What to watch with FT3 and fatigue?

When monitoring FT3 in the context of fatigue, focus not only on the absolute value but also on the TSH/FT4 ratio and potential conversion issues. Low FT3 with normal TSH may indicate impaired T4-to-T3 conversion (e.g., due to zinc deficiency, chronic stress, or low-calorie diets). Reverse T3 can also be elevated and block T3 action. Important: single FT3 measurements vary diurnally and are insufficient for diagnosis. Evidence is primarily clinical practice, not strong trials. Caveat: many fatigue causes are non-thyroidal (iron deficiency, sleep disorders, depression). An endocrinologist should interpret values holistically.