How are FT4 and Müdigkeit related?

FT4 (free thyroxine) is a thyroid hormone whose blood level indicates thyroid function. Low FT4 (hypothyroidism) is strongly associated with fatigue, lethargy, and lack of energy due to slowed metabolism. High FT4 (hyperthyroidism) can also cause fatigue, often accompanied by sleep disturbances and restlessness. Thus, the relationship is U-shaped: both low and high FT4 can lead to tiredness. However, fatigue is a non-specific symptom with many possible causes. A single FT4 value without TSH and clinical assessment is insufficient for diagnosis. The evidence is strong (human-strong) based on decades of clinical and epidemiological research, but individual factors like stress, diet, and sleep quality modulate the effect. Caveat: Normal FT4 does not rule out thyroid dysfunction, and self-interpreting lab values without medical guidance is not recommended.