Evidence check: FT3 in the context of weight loss

FT3 (free triiodothyronine) is a thyroid hormone that influences basal metabolic rate. In hypothyroidism, low FT3 can contribute to weight gain or difficulty losing weight. However, FT3 alone is not a strong predictor of weight loss outcomes in healthy individuals. Studies show that low-normal FT3 is associated with a slightly lower resting energy expenditure, but effect sizes are moderate. Testing FT3 may be useful if symptoms of thyroid dysfunction are present (fatigue, cold intolerance, unexplained weight changes). Without clinical indication, using FT3 as a standalone 'wellness' marker for weight loss is not evidence-based. The evidence level is human-moderate: observational studies exist, but no RCTs demonstrate benefit of FT3 screening for weight loss in asymptomatic individuals. Caveat: normal FT3 does not rule out thyroid dysfunction; TSH and fT4 should also be measured. A single FT3 test is insufficient for clinical decisions.