Which blood tests check the cause of fatigue
Tests worth discussing with your doctor
Complete blood count (CBC) — shows red and white blood cell counts, haemoglobin and haematocrit; lowered levels can point to anaemia or iron deficiency, which often causes fatigue
Ferritin — shows the body's iron stores; low ferritin is linked to fatigue, weakness and shortness of breath even when haemoglobin is still within the normal range
TSH (thyroid-stimulating hormone) — helps assess thyroid function; both an underactive (hypothyroidism) and an overactive (hyperthyroidism) thyroid often cause fatigue
Source: MedlinePlus — TSH (Thyroid-Stimulating Hormone) Test
Free T4 — checked in addition when TSH is out of range, to get a clearer picture of thyroid hormone production
Glucose — helps rule out diabetes or impaired glucose tolerance; both too much and too little blood sugar can cause fatigue
Source: MedlinePlus — Blood Glucose Test
Potassium — an electrolyte important for muscle and nerve function; an imbalance can cause weakness and fatigue, so it's included in the general metabolic panel
Sodium — reflects the body's fluid balance; changes in sodium can be one of the causes of nonspecific fatigue, so it's checked alongside other electrolytes
ALT (a liver enzyme) — tested as part of a general metabolic panel; this test may be ordered when evaluating a nonspecific symptom like fatigue, since it can have many different causes
Cortisol — long-lasting fatigue combined with muscle weakness or weight changes can be linked to an adrenal gland disorder, so cortisol levels may be checked
Source: MedlinePlus — Cortisol Test
Sources
- MedlinePlus — Complete Blood Count (CBC) — 2026-09-06
- MedlinePlus — Ferritin Blood Test — 2026-09-06
- MedlinePlus — TSH (Thyroid-Stimulating Hormone) Test — 2026-09-06
- MedlinePlus — Thyroxine (T4) Test — 2026-09-06
- MedlinePlus — Blood Glucose Test — 2026-09-06
- MedlinePlus — Comprehensive Metabolic Panel (CMP) — 2026-09-06
- MedlinePlus — Cortisol Test — 2026-09-06
Why these tests
Chronic fatigue is one of the least specific symptoms in medicine: it can be caused by dozens of different things, from lack of sleep to anaemia, a thyroid disorder, diabetes, changes in kidney or liver function, an electrolyte imbalance, or an adrenal gland disorder. Blood tests don't show the cause of fatigue directly, but they help a doctor rule out or confirm the most common treatable organic causes — for example iron-deficiency anaemia or an underactive thyroid (hypothyroidism), whose symptoms (fatigue, weight changes, weakness) often overlap.
A complete blood count shows whether there are signs of anaemia or inflammation; ferritin shows whether iron stores are adequate; TSH and free T4 show whether the thyroid is working properly; glucose checks for a problem with sugar metabolism; and electrolytes and liver enzymes show whether the kidneys and liver are functioning normally, since problems there can also show up as fatigue. It's important to understand that normal test results don't disprove the fatigue itself and don't rule out causes that blood tests can't show — for example sleep disorders, psychological strain, or lifestyle factors. That's why it's always worth discussing the results with a doctor together with how long the symptoms have lasted, their character, and any other findings.
When not to wait
- Fatigue accompanied by sudden weight loss with no clear cause
- Fatigue is accompanied by prolonged fever, night sweats, or swollen lymph nodes
- Fatigue is sudden and severe and interferes with daily activities for more than a few weeks
When to seek care immediately
- Shortness of breath, chest pain, or a rapid or irregular heartbeat appears
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
Frequently asked questions
›Are these tests free?
When a family doctor evaluates fatigue symptoms and sees a medical indication for testing, they can order the tests free of charge for people insured under Lithuania's compulsory health insurance fund (PSDF); without an indication, you can also have the tests done privately in a lab at your own cost.
›Who should I see about fatigue tests?
Start with your family doctor (the GP you're registered with under the public system), who will assess your symptoms and their duration and, if needed, order blood tests or refer you to the appropriate specialist.
›Which blood tests are usually done for fatigue?
Doctors usually order a complete blood count, ferritin, TSH and free T4, glucose, electrolytes (sodium, potassium) and liver enzymes — these tests help rule out the most common organic causes of fatigue.
›How long does fatigue need to last before getting tested?
If fatigue lasts more than a few weeks and doesn't improve with rest or lifestyle changes, it's worth discussing with a doctor whether blood tests are needed.
›If the test results are normal, does that mean the cause of fatigue isn't serious?
Not always — normal blood test results don't rule out every possible cause, such as sleep disorders or psychological strain, so it's best to discuss further evaluation with a doctor.
›Do I need to fast before the tests?
For glucose and some other tests, fasting before the sample is taken can matter — it's best to confirm the specific requirements with the lab or your doctor before the test.
Related situations
How to prepare
This site is not an emergency service. If severe chest or head pain, breathlessness, confusion, difficulty speaking, heavy bleeding or another acute problem has come on suddenly — do not wait for tests, call 112.
This information is not a diagnosis and does not replace a consultation with a doctor. Reference ranges depend on the laboratory, your age and sex — always compare your result with the range printed on your own report.