Mileage may vary. Under conditions of stress (such as the various dead sports players), or if you have pre-existing cardiac system weaknesses (which you may or may not know about) yes, it can kill you, or do lasting damage. If you have any kind of imbalance in your thyroid or other glands, it also has the potential to cause lasting problems.
And it is a powerful substance; it's possible to overdose on it, which will give you all sorts of unpleasant symptoms, including hallucinations, racing or irregular heartbeat, and more.
However, many people take it in the recommended doses with no problem at all. I don't think it's any more dangerous than the general run of most over-the-counter drugs (even aspirin can cause death in some cases, after all) but it is more subject to abuse, I think -- a lot of people don't respect the strength of it, either alone or stacked, thinking "it's just a supplement".