The jury is still out in regards to treatment of a multicystic goiter in a patient with normal thyroid function, please post at the adult thyroid expert's forum for their opinion, at 18 I would recommend you see an adult endocrinologist, have your labs checked and they can consider further testing and follow you closely.
I am thinking if your labs are fine you will not need surgery as long as the ultrasound is fine and exam correlates. You can find an adult endo at AACE.org and click on find an endocrinologist and type in your zip code (that is if you live in the US)
Also, Im wondering if maybe i should get checked for Hashimoto's Thyroiditis? Ive read that thyroid levels in people with Hashimoto's doesnt always come back high or low but it can sometimes show up as normal levels.
I have never been tested for that or for Grave's and im wondering if i should??