You don't need RegEx - or even a macro - for that. A wildcard Find/Replace, with:
Find = <th[eis]{1,2}>
Replace = that
will do. That said, it's not apparent how your font sizes or reference to titles (both of which can be accommodated in a wildcard Find/Replace) have to do with what you're doing, as your code discloses nothing about that.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]