To illustrate: If the T3 levels were to be kept steady enough, even people who don’t need T3 therapy would not have side effects from exogenous T3 (provided their temps were kept close to 98.6). Because when people are given T3 their own systems compensate (through negative feedback inhibition of the pituitary) by making less T3 of their own. If enough T3 were given by mouth to healthy people, their own systems would be completely suppressed and their T3 levels would be completely replaced with exogenous T3. If their temperatures were not too high or too low, then how they would feel while replaced on the exogenous T3 would mainly depend on how close to endogenously steady the T3 levels could be kept.