Evidence check: FT3 in the context of Schlaf

FT3 (free triiodothyronine) is the active thyroid hormone and plays a key role in metabolism, thermogenesis, and circadian rhythm regulation. Low FT3 levels have been associated with fatigue, low energy, and poor sleep quality in observational studies, but the evidence for a direct causal link to sleep disorders is moderate. Subclinical hypothyroidism (low FT3 with normal TSH) may contribute to sleep complaints, yet no robust interventional trials confirm this. FT3 levels fluctuate diurnally and are influenced by diet, stress, and medications. A single measurement from a home test (e.g., MyBody-X) provides only a snapshot and is insufficient for clinical decision-making. Persistent sleep issues warrant a medical evaluation including TSH, FT4, and clinical history. The evidence base is human-moderate, relying mainly on observational data.