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    Radiation for breast cancer can cause fatigue in people with or without Graves although it usually only lasts for several months. I had orbital radiation for 14 days for Graves ophthalmopathy with no ill effects but the dose of radiation must be much smaller than for cancer. Maybe your hormonal levels have changed too. I do not think Graves has anything to do with it unless your thyroid levels have changed putting you in the hypothyroid range.