I went back to the endocrinologist this afternoon and he said that I definitely don't have cancer.
It's just ("just") a degenerative nodule (or somesuch) that he's pretty sure has been caused by the lithium. He doesn't think that stopping lithium is the answer, thankfully. I just couldn't deal with that.
He did say that my thyroid hormone level is still low even though I've been taking the Synthroid. So, I'm going to take more and come back in two months.
The goiter should continue to shrink as I have more of the thyroid hormone.
I do have more energy. As in, I only want to sleep 50% of the time and don't fall asleep in class every day. And I've cut back on the caffeine.
So, hooray! Hooray!