I can't comment on the need for a scan. If the goiter is very uncomfortable, I think I'd at least ask for a closer examination. I believe... but don't quote me... general diffuse enlargement is hardly ever treated surgically, but could be an option for problem ones.
However, I looked up some information for you about iodine deficiency in Mary Shomon's book. A deficiency can indeed cause a goiter, but most doctors will tell you there is no iodine deficiency in the U.S. That appears to be changing, however. According to the National Health and Nutrition Examination Survey (NHANES I), which took place from 1971 to 1974, just 2.6% of U.S. citizens had iodine deficiency. The follow-up NHANES III, conducted from 1988 to 1994 found that 11.7% are deficient. Researchers don't have a real explanation for it, but low-salt diets may play a role and food manufacturers may be adding less iodine. They're still trying to sort it out. I read another source that said farm acreage is becoming depleted of trace minerals, so less ends up in the food supply.
Also... you didn't say if your hypo is caused by Hashimoto's disease. It's generally accepted that iodine will make it worse, not better. So beware of iodine if that's your case.
"We can complain because rose bushes have thorns, or rejoice because thorn bushes have roses." Abraham Lincoln