What to watch with FT3 and weight loss?

The question about FT3 (free triiodothyronine) and weight loss is not addressed in the provided context. From general medical knowledge: FT3 is the active thyroid hormone that regulates basal metabolic rate. During caloric restriction, FT3 can decrease (euthyroid sick syndrome), potentially slowing metabolism and hindering weight loss. Low FT3 may indicate insufficient calorie intake or hypothyroidism. However, FT3 alone is not a reliable predictor of weight loss success – metabolic adaptation is multifactorial. Consumer FT3 tests have high variability and should not be interpreted in isolation. A medical workup including TSH, fT3, fT4, and possibly reverse T3 is more appropriate. Without context-specific evidence, the evidence grade is unclear. Beware of marketing claims that promote FT3 as a standalone 'fat-burning indicator'.