Constant fatigue: 8 tests that help find the cause

In shortFor fatigue lasting more than a month and not relieved by rest, test: a complete blood count and ferritin (iron and anemia), TSH (thyroid), vitamin D and B12, glucose, CRP (hidden inflammation); cortisol — only as ordered by a doctor. This set finds the cause in most cases; if it is clean, the search moves to sleep apnea, depression, and post-infection states.

When fatigue is a symptom, not a lifestyle

Tiredness after a hard week is normal; fatigue that does not lift for a month and is not fixed by sleep or vacation is a symptom. Before blaming age and "that's life," check the reversible causes — and nearly all of them are laboratory ones.

The logic is simple: the most frequent culprits — iron deficiency, hypothyroidism, low vitamin D, hidden diabetes, and smoldering inflammation — are visible in ordinary, inexpensive tests. Start there.

Iron and thyroid: the two prime suspects

Hidden iron deficiency is the champion cause of fatigue in women: hemoglobin is still normal while ferritin is already at the bottom. So a blood count without ferritin is half the picture — test the pair, or early-stage deficiency walks past unnoticed.

Hypothyroidism is the second champion: a slowed metabolism brings sleepiness, chilliness, swelling, and a "cotton-wool" head. A single TSH line answers that question; if it deviates, the doctor adds free T4 and TPO antibodies.

Vitamins, sugar, and inflammation

Deficiencies of vitamin D and B12 produce a similar picture of exhaustion and brain fog — in northern latitudes low vitamin D is found in most people from October to May. Both tests cost little and immediately clarify: a deficiency is closed, no deficiency — keep searching.

Fasting glucose excludes diabetes, whose fatigue begins long before diagnosis, and CRP reveals hidden inflammation — from a chronic focus to an autoimmune process. Do not test cortisol without a doctor's order: its level depends on the hour and the stress of the waiting room, and a random number only confuses.

If the tests are clean

A clean basic set is also a result: it shifts the search to non-chemical causes. First candidate — sleep apnea: snoring, witnessed pauses in breathing, and daytime sleepiness point to a sleep specialist, not a laboratory.

Next — depression and anxiety, which disguise themselves as physical fatigue, the aftermath of past infections, and a plain lack of movement: paradoxically, a sitting life tires more. The laboratory has done its job here — bodily causes are excluded.

The basic set for fatigue

Complete blood count plus ferritin — iron and anemia
TSH — the thyroid
Vitamin D and B12 — the deficiencies of exhaustion
Fasting glucose — hidden diabetes
CRP — hidden inflammation; cortisol — only as ordered

Markers covered in this article

Ferritin TSH (thyrotropin)Vitamin D Vitamin B12 Blood glucose C-reactive protein (CRP)Cortisol Hemoglobin

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Frequently asked questions

Hemoglobin is normal — can iron be skipped?

No: iron stores (ferritin) run dry long before hemoglobin falls. Hidden deficiency already produces fatigue and hair shedding with a "perfect" CBC — which is why ferritin is tested.

Will pharmacy vitamins help without testing?

Vitamin D at a standard winter dose is a reasonable guess, but guessing at everything else leads nowhere: fatigue from hypothyroidism or diabetes is not treated with vitamins. Testing the base once is cheaper.

When is fatigue urgent?

Alarming combinations: fatigue with weight loss without dieting, night sweats, fever, enlarged lymph nodes, or shortness of breath. Tests here are done this week, not "someday."

Sources

Ferritin Blood Test: MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
Ferritin blood test: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
TSH (Thyroid Stimulating Hormone) Test: MedlinePlus Medical Test — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.
TSH test: MedlinePlus Medical Encyclopedia — MedlinePlus, National Library of Medicine. Accessed: August 29, 2026.