welche vitamine fehlen bei müdigkeit — when to see a doctor?

Fatigue can be caused by deficiencies in vitamin B12, vitamin D, iron, or folate, but also by thyroid disorders, sleep apnea, or psychological stress. A blood test ordered by a physician (e.g., complete blood count, ferritin, B12, 25-OH vitamin D) is the only reliable way to confirm a nutrient deficiency. Consumer tests often lack standardization or clinical validation. When to see a doctor? If fatigue persists for more than two weeks despite adequate sleep and diet, or if accompanied by shortness of breath, pallor, palpitations, concentration problems, weight changes, or low mood. Also seek medical advice if you have a known condition (celiac, Crohn's, vegan diet without supplementation) that increases deficiency risk. Self-supplementing with high doses can cause side effects or mask underlying diseases. Evidence: practice (clinical guidelines, medical consensus). No specific sources from the provided context.