Fatigue: 7 medical causes your doctor should investigate

Dipl. med. Charalampos Romanos, FMH-certified physician · Published: April 13, 2026 · Last medically reviewed: 14 May 2026 · 5 min read

Fatigue is one of the most common reasons patients visit our practice. "I sleep enough, but I'm still constantly exhausted" — I hear this several times a week. What many don't realise: chronic fatigue often has medical causes that can be identified through targeted blood work and a thorough history.

1. Iron deficiency — even without anaemia

A ferritin level below 30 µg/l can already cause fatigue — even when haemoglobin is still normal. I see this regularly, especially in women before menopause. A simple blood count isn't enough; ferritin, transferrin saturation, and CRP are essential.

2. Underactive thyroid (hypothyroidism)

TSH, fT3, and fT4 — these three values paint a clear picture. Subclinical hypothyroidism (elevated TSH with normal fT4) is frequently overlooked but can explain exhaustion, cold sensitivity, and weight gain.

3. Vitamin D deficiency

In Zurich, over 60% of my patients have vitamin D levels below 30 ng/ml — especially during winter months. A deficiency affects not just bone health but also energy, mood, and immune function.

4. Insulin resistance and blood sugar fluctuations

Post-meal fatigue, afternoon energy crashes, sugar cravings — these can indicate insulin resistance. Fasting glucose alone isn't enough. We also measure HbA1c, fasting insulin, and calculate the HOMA index.

5. Vitamin B12 deficiency

Particularly with vegetarian or vegan diets, proton pump inhibitor use (PPIs), or in older patients, I frequently see low B12 levels. Active B12 (holotranscobalamin) is a more reliable marker than total B12.

6. Chronic stress and cortisol patterns

Prolonged stress can alter cortisol patterns. The result: morning exhaustion, afternoon energy dips, but evening restlessness. A cortisol day profile (saliva test) provides clarity here.

7. Sleep quality — not just duration

Eight hours of sleep won't help if the quality is poor. Snoring, breathing pauses, or restless sleep can indicate sleep apnoea. When suspected, we arrange a sleep study.

Which blood tests are important for fatigue?

A sensible fatigue work-up goes well beyond a standard full blood count. It typically includes ferritin, TSH, fT3/fT4, vitamin D, active vitamin B12 (holotranscobalamin), folate, HbA1c, fasting glucose, CRP, liver enzymes, creatinine, and electrolytes. Depending on history we add a salivary cortisol curve and sex hormones. The full list with medical context is in our companion article Which Blood Tests for Fatigue — 14 Lab Markers.

When to see a doctor about fatigue?

You should see a doctor about fatigue when the exhaustion lasts longer than 4 weeks, clearly limits your daily function, or comes with other symptoms. Fatigue on its own is usually harmless — but when it cannot be explained by sleep or follows a clear timeline, it belongs in a systematic medical work-up.

Specifically, I recommend a GP appointment in the following situations:

• Duration: Exhaustion for more than 4 weeks without a clear cause.
• Severity: You can no longer manage your usual work, daily life, or exercise.
• Accompanying symptoms: Unintended weight loss or gain, hair loss, dizziness, shortness of breath, persistent pain, new fever, swollen lymph nodes.
• Neurological: Poor concentration, "brain fog", tingling in hands or feet, mood changes.
• Sleep: Not refreshed despite enough sleep — or loud snoring with breathing pauses (possible sleep apnoea).
• Post-infection: Fatigue that persists more than 6 weeks after a viral illness (e.g. flu, EBV, COVID-19).

Acute warning signs that need immediate medical attention: chest pain, sudden shortness of breath, sustained high fever, altered consciousness, bloody or black stools. In these cases, the nearest emergency department or the Swiss emergency number 144 is the right contact — not next week's GP appointment.

At my GP practice next to Zurich main station, persistent fatigue starts with a thorough history and a structured check-up covering exactly these parameters. Which blood tests are sensible is described in our companion article Which Blood Tests for Fatigue.