Is FT3 useful for gut health?

The question of whether FT3 (free triiodothyronine) is useful for gut health cannot be answered from the provided context. The given information covers cardiorespiratory fitness, microbiome tests, and the MC4R gene, not thyroid hormones. In general, FT3 is a thyroid hormone that regulates metabolism, but its direct role in gut health is not well established. There is some evidence of interaction between thyroid function and the gut microbiome, but the evidence is weak, mostly from animal or associative studies. An FT3 test alone is not a validated tool for assessing gut health. If thyroid or gut issues are suspected, medical evaluation is necessary. The evidence is therefore rated as 'unclear'.