Is FT4 useful for Müdigkeit?

FT4 (free thyroxine) is a key thyroid marker, but it is not sufficient on its own to evaluate fatigue. Fatigue has many causes—sleep deprivation, iron deficiency, thyroid dysfunction, and more. In hypothyroidism, FT4 is often low, but TSH is the more sensitive screening test. Isolated FT4 can be normal in subclinical hypothyroidism. Guidelines recommend TSH as first-line; FT4 is used for confirmation. A home FT4 test alone cannot diagnose or rule out thyroid disease. For persistent fatigue, a doctor should interpret results in context and may order a full thyroid panel (TSH, FT3, FT4, antibodies). Evidence is strong for thyroid-related fatigue, but FT4 alone has limitations. Caveat: single measurements can be misleading due to circadian variation or biotin interference.