Evidence check: FT4 in the context of Müdigkeit

FT4 (free thyroxine) is a thyroid hormone often measured in the context of fatigue. Both hypothyroidism (low FT4) and hyperthyroidism (high FT4) can cause fatigue, but a single FT4 value is not diagnostic. TSH is the primary screening test for thyroid function. Reference ranges vary by lab and population. Consumer tests may include FT4, but interpretation requires a physician, as factors like medications and illness can affect levels. Evidence is based on clinical practice, not controlled trials. Caveat: self-medicating with thyroid hormones is dangerous. Persistent fatigue warrants a full thyroid panel (TSH, fT3, fT4, antibodies) under medical supervision.