Is FT3 useful for Müdigkeit?

FT3 (free triiodothyronine) is a thyroid hormone that can be useful in diagnosing thyroid disorders. However, for fatigue as a nonspecific symptom, FT3 alone is rarely diagnostic. Standard first-line tests are TSH and FT4. FT3 may be helpful when T3 thyrotoxicosis is suspected or when monitoring T3 therapy. Without additional signs or risk factors, isolated FT3 testing for fatigue lacks strong evidence. Caveat: fatigue has many causes (sleep deprivation, stress, iron deficiency, depression, etc.), so FT3 should only be interpreted within a full thyroid workup.