What role does FT3 play in Abnehmen?

FT3 (free triiodothyronine) is the active thyroid hormone that increases basal metabolic rate, thermogenesis, and fat oxidation. During caloric restriction, FT3 levels often drop (euthyroid sick syndrome), which can slow metabolism and hinder weight loss. Observational studies suggest that higher FT3 within the normal range is associated with greater weight loss, but causality remains unclear—it may be a consequence rather than a driver. Importantly, using thyroid hormones for weight loss is dangerous and not indicated, as it can cause thyrotoxicosis, arrhythmias, and muscle wasting. Evidence is primarily mechanistic and epidemiological; robust RCTs are lacking. A blood test for FT3 can be useful for thyroid evaluation but not as a standalone predictor of diet success. Genetic variants (e.g., in FTO or thyroid-related genes) may influence FT3 levels and metabolism, but the effect is small on an individual level. In summary, FT3 plays a role in energy balance, but weight loss should not be pursued by manipulating FT3 levels.