Is multinodular goiter preventable?

Unknown.: Unfortunately, the reasons for nodule development in the thyroid gland are not well understood.

Generally not: I suppose if you had your thyroid removed at birth, or took thyroid replacement since childhood it would be possible -- but since over 50% of the population that is otherwise normal has multiple thyroid nodules when they die of old age -- it's probably not preventable, but at least is not usually that significant a problem. Make sure it's followed properly, however.