Is FT4 useful for sleep?

FT4 (free thyroxine) is a thyroid hormone primarily regulating metabolism. There is no direct evidence that FT4 is useful for sleep per se. However, thyroid dysfunction (hyper- or hypothyroidism) can affect sleep quality—e.g., causing restlessness, night sweats, or fatigue. Measuring FT4 is relevant when thyroid disease is suspected, but not as a general sleep biomarker. The evidence for FT4's standalone utility for sleep is weak (mechanistic, indirect). Without clinical indication, FT4 testing for sleep is not recommended. Caveat: Interpretation should be done by a physician, as reference ranges vary individually.