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Yes, it causes you to react as if you have Graves disease if you get too much.

Also, I should mention that if you start the meds too fast... rather than ease into it, it's like jump starting your system with too big a jolt and can throw you into an emergency crisis as it shocks the heart and other organs. Even as "off" as I was when diagnosed, they started me on the lowest dose and slowly added to it every 6-8 weeks over many months time until I got to a normal level. It meant having blood tests each time to see where I was at, but is the only safe way to go.