How do enlarged thyroids become unenlarged?

Depends on the cause: Some enlarged thyroid glands may need radiation , some need medications, some need surgery, some need both. Let an endocrinologist evaluate your thyroid and find out the cause first. That will determine how to reduce the size, but it's more portent to normalize the function of the thyroid and make sure there is no cancer.

Supply iodine: When the thyroid is enlarged due to iodine deficiency i.e. A goiter: it will gradually decrease in size by supplying dietary iodine in appropriate amounts.

Depends: Enlarged glands (goiters) can be due to over or underactivity of the glands. See an endocrinologist to find out which you have.