The most common cause of hyperthyroidism in females is Graves disease, which can be "triggered" by an infection, stress, etc. and occasionally runs in families. However, I was just diagnosed with Graves two months ago (with a TSH that was undetectable, high T4, and high thyroid antibodies - a classic trio) and no one in my family had thyroid problems that I know of. Being hyperthyroid can cause a lot of nasty issues, most of which I experienced firsthand - rapid heart beat, palpitations, sweating, diarrhea or increased BMs, fatigue and insomnia (there's a paradox!) - ugh it was NO fun. But now I'm being treated with anti-thyroid meds and feel much better.