I have found that any of the brand names and the
generic can make arrythmias worse. I have been on
generic, Synthroid and Levoxyl at one time or another, and they all cause increased episodes of tachycardia. In my case, I think it's the actual medicine in the pills that causes the tachy, not the fillers. I'm also on a beta blocker to control the tachy.
My guess is that it's your increase that has caused the PVCs, not the generic versus Synthroid.