This is a metabolic process, not a thyroid process, and can be affected by any number of factors, including vitamin/mineral imbalances, general
health, age, stress, trauma (just to name a few).
It would be great if you posted your actual FT3, FT4 and TSH (with reference ranges). If your FT3 is really high, you might ask your doctor about lowering your Armour a bit and adding a little synthetic T4 to get a better FT3/FT4 balance.