I'm living on half a thyroid... the other lobe was cut out because it grew numerous cysts and grew a large goitre approx 10+ years ago.
Rational for putting me in synthroid is that "they" wanted to make sure I didn't go hypothyroid after the operation and grow lumps on the remaining thyroid lobe. I would have been far better off without synthroid and straight onto iodine; but ofcourse there are no doctors that smart down here yet.
Now, you could say I have a synthroid habit and a lazy thyroid from years of supplementation. I'm trying to make it work and encouraging it slowly by reducing my synthroid: seems to be working thus far.