Evidence check: FT3 in the context of stress

FT3 (free triiodothyronine) is the active thyroid hormone regulating metabolism. Chronic stress can suppress the conversion of T4 to T3 via the HPA axis and elevated cortisol – a phenomenon known as 'low T3 syndrome' or 'euthyroid sick syndrome'. Human studies (e.g., in critical illness, overtraining, or psychological stress) show that FT3 can drop while reverse T3 rises. However, this is not a validated diagnostic marker for stress in healthy individuals. A single FT3 measurement without TSH, T4, and clinical context is insufficient. MyBody-X does not offer FT3 testing; it is a blood biomarker, not a genetic test. If you experience stress-related symptoms, consult a physician for a full thyroid panel. Caveat: Low FT3 can also result from caloric restriction, inflammation, or medications. Evidence is moderate but not strong enough for standalone stress assessment.