Evidence check: FT4 in the context of Schlaf

The query concerns FT4 (free thyroxine) in the context of sleep. The provided knowledge context contains no direct evidence linking FT4 to sleep. Generally, thyroid hormones influence sleep-wake regulation: hyperthyroidism (elevated FT4) can cause insomnia, while hypothyroidism (low FT4) is associated with fatigue and increased sleep need. These associations are based on clinical observation and physiological mechanisms, not on specific genetic or microbial markers from the given data. The evidence grade is therefore 'mechanistic' or 'unclear', as no direct studies are cited in the context. Caveat: A single FT4 measurement without other thyroid markers (TSH, fT3) is insufficient for sleep assessment. For sleep disturbances, a medical evaluation including thyroid function is recommended.