I used to dabble in the bodybuilding industry. I was actually a consultant for non-natural steroid regimens and programs, and I used almost all of the drugs myself as well. I always heard that rumor of testosterone, as well as taking t3(cytomel etc) can shut down your natural thyroid production forever. But I've never seen one case, including my own, where it has actually happened...not one. A persons body will correct the imbalance over time, usually within 3 months, in a very bad case, 6-8 months.