Tub Drain

How do I break loose the tub drain? It is one of the kind with threads that should just unscrew from the pvc, but it won't budge. I don't want to do anything else that would damage the tub. My guess is that when it was installed they used some kind of sealant.

I would try pouring some boiling water down the drain to heat the fitting and then try removing it with the special wrench made for the job.

Thanks. I did not realize there was a special wrench available. I bought one and managed to get the old drain out. It was sealed with some kind of gunk, but I gave way without having to use the boiling water trick. Thanks again. I hated the thought of calling a plumber for something like this.