Edit: Nevermind I just realized you said selenium made it worse.
I took 400 mcg of selenium (l-selenomethionine)a day for a few weeks before starting Iodine to resolve my thyroid pain during Iodine and 600 mgs a day of NAC helped with this. Sometimes it returned if I went down to 200 mcg. There are other co-supplements needed during Iodine supplementation. Do you have hashimoto's?