What role does FT4 play in Schlaf?

Free thyroxine (FT4) plays a modulatory role in sleep via the hypothalamic-pituitary-thyroid axis. Thyroid hormones influence circadian rhythms, sleep architecture, and thermoregulation. Both hyperthyroidism (elevated FT4) and hypothyroidism (low FT4) are associated with sleep disturbances such as insomnia, fragmented sleep, or excessive daytime sleepiness. However, the direct causal effect of FT4 on specific sleep stages (e.g., slow-wave or REM sleep) remains unclear. Most evidence comes from observational studies or mechanistic animal models. There are no robust clinical guidelines defining an optimal FT4 range for sleep health. Therefore, in individuals with sleep complaints, a comprehensive evaluation is warranted before attributing symptoms to thyroid function alone. The evidence level is mechanistic/unclear. Caveat: FT4 fluctuations can affect sleep, but routine testing without clinical suspicion is not recommended.