far from similar. synthroid is t4 as explained earlier. yes t4 is much less potent than t3 but t4 can cause more problems such as hypothyroidism. how? t4 has a very long half life as compared to t3.
on the other hand t3 can be dangerous too. there is a condition known as thyroid storm that needs to be avoided. if one overdoses on t3 than you may see this happen, symptoms include vomiting, sweating, et. al. the end result can be fatal, so if you suspect this, than check yourself into the ER