Evidence check: FT3 in the context of Müdigkeit

FT3 (free triiodothyronine) is commonly included in thyroid panels, but as a standalone marker for fatigue the evidence is weak. Fatigue has many causes (iron deficiency, sleep disorders, stress, depression). A normal FT3 does not rule out thyroid dysfunction; TSH and free T4 are the primary screening tests. Some functional medicine promotes FT3 to assess peripheral conversion, but RCTs are lacking. DTC tests provide screening-level values, not a diagnosis. Caveat: Do not self-medicate with thyroid hormones based on FT3 alone – risk of thyrotoxicosis. Evidence grade: practice (clinical routine) / mechanistic (conversion hypothesis).