What are the key points in brief?
- Fatigue is the main or a secondary reason in 10 to 20 percent of all consultations in general practice1.
- The most common causes of persistent fatigue are sleep disorders, breathing disorders during sleep, depression and severe psychosocial strain1.
- A conversation, a physical examination and simple lab tests come first1.
- If the cause stays open, follow-up appointments can help avoid over-investigation1.
How often is fatigue a reason to see a doctor?
Fatigue is the main or a secondary reason in 10 to 20 percent of all consultations in general practice1.
Which causes are common?
Fatigue can have many causes. Conditions that are already known may also have worsened1. According to a review in Deutsches Ärzteblatt, the most common causes of persistent fatigue are:
- sleep disorders and breathing disorders during sleep1
- depression, in 18.5 percent of cases1
- severe psychosocial strain1
Anemia and other physical causes are less common. They accounted for 4.3 percent1.
What can a practice check?
This list summarizes what a medical work-up can include. It is the same for everyone.
- A medical conversation about symptoms and their course (medical history)1
- A physical examination1
- Simple lab tests, for example blood count, blood sugar, an inflammation marker, liver values and thyroid value (TSH)1,2
- Common causes of persistent fatigue: sleep disorders, breathing disorders during sleep, depression and severe psychosocial strain1
- Conditions that are already known and may have worsened1
- Anemia and other physical causes – they are less common1
- Further tests, if additional symptoms or findings give reason for them1
- Follow-up appointments, if the cause stays open at first1
The lab examples come from a basic check that the French National Academy of Medicine describes for medical prevention2. Which values make sense in an individual case is decided by the practice.
What happens if the cause stays open?
If the cause is unclear at first, watchful waiting and regular follow-up appointments can help. This can prevent the search from narrowing to physical causes only and prevent too much testing1.
Where does NAD+ stand in this work-up?
According to a review, the first steps are a conversation, a physical examination and simple lab tests1. Normal NAD+ blood values are so far not well known3. More on this is on the page Can NAD+ be measured reliably in blood?.
The five-step preparation helps you note your observations and gives you suggestions for questions to ask your doctor.
Which sources does this page rely on?
- Maisel P, Baum E, Donner-Banzhoff N (2021). Fatigue as the Chief Complaint. Deutsches Ärzteblatt International. DOI: 10.3238/arztebl.m2021.0192
The paper summarises how family doctors may assess persistent fatigue. It is a review based on a literature search. Its authors suggest that history, physical examination and simple laboratory tests may usually be enough, with further tests mainly when other findings point that way.
- Académie nationale de médecine (2026). Approche clinique de la longévité en bonne santé. Report of 13 January 2026, in French. www.academie-medecine.fr/wp-content/uploads/2026/01/RAPPORT-Approche-clinique-de-la-longevite-en-bonne-sante-APRES-VOTE.pdf
The report asks what medical practice can do today to support a long and healthy life. It is a position paper of the French National Academy of Medicine. Based on current knowledge, it does not recommend measuring biological age or using anti-ageing agents in practice and gives priority to lifestyle.
- Euro L et al. (2026). Dynamics of blood NAD and glutathione in health, disease, aging, and under NAD+-booster treatment. Redox Biology. DOI: 10.1016/j.redox.2026.104343
The study developed a standardised measurement of NAD and glutathione from a single whole-blood sample. It is a methods and cohort study with 299 healthy adults. Levels appeared stable with age and rose to a different extent from person to person under NAD+ boosters.