If I were you I would get a potassium supplement if the doc will prescribe one for a short period to get your K+ level up. I have mulitple problems with my thyroid with the thyroid stimulating hormone in the several hundreds but the docs thought it use a fluke. It was not regardless of them coming back with some other value somewhere close to the norm. I had alot of thyroid problems and although I got better I continue to have some problems similar to yours with the cramping and muscle problems etc. I chose to seek an alternative MD where I used armour thyroid medication rather than synthroid because its synthetic and people have too many problems with it. It is believed that a good majority of people are hypothyroid. But the fact you have a goiter indicates that your body is overproducing or not able to utilize your thyroid hormones and why that is needs to be addressed.

You also need to be very careful about taking iodine. And yes it has been used for simple goiters. Please let your doc recommend iodine if he feels it will stop further growth, but it will not lessen the size from what I understand. Hopefully you will seek assistance from a good endocrininologist. And if you haven't already researched info on the net yet, please do so. My thyroid was getting enlarged but I did not have a goiter. For that I took the armour, checked my temps, and came off it after a short time or just titrated it because I am more sensitive to drugs. While taking it my thyroid could rest and allow all the thyroid hormones to circulate, just in case I was not producing the t2,3,4, etc I needed and then coming off it to see if my thyroid will or could now pick up the slack. I personally don't believe we should stay on any medications, and believe that the body knows how to heal itself if we only give it what it needs and a chance to heal. good Luck, hope something I have said will help.