I am hypothyroid and came to the same conclusion. I drink decaf coffee, partly because I can't tolerate black or green tea. I also discovered through 23andMe that I am a slow metabolizer, so caffeine stays in my system a long time. With thyroid disease it seems you have to remove all stimulants and chemicals and many foods before you can figure out what the issue is, then slowly reintroduce. Thankfully, naturopathic doctors are getting better at treating it.