I iron hot with huge amounts of steam and have never had any problem with a polyester thread. I don't think it's a problem unless, like Rosemary says, you use a very cheap or very old thread. But you wouldn't want to use a cheap or old cotton thread, either, because it might handle the iron, but wouldn't handle wear and tear.

I admit that I have had trouble with cheap polyester thread disappearing whilst ironing (I was using it for basting clothes) But I have never had any problems with Superior's Bottom line and I tend to iron hot. Check out Bob's information video's posts about their threads at Superior, that should help to put your mind at rest.

I have become addicted to english paper piecing and have started making a hexagon quilt. I have been using both Superior Bottom Line thread and Deco Bob for the whip stitching, both of which are very nice and blend well. My question is that I have heard that you should not use polyester thread if you are going to iron it. I have been ironing my sections as they are completed and haven't had a problem so far. But I am concerned that the thread will fall apart if I continue to iron it over time. Does anyone have any experience with this?