You need to take the companion supplements and do salt pushes to get the bromide out. Your symptom is a common one as the thyroid softens up to accept the iodine. This link has the Iodine protocol.
http://breastcancerchoices.org/iprotocol.html

May be you need to start on a little lower dosage and work up from there.

How much selenium and what form? You should be taking 200mcg minium of selenomethionine. And you should be doing salt pushes to help flush out the bromide that has been mobilized, otherwise your body will have a difficult time flushing it out. Taking NAC (N-acetyl-cysteine) 1-2 caps a couple times a day will assist the liver and kidneys in detoxing. Head on over to the Iodine Supplementation forum for more support. It's been the experience of many that higher doses will help detox symptoms. I don't know why this is the case, but many have reported it. I don't know what the experience of hashimoto's folks are, but if you post and ask for feedback from people with hashimoto's I am sure you will get a response.

Make sure you are taking vitamin D, magnesium and boron along with the other companion supplements.

In addition to the above, hashimoto's and other auto-immune disorders seem to improve with a gluten free diet.

Consult a do9ctor on this...

See Dr. Lawrence Wilson's site about how to get idodine from kelp, and in what brands, and why Iodine alone can be problematic. Don't expect most doctors to have a clue about Iodine one way or another.