I had times of neck discomfort at one time even though I was taking all the recommended nutrients. However this only happened when my adrenal glands where called on to produce hormones such as cortisol. This was most common due to hypoglycaemia. I fixed it by adding a fair bit of pantothenic acid (vitamin b5). That stopped it happening for me. I was already getting a fair bit of supplemental b5 but not near enough. Chances are that my body had insufficient B5 and was using up other nutrients trying to cope.
Weight gain indicates that your thyroid is under functioning probably due to your thyroid shutting down due to the Iodine without the other needed nutrients needed to complete the cycle. That could be due to a deficiency of an important nutrient required for conversion/activation of the thyroid hormones.