I hear what you are saying with the Thyroid but if you cycle up and then mirror the cycle when you come off, you should be fine. I am not a big fan of CLEN as I found it to make me pretty jittery, have nasty headaches, and I didn't really see much on the results end. I was on a pretty strict diet so I think for the most part, my losses were due to that rather than the CLEN. Lastly, messing with my heart seems to be a lot more dangerous than messing with my thyroid. Lets look at the extreme examples of messing with each. Mess with you thyroid you could end up on medication for the rest of your life. Mess with your heart, you could end up dead. Seems like a no brainer to me... WWW.ANABOLICREVIEW.COM has great studies on each. Also check out their message boards as they have many opinions from many people who have tried the different drugs.