Firstly, I suppose the question is what type of surgery is it you are looking to have (band/sleeve/bypass) as reading here and from my own experiences, it seems that some surgery has greater issues with taking medications than others (of course, it always changes at the individual level).
For me (sleeve), I crushed what meds I couldn't get in liquid form and ate them with v.runny yogurt and after a couple of weeks was back to taking them normally. I wouldn't have been able to "gulp" them down with a big glug of water (and still don't take big glugs of it).
For the specific question re thyroxine, I believe the liquid form of that was Evotrox, but I think it was withdrawn late last year/early this year - I'm sorry to say I don't know if there is an alternative brand these days, but either your gp or (fingers crossed) your provider should be able to help you out with that.