commission if you order through this link.
If you are going to test for celiac, do it before cutting out gluten. Going gluten-free lowers the antibodies and lets the gut lining heal, which is the point of doing it but also means both the blood test and the biopsy can read clear when you do have it. The only way back is to eat gluten again for weeks before testing.
This is the most common outcome, and the reason so many people search for exactly this phrase. Two different things are going on, and they need separating.
The first is that normal is a wide band drawn from the whole population, and low-normal can still be low for you. Ferritin is the clearest example on the site. Plenty of labs flag it only below 15, the World Health Organization puts iron deficiency below 30, and the two trials that reduced fatigue with iron enrolled women below 50 [3]. Those are three different numbers answering three different questions, and a result of 22 gets reported as normal against the first one.
The second is harder to hear and more useful. The largest causes of persistent tiredness are not blood-test-shaped, and no panel will find them.
A clear panel is worth something. It removes the explanations that would otherwise be used to dismiss you, and it means the next conversation can start somewhere other than the beginning.
The most asked question in this whole category, in one form or another: what am I lacking? The answer is narrower than the supplement aisle suggests. Three deficiencies are worth measuring, and there is no fourth waiting to be found.
None of this works in reverse. A vitamin only helps someone short of that vitamin, so the question is which one you are actually low in, not which one is best for fatigue. That is the difference between the list above and the list below.
Fatigue is the most heavily monetized symptom there is. It is common, it is miserable, and claims about it are hard to disprove, because everyone feels better some days than others. These are the things being marketed hardest to people in your position.
Real adrenal disease exists, and it is a different thing entirely. Addison's disease is rare, serious, and diagnosed properly rather than with a saliva kit.
Some presentations should not go through a self-ordered panel first.
The question people arrive with is usually why they are tired when their results were normal. Part of the answer is that normal is a wide band and low-normal can still be low for you. Part of it is that the biggest causes were never going to show up in blood. If you have your numbers, seeing where each one sits against the range the evidence supports is the next thing worth doing.