ok, ive brushed up. understand now.
t3 dosen't cause an overactive thyroid. an overactive thyroid would simply produce ecessive t4 which is converted to t3.
t3's physiologic effects are manifested @ the cellular level (increasing metabolism). introduction of exogenous t3 will be sensed by the hypothalamus = suppressed trh. suppressed trh causes the pituitary to suppress tsh production.
while suppressed tsh (in the absence of exogenous t3) would certainly lead to hypothyroidism, it's not the case when an abundance of exogenous t3.
thanks macro. k to you!