Is FT4 useful for Schlaf?

FT4 (free thyroxine) is a thyroid hormone that regulates basal metabolism and many bodily functions. Thyroid disorders (hyper- or hypothyroidism) can cause sleep issues – e.g., difficulty falling asleep in hyperthyroidism or fatigue in hypothyroidism. However, FT4 alone is not a specific marker for sleep quality. In sleep medicine, TSH, possibly FT3, and clinical symptoms are primarily evaluated. A self-ordered FT4 blood test may provide hints but does not replace medical diagnosis. The evidence for a direct benefit of FT4 for sleep problems is weak (mechanistic, no large studies). Caveat: normal FT4 levels do not rule out sleep-related issues. For persistent sleep complaints, consulting a physician is recommended.