Hello and welcome!
The muscle weakness is common with Graves', but should continue to improve as your daughter's levels get to a place where they are normal and stable.
Personally, I've had both knee issues and shoulder issues in the years since my Graves' diagnosis, and I've heard anecdotal stories from others as well. I've also noticed a decrease in range of motion during this time, but I spend a lot of the day sitting, *and* I'm a lot older than your daughter!
I wish this was an area where we had more info, but some researchers are beginning to explore this connection; for example, here's a piece on tendon issues and thyroid dysfunction:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3838329/
For more general info on Graves' disease in children, check out the videos and playlists on our YouTube channel. There are videos from Dr. Scott Rivkees and Dr. Michael Gottschalk, as well as a playlist (on a separate “Playlist” tab) featuring several doctors from The Children's Hospital of Philadelphia:https://www.youtube.com/user/GravesAndThyroid/videos
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Wishing you and your family all the best!