Good news! For now, at least.
I spoke to Spencer's Pediatrician and she is not concerned about his thyroid at the moment. While his TSH level IS slightly elevated, his T4 (I guess they work together to make the thyroid do what it needs to) was normal. This I knew from his doc at the Oncology Clinic but I didn't really know what it MEANT.
I guess what it means is that his Pediatrician isn't worried. She's comfortable with the numbers. She'll keep an eye on it, retesting him at his 1 year appointment in August (she would've tested his thyroid then anyway) but as long as the numbers don't drastically change for some reason, she's happy. If she's happy, then I'm happy.
Right now I'm REALLY happy. No extra doctors and tests for Spencer!