Is FT3 useful for sleep?

FT3 (free triiodothyronine) is a thyroid hormone that regulates basal metabolic rate and energy homeostasis. Direct evidence that FT3 measurement is useful for sleep optimization is lacking. While thyroid dysfunction (e.g., hyperthyroidism) can disrupt sleep, isolated FT3 values without TSH and fT4 are clinically limited. Consumer tests marketing FT3 as a 'sleep key' often overinterpret correlational studies. The evidence is weak: no randomized controlled trials demonstrate a causal link or benefit of FT3 testing for sleep improvement. FT3 levels also fluctuate diurnally and are influenced by diet, stress, and medications. Therefore, standalone FT3 testing for sleep is not recommended. Comprehensive medical evaluation is advised for sleep issues.