If my thyroid hormone levels are "normal, " is there any harm in taking a T3 (liothyronine) compound? And then what if I stop the T3 (liothyronine) after 2 months for example?
Depends. There is no harm if you are monitored correctly. Too high of a dose can indeed cause atrial fibrillation and/or osteoporosis. The TSH test is not sufficient to monitor thyroid without measuring the entire thyroid hormone panel which would tell if you needed T3 (liothyronine) supplementation. Remember "normal" lab values follow a bell curve so normal may not be optimal for you.
Don't do it. If your thyroid function tests are "normal", do not risk messing them up by taking unnecessary t3 (liothyronine). If your thought is to use it for weight loss, find another more acceptable, less potentially harmful way—like limiting food portion sizes, choosing lean meats, colorful fruits & veggies, and exercising.
Yes there is harm. It can cause a type of irregular heartbeat called atrial fibrillation. In people with heart disease (even undiagnosed), it can lead to a heart attack. If taken for a long time it can lead to osteoporosis. If your thyroid hormone levels are normal, there is no reason to take thyroid hormone.