Re: Was hypo, now hyper?
There's a difference between a radioctive iodine scan and ablation. You've most likely been reading about ablation. The scan only uses a very tiny dose of radioactive iodine in order to measure the function of the thyroid and get a good picture of it. I'm not for ablation either but I have had 4 scans with no ill effects. A scan in your daughters case would be very, very helpful.
A negative TSI test does not totally rule out graves. There are a couple other obscure antibodies that can also be positive in graves. I suspect the endo is considering the possibility of toxic multinodular goiter and a scan will show that if it's there. Your daughters lab's clearly show that she is hyperthyroid. Weight gain isn't terribly unusual for hypers either but it mostly happens with the younger set.
It's terribly difficult to understand the thyroid when you're first starting out. I do hope they have prescribed some type of medication to help, either beta blockers or anti-thyroid meds? Being hyper myself, I know what kind of hell she's probably going thru and I don't know what I'd do without my meds. You sound like a wonderful mom in doing all this research to help her. You've also probably found about the best place to start. There's a lot of experience wandering around here. If you can talk your daughter into joining us, it might make her feel better knowing she's got people who understand what she's going thru. I first got real sick when I was about 23 and it was hard going it alone.
My suggestion would be for her to get the scan and get a copy of the actual report. Post the results here and we can give you a hand and help guide you thru this maze.