I am not sure what you mean by that. We dont go by whether your thyroid is function at x%. There is a range that we consider and you are either in range or not. If you are low as in out of range of what is considered normal or at the low low end of what is considered normal and you have symptoms of hypothyroidism such as fatigue, excess weight gain, cold intolerance etc then we start thyroid therapy. I will tell you that 90% of endos and other physicians prescribe T4 and not T3. That is because first it is cheaper and second it is the inactive compound since T4 converts to T3 in the body and easier to control and have a wider range and most importantly has a long half life so you only need to dose T4 once a day even if you are taking 250mcg a day. But T3 since it is the active compound has a shorter half life to the point that if you are taking 75mcg daily, it works much better and less side effects on heart where you need to take it three times a day at 25mcg each. For the same reason T3 works much better than T4 in fat loss etc since it is the active compound and not dependent on your body to convert T4 to T4.
Thyroid is a strong strong little organ. You really cant shut it down permanently. I have had patients that have been on 75mcg of cytomel for two years straight and after a proper tapering off their thyroid came right back to normal. You def dont want to go from 75mcg to nothing over night. You want to taper down over a couple of weeks so your body can sense a decrease in T3 and start to produce TSH. Hope that makes sense.