Fatigue: 7 medical causes your doctor should investigate

Dr. Charalampos Romanos · April 13, 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.

When to see a doctor?

If fatigue persists for more than 4 weeks, limits your performance, or is accompanied by other symptoms (weight changes, hair loss, dizziness), I recommend a systematic evaluation. In our practice, we start with a structured check-up that covers exactly these parameters.