It really depends on if your thyroid is functioning or not
Taking thyroid hormone due to a condition where you hypothalamus - pituitary - thyroid negative feedback loop is simply not functioning , then there may be no harm in adding T3. For people who have had thier thyroid removed, then I also think that you may not have much to worry about
The issues are
1. side effects from the thyroid drugs which can be worse then the conditions they were meant to threat
2. shutdown of a perfectly working thyroid
If one and 2 can be controlled, then y all means , add some synthetic T3