Which lifestyle factors change FT4?

FT4 (free thyroxine) is influenced by several lifestyle factors, but effects are generally modest. Key factors include: iodine and selenium intake (essential for thyroid hormone synthesis), chronic stress (cortisol elevation can suppress TSH and FT4), smoking (may slightly increase FT4), excessive alcohol (can lower FT4), intense endurance exercise (transient increase, long-term stable), sleep deprivation (can alter TSH and FT4), and caloric restriction (reduces FT4). Evidence is mostly human-observational, not from RCTs. Importantly, FT4 is tightly regulated by the HPT axis; large changes usually indicate thyroid pathology rather than lifestyle. MyBody-X biomarker tests can measure FT4, but interpretation requires clinical context. Do not self-supplement iodine or selenium without medical guidance.