Constantly tired and drained: what commonly causes it and what rarely does
The key points at a glance
Constant fatigue almost never has a single cause. Most often, it is due to too little or poor-quality sleep, a strain that lasts longer than expected, or a medication. Much less often, it is caused by iron deficiency, vitamin B12 deficiency, or an underactive thyroid. A blood test shows whether one of these values is involved. It does not answer the question of why someone is tired.
This article ranks causes by frequency, not by severity. It begins with everyday reasons because they explain most cases, and then identifies the point at which fatigue should be medically evaluated. Whenever an institution cites a figure, it is given here with the institution's name and year. Where no figure is available, that is stated too.
You will first learn why fatigue is so difficult to attribute. This is followed by warning signs, sleep as the most common explanation, the three nutrients and the thyroid, an overview of all groups of causes, and two weeks of observation to help prepare for a conversation with a doctor. The limitations come at the end.
What to expect in this article
1. Why constant fatigue is so difficult to attribute
2. When fatigue belongs in a doctor's office
3. Sleep: the explanation that is most often correct
4. Iron, vitamin B12, and vitamin D: what a blood count shows
5. The thyroid: why the TSH level is measured first
6. The overview: which cause can be identified by what
7. Daily life, mood, and medications: causes without a laboratory value
8. Observe for two weeks before getting tested
9. What a blood test shows here and what it does not
10. Who should consider a blood test and who should not
11. Limitations: what blood values do not reveal about fatigue
12. What you can change this week
Frequently asked questions
Sources
Why constant fatigue is so difficult to attribute
Fatigue is not a finding but a description. It occurs at the end of many different processes, which is why the search for a single cause so often leads nowhere.
The body signals an imbalance through fatigue: more is being used than is replenished, or recovery does not keep pace with the strain. This signal does not reveal what is causing it in a particular case. Sleep deprivation feels just like iron deficiency, and both feel like a week with too many appointments.
In addition, several factors can act at the same time and reinforce one another. People who sleep poorly eat more irregularly. Those who eat irregularly absorb less iron. Those who absorb less iron sleep worse. In retrospect, it is rarely possible to say where this chain began.
Three documented orders of magnitude
about one-third
of people have trouble falling asleep or staying asleep (IQWiG, 2024)
5 to 10%
of people in Europe have an iron deficiency (IQWiG, 2023)
5 in 100
People in countries such as Germany have an underactive thyroid (IQWiG, 2024)
These three figures put the issue into perspective. Sleep problems affect many times more people than iron deficiency or an underactive thyroid. Anyone who thinks of a rare laboratory value first when experiencing fatigue is statistically looking in the least likely place.
Key message
Fatigue is a result, not a cause. The sensible order is therefore: first rule out the common explanations, then investigate the rare ones—not the other way around.
The difference between tired and exhausted
In everyday usage, both words are used interchangeably, but they are not the same when making an assessment. Fatigue improves with sleep. Exhaustion does not: waking up just as drained after eight hours as when you went to sleep describes something different.
This distinction is the most useful question you can ask yourself. It costs nothing, requires no test, and separates two groups of causes.
If your energy returns after a free weekend, the cause is more likely to lie in everyday life. If it remains absent for weeks, regardless of how much someone sleeps, it is worth looking at values that cannot be felt.
Why the search often began with a doctor
Many people researching this topic have already visited a doctor. The complete blood count was unremarkable, and the symptoms remained. That does not mean they are imagined: it means the obvious values were checked and others were not.
A standard blood count includes neither ferritin nor vitamin B12, neither vitamin D nor the free thyroid hormones. It is not wrong; it is simply narrower than many people assume. Knowing the gap allows you to ask specifically about these tests at your next appointment.
When fatigue belongs in a medical practice
This chapter deliberately comes before all the causes. A text that puts the criteria for stopping at the end fails to reach the very reader it is intended for.
These observations should be medically evaluated
Fatigue together with unintentional weight loss
when weight drops without any change in eating or exercise
Fever, night sweats, or swollen lymph nodes
for several weeks, without any apparent infection
Shortness of breath or a racing heart during activities that used to be easy
Climbing stairs, carrying shopping bags, taking a brisk walk
Noticeable paleness or visible blood in the stool
both can indicate blood loss that explains the fatigue
Persistent low mood or loss of pleasure
for more than two weeks, especially when nothing brings joy anymore
Falling asleep during the day in situations that require attention
at the wheel, in conversation, at work – this is no longer just sleep deprivation
This list is not a substitute for an examination, and it is not a diagnostic framework. It indicates the point at which continuing to read would be the wrong answer to the right question.
For everything else, the order is reversed: observe first, then measure, then talk. That is exactly what the rest of this article is designed for.
Sleep: the explanation that applies most often
The most common cause of fatigue is also the least remarkable. It is nevertheless overlooked because most people consider their sleep sufficient as long as they spend the usual number of hours in bed.
The Institute for Quality and Efficiency in Health Care (IQWiG) states that adults need approximately seven hours per night, while people over 80 need only about six (2024). The need is therefore not fixed and decreases with age.
What matters, however, is not duration alone, but whether sleep is restorative. The IQWiG draws the line between poor sleep and a sleep disorder using a verifiable criterion:
Documented source
“Anyone who sleeps so poorly on at least three nights a week for more than a month that their quality of life is impaired has a sleep disorder.”
Institute for Quality and Efficiency in Health Care (IQWiG)
Sleep problems and sleep disorders (insomnia), as of 2024
The statement provides two figures that can be checked: three nights per week and more than one month. Anyone who finds both apply to them has probably already found the most likely explanation for their fatigue—without a single laboratory result.
According to the same institute, about 6 in 100 people in Germany have a sleep disorder in this sense (2024). By contrast, around one-third are familiar with problems falling asleep or staying asleep.
When enough sleep still isn't enough
There is one case that is overlooked particularly often: eight hours in bed, yet the energy is still gone in the morning. Common reasons include pauses in breathing during sleep, restless legs, or sleep fragmented by waking up during the night.
This is usually noticeable from the outside: anyone who sleeps next to someone with loud, irregular snoring and pauses in breathing is more likely to notice it than the affected person themselves. This observation belongs in a doctor's office because it requires an examination, not a dietary supplement.
Genetics also play a role here. How quickly someone breaks down caffeine and how strongly it disrupts their nighttime sleep is partly determined by their genetic makeup. How these factors are connected is explained in Genes and Sleep—this article focuses on causes that can be identified through blood tests.
Two habits that shorten your sleep without you noticing
Alcohol in the evening makes it easier to fall asleep but worsens the second half of the night. Sleep becomes lighter, waking becomes more frequent, and in the morning there is no sense of recovery even though the number of hours is adequate. Anyone who regularly drinks in the evening has an explanation that requires no laboratory test.
With caffeine, timing matters more than quantity. For many people, coffee at four in the afternoon affects them well into the night, and exactly how long it lasts varies considerably from person to person. Anyone who wants to test this can move their last cup to the morning for two weeks and compare their notes.
Both points sound like generalities, and yet both are rarely checked. The reason is the same as with the medication: What has been part of the routine for years is not suspected as a cause.
Iron, vitamin B12, and vitamin D: what a blood count shows
Three nutrients regularly come up in connection with fatigue. They are not the most common causes, but they are the ones that can be measured—and that explains their prominence online.
Iron: the value with the strongest evidence
According to IQWiG, adults have approximately 3 to 5 grams of iron in their bodies (2023). In Europe, 5 to 10 percent of people have iron deficiency, and 2 to 5 percent have iron-deficiency anemia. Women before menopause are affected more often than men.
The institute lists fatigue and unusual exhaustion after exertion among the symptoms of more severe iron deficiency, along with pale skin, difficulty concentrating, headaches, brittle nails, and hair loss (2023). Anyone who notices several of these symptoms together has a specific reason to get tested.
The same source briefly explains how iron deficiency is diagnosed: Doctors diagnose iron deficiency with a blood test (IQWiG, 2023). A single value is rarely enough—ferritin, iron, and transferrin only provide a complete picture together, because ferritin can also be elevated during inflammation when the stores are empty.
Vitamin B12: groups with increased needs
Vitamin B12 is found almost exclusively in animal-based foods. People who follow a vegan diet, eat vegetarian for an extended period, or take certain stomach medications therefore have a higher risk of inadequate intake.
The liver’s stores last a long time, often several years. That is why a B12 deficiency develops gradually and is rarely linked to a clear event. Someone who changed their diet three years ago may no longer think of it when experiencing symptoms today.
Vitamin D: expectations exceed the evidence
Vitamin D is mentioned most often in connection with fatigue and is supported by evidence least often. A low level is common during the winter months, and it can be measured reliably. Whether it explains the fatigue in an individual case cannot be inferred from this alone.
The honest way to deal with this is to follow an order: The value is measured because it is part of the assessment, not because it provides the answer. Anyone who checks it alone and skips everything else has not answered the question, but merely narrowed it.
Why taking supplements without measuring is the worse approach
The obvious thought is: Instead of measuring, I will simply take a supplement and see whether I feel better. That sounds practical and has three disadvantages that are rarely considered.
First, starting supplementation changes the value you later want to measure. If you have taken iron for three months and then have your ferritin measured, you are measuring the supplementation, not the initial state. Afterward, it is no longer possible to determine whether a deficiency existed at all.
Second, the three nutrients are not interchangeable. A vitamin D supplement does nothing for iron deficiency, and iron does nothing for an underactive thyroid. Anyone who takes the wrong thing on suspicion loses time and assumes the cause has been ruled out.
Third, iron in particular is not harmless and cannot simply be taken as a precaution. Taking it without a confirmed deficiency should be medically supervised. Measuring is therefore not the more cumbersome route, but the shorter one.
The thyroid: why the TSH level is measured first
The thyroid controls how quickly the metabolism works. When it slows down, everything slows down—and people usually describe this as fatigue.
IQWiG lists weakness and fatigue, lack of energy and apathy, as well as skin changes and constipation, among the symptoms of an underactive thyroid (2024). In countries such as Germany, approximately 5 in 100 people have an underactive thyroid.
TSH is a control signal, not a thyroid hormone
TSH is not produced in the thyroid but in the pituitary gland. It is the instruction to the thyroid to produce more. IQWiG describes the mechanism as follows: As soon as the thyroid becomes less active, the pituitary gland responds by increasing TSH production to stimulate the thyroid more strongly (2024).
This is the logic behind the testing. An elevated TSH level in the blood may be a sign of an underactive thyroid (IQWiG, 2024)—and because the signal rises earlier than the hormones themselves, initially only TSH is often measured.
What fT3 and fT4 additionally show
fT3 and fT4 are the free thyroid hormones—that is, the fraction that can actually take effect. If they are too low, this may indicate an underactive thyroid; if they are elevated, it points more toward an overactive thyroid (IQWiG, 2024).
For interpretation, this means: TSH shows that something is wrong. fT3 and fT4 show how severe it is. A result including all three values is therefore more informative than one with TSH alone, even though TSH is sufficient as a first step.
One point that comes up in every discussion of test results yet is rarely taken in: reference ranges depend on the laboratory, method, and age. Two laboratories can classify the same value differently without either one being wrong.
The borderline case over which there is the most debate
There is one pattern that affects many people with fatigue: the TSH value is elevated, but the thyroid hormones are still within the normal range. Medically, this is called latent hypothyroidism, and it usually causes no symptoms.
According to IQWiG, each year 2 to 5 out of 100 people with latent hypothyroidism develop hypothyroidism with symptoms (2024). For the others, the result remains abnormal without clinical significance.
This leads to an uncomfortable insight: a mildly elevated TSH value usually does not explain existing fatigue. Whether and when it should be treated is a medical judgment and not a question that a finding alone can answer.
The overview: which cause can be recognized by what
The following table compares five cause categories according to the same three criteria. It provides orientation and is not a diagnosis: in each row, it first states how the category can be recognized in everyday life, then whether and how it can be measured, and finally what makes sense next.
| Cause category | How it can be recognized in everyday life | Whether it can be measured | What makes sense next |
|---|---|---|---|
| sleep | less sleep than one needs, frequent waking, snoring with pauses in breathing; energy returns over a free weekend | not in the blood; identifiable through a sleep diary or a sleep-lab examination | record sleep times for two weeks before measuring |
| iron status | paleness, difficulty concentrating, brittle nails, hair loss; often with heavy menstrual bleeding or a meat-poor diet | yes, through ferritin, iron, and transferrin together – a single value is not enough | Have it measured and have the findings medically interpreted; do not take supplements without a confirmed need |
| thyroid | lack of drive, feeling cold, dry skin, constipation, weight gain without changes in diet | yes, initially through TSH; fT3 and fT4 show how severely function is affected | Have TSH measured; seek medical evaluation if the value is abnormal |
| vitamin status | gradual onset without a clear event; increased risk with a vegan diet, in winter, and with advancing age | yes, through vitamin B12, vitamin D3, and folate – all three are snapshots | measure together with iron status, not individually one after another |
| strain, mood, medications | persistent pressure for weeks, loss of pleasure, a newly started medication; recovery does not occur over the weekend | no – no blood value can capture this; the package leaflet and a conversation are more informative | Have medications reviewed; seek medical help if low mood persists |
There is no reliable information from a German institution on how often each of these groups is actually the cause of persistent fatigue. That is why this table contains no percentage, but rather the order: the most common explanation first.
Everyday life, mood, and medications: Causes without a laboratory value
The last row of the table is the most uncomfortable because it provides no result you can print out. It is nevertheless the one that explains many cases.
The package insert as the first place to look
Fatigue is one of the most common side effects of all. It is listed in the package inserts of many blood pressure medications, allergy medications, antidepressants, and painkillers, and it often begins only weeks after the first day of use.
The test for this costs nothing: When did the fatigue begin, and what changed in the four weeks before that? Comparing these two dates reveals a connection or rules one out. Even then, a medication should only be stopped or changed under medical supervision.
When exhaustion is a mood
Persistent low mood makes you tired, in a physically noticeable way. The difference from sleep-deprivation fatigue is that sleep does not help and that something else usually comes along with it: interest and enjoyment decline, often even before the fatigue becomes noticeable.
This cause cannot be measured away, nor does it become any smaller when a blood count is unremarkable. Anyone who suspects it in themselves is better served by a conversation than by another test.
The sustained burden no one counts as a burden
Caring for a relative, a toddler with restless nights, moving house, a change at work: These things do not appear in any test result, and they are rarely identified as causes because they are considered normal.
But “normal” does not mean without consequences. Anyone who gets less recovery than strain for months becomes tired—and no nutrient can change that.
Chapter at a glance
Three causes of fatigue do not show up in any blood test: a medication, low mood, and a sustained burden that is accepted as normal. Together, they are more common than all the laboratory findings discussed in this article. Anyone who has not checked for them before testing risks an unremarkable result while feeling no better—and that is exactly the experience many people have when they open this article.
Observe for two weeks before getting tested
The following four steps prepare you for a conversation at a medical practice; they do not replace it. They expressly do not apply to the signs in Chapter 2: If you have weight loss, shortness of breath, blood in your stool, or persistent low mood, the appointment comes at the beginning, not at the end of the two weeks.
Record sleep times
When you went to bed, when you woke up, and how often you were awake at night. Two numbers per day are enough.
Rate your energy during the day
A number from 1 to 10, in the morning and afternoon. No explanation, no evaluation.
List medications
Everything taken regularly, with the start date. This also includes non-prescription preparations.
Taking the note into the conversation
Fourteen lines on paper shorten the medical history and replace memory.
The value of these two weeks lies less in the result than in the clarity it provides. By writing things down, you can see on the fourteenth day whether the fatigue was truly present throughout or absent on certain days—and which days those were.
The note also helps during the conversation itself. When asked how long this has been going on, a piece of paper with fourteen lines is a better answer than a guess.
What a blood test shows here—and what it does not
At this point, an honest statement is needed: A blood test shows whether a nutrient deficiency or a thyroid value is contributing to the fatigue. It does not show why someone is tired. That assessment comes from a conversation with a medical practice, not from a result or an article.
Testing becomes worthwhile in a specific situation: The symptoms have persisted for weeks, the everyday explanations from Chapter 7 have been checked, and one piece of data is missing. The values measured are described in more detail in Testing for Nutrient Deficiency; this article focuses on how to put fatigue into perspective.
Two tests from mybody®x (MYBODY Lab GmbH) cover the values discussed in this article. They differ in their focus.
Capillary blood test
VitalCheck | Nutrient & Mineral Test Complete
Measures vitamin D3, B12, and folic acid, as well as iron status through ferritin, iron, and transferrin, along with calcium, magnesium, phosphate, selenium, zinc, cholesterol, long-term blood sugar, and CRP. What the test does not do: It does not include thyroid values, electrolytes, or heavy metals. It provides a snapshot, not a diagnosis—an abnormal value means that a closer look may be worthwhile.
Lab analysis 3–5 business days after sample receipt
Capillary blood test
Women’s Wellness Check | Women’s Health Test
Provides a complete thyroid profile with TSH, fT3, and fT4, along with ferritin, vitamin B12 and vitamin D3, as well as cortisol, prolactin, SHBG, and organ markers—18 values from one blood sample. What the test does not do: It does not measure cycle hormones, and it provides a single cortisol value at the time of sampling, not a daily profile. A result is a basis for a conversation, not a diagnosis.
Lab analysis 3–5 business days after sample receipt
A blood test shows whether a nutrient deficiency is involved. It does not show why someone is tired.
Who a blood test makes sense for and who it does not
A measurement is not an answer to every kind of fatigue. It is an answer to a specific question, and the two columns below distinguish between the cases.
Makes sense for you if …
the fatigue has lasted for weeks, you have checked the everyday explanations, and one data point is missing.
only a complete blood count was done at your last appointment, and ferritin, B12, or thyroid values were not included.
you want to change your diet and track the progress with one measurement before and one afterward.
Probably not if …
you notice one of the signs from Chapter 2 in yourself. Then making an appointment is the right order of things, not taking the test.
you have been sleeping too little for four nights. The answer is in the calendar, not the laboratory.
you expect a diagnosis. The result provides values, not a disease name.
Limitations: what blood values do not tell you about fatigue
A laboratory value is an indication, not a diagnosis of a person. IQWiG puts it succinctly in its guide to laboratory values: A disease usually cannot be inferred from a laboratory value alone (2025). This applies twice over to fatigue because there is no fixed connection between a value and a symptom.
Two people with the same ferritin level may feel completely different. Conversely, some people have unremarkable values and severe exhaustion. Neither is a contradiction; both are normal.
The available data also has limitations. Institutions cite specific frequencies for sleep disorders, iron deficiency, and hypothyroidism. There is no comparable figure from a German institution for how often each cause is behind persistent fatigue. That is why this article does not include one.
And the tests have the limitation stated in the product cards: They provide a snapshot. Today’s value says nothing about what it was three months ago—and fatigue develops over months, not in a single day.
What you can change this week
If you take away one action from this article, let it be this: For fourteen days, write down your sleep times and, next to them, a number from 1 to 10 for your energy in the afternoon. Two lines per day, nothing more.
The reason is unremarkable. Most questions about fatigue become clear the moment the connection between night and afternoon becomes visible. What remains afterward is what can be discussed at a doctor’s office—with a note instead of an estimate.
The starting point was the question of what lies behind persistent fatigue. The most honest answer is: usually several things at once, and rarely what people look for first. That is precisely why the order matters.
Frequently asked questions
Why am I constantly tired and exhausted even though I get enough sleep?
Spending enough time in bed does not mean getting enough restorative sleep. Common reasons include fragmented sleep, breathing interruptions at night, or a strain that uses up more during the day than the night replaces. Only afterward should measurable causes such as iron deficiency or an underactive thyroid be considered. For adults, IQWiG states that approximately seven hours of sleep are needed (2024)—individual needs may differ.
Which blood values should I have checked if I have persistent fatigue?
It is useful to assess iron status using ferritin, iron, and transferrin together, as well as vitamin B12, vitamin D3, folate, and the TSH level. A complete blood count contains none of these values. Ferritin alone is not sufficient because it can be high during inflammation even when iron stores are depleted. Which values are needed in an individual case is decided by the medical practice based on the symptoms.
When should persistent fatigue be medically evaluated?
Immediately if unexplained weight loss, fever lasting several weeks, shortness of breath during light exertion, noticeable paleness, blood in the stool, or persistent low mood occur as well. The same applies if you fall asleep during the day in situations that require attention. Without these signs, observing for two weeks is the more sensible first step.
Can an at-home test find the cause of my fatigue?
No. A blood test shows whether a nutrient deficiency or a thyroid value is involved. It does not show why someone is tired, and it does not assess sleep quality, medication effects, or low mood—three causes that together are more common than all laboratory values. An abnormal result is a reason to have a conversation, not a diagnosis.
How are the thyroid and fatigue connected?
The thyroid sets the pace of metabolism. If it works too slowly, IQWiG lists weakness and fatigue, lack of drive, and apathy among the symptoms (2024). In countries such as Germany, approximately 5 in 100 people have an underactive thyroid. TSH is measured first because functional disorders are particularly detectable there at an early stage; fT3 and fT4 then show how severely function is affected.
Next step
Observe first, then measure
If, after two weeks, everyday explanations have been ruled out and a data point is missing, a blood test provides values that are not included in a complete blood count. It does not replace medical evaluation.
VitalCheck Complete Women’s Wellness CheckRead more
You might also be interested in
Which values are measured, how samples are collected, and what a report reveals.
The genetic side: Caffeine metabolism, sleep patterns, and what a DNA analysis reveals about them.
Sources
- Institute for Quality and Efficiency in Health Care (IQWiG): Sleep problems and sleep disorders (insomnia) (as of 2024) – gesundheitsinformation.de
- Institute for Quality and Efficiency in Health Care (IQWiG): Iron deficiency and iron-deficiency anemia (as of 2023) – gesundheitsinformation.de
- Institute for Quality and Efficiency in Health Care (IQWiG): Hypothyroidism (underactive thyroid) (as of 2024) – gesundheitsinformation.de
- Institute for Quality and Efficiency in Health Care (IQWiG): Understanding thyroid examinations (as of 2024) and Creatinine clearance (as of 2025) – gesundheitsinformation.de
The verbatim definition of a sleep disorder comes from source [1], as do the information on sleep requirements by age and the prevalence of sleep problems. All figures on iron and iron deficiency, as well as the list of symptoms, come from [2]. The prevalence of hypothyroidism, the list of symptoms, and the description of the TSH mechanism come from [3]. The statements about why TSH is measured first and what fT3 and fT4 add come from [4]; this source also contains the statement that a disease generally cannot be inferred from a single laboratory value alone, taken from the guide to creatinine clearance, where it is worded generally for laboratory values. Information on the price, sample type, and scope of analysis of the tests mentioned comes from mybody®x product pages, accessed on 28/08/2026; processing times follow the central guideline for blood tests. All sources were accessed and reviewed on 28/08/2026.
mybody®x Editorial & Specialist Team
Blood analysis interpretation Laboratory diagnostics Nutritional science Nutrigenetics
This article was created by the mybody®x editorial and specialist team. The team combines expertise in blood analysis interpretation, laboratory diagnostics, and nutritional science. Everyone who contributes to it is listed on the authors' page.
Published on 15/01/2026 · Last updated on 28/08/2026
The content is intended for general information and does not replace medical advice, diagnosis, or treatment. Reference ranges depend on the laboratory, method, and age—the information on your report is always decisive.
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