Selenium is essential in the conversion process of T4 to T3. Its also critical for the handling of the toxic byproducts that are generated from thyroxin creation and conversion. One of the reasons you hear about people having thyroid issues from taking Iodine is because they are not taking or getting enough selenium in their diet. One without the other can cause problems.
As for your side effects largely symptoms of detox but really you need to share what "type" of liguid Iodine your using and the dose.