How are FT4 and fatigue related?

FT4 (free thyroxine) is a thyroid hormone that regulates metabolism. Both low and high FT4 levels can cause fatigue. In hypothyroidism (low FT4), fatigue is a hallmark symptom; in hyperthyroidism (high FT4), fatigue often results from increased metabolic rate and sleep disruption. However, the provided context does not contain specific studies on FT4 and fatigue. The evidence is based on established clinical endocrinology, not on the given sources. For consumer tests, FT4 alone is insufficient – TSH and possibly fT3 are needed for a full picture. Fatigue has many causes (iron deficiency, sleep, stress), so an abnormal FT4 should always be evaluated by a physician. Home test kits have limited diagnostic accuracy and cannot replace professional lab work.