My mother also had this, she had the overactive one first, and was treated with the iodine etc, now it has changed to the underactive one,it is indeed a tricky thing to treat,she is on daily medication, and she swears things are not quite right for her at the moment,although she would need to re-visit a specialist to find this out for sure.

Do take it seriously and get the right kind of treatment, it affects a lot of things going on in your body, I have not heard of a specific diet for it either, but if you find out let me know ok, take care and HUGS.