Low free T3 or free T4 levels, even with normal TSH, can indicate an underperforming thyroid and negatively impact metabolism, body temperature, and reflexes.

“what we often see in people on low carb diets is that they're T4 free or they're T3 free drop TSH remains within the normal range and for the longest time I ignored this but I really don't think we can having a low T3 or low T4 is bad you want your body to be working on all cylinders you want all of your V6 motor cylinders to be firing you want your body temperature to be robust you don't want to be cold you want your Achilles tendon reflexes to be good there are all sorts of clinical things that you can do to assay how good your metabolism is and you want all of that to work well”