I was diagnosed with Grave's Disease 15 years ago, had radioiodine treatment which destroyed my throid gland, and now take thyroxine replacement as a result. I find I function very well at the levels I take, but would like to be allowed a higher dose to be that bit sharper (I'm told by my doctor that is not uncommon, but that I have to stay within the accepted given normal levels...)!
My diagnosis was a case of "when, not if" as my family has a strong incidence of thyroid problems - my mother, father, father's twin sister, my maternal grandmother, my half-sister on my father's side, and now my sister (although she is underactive - rather than overactive).
My father went on to deveop ulcerative colitis, but he was a man who very much internalised his stresses, and equally hid from his problems - his IBD could have been treated differently had he chosen to do so, but he didn't and it went on to full blown ulcerative colitis which again he could have had surgery for but he chose not to do so....
My mother also has Parkinson's Disease, which has no family history that we know of.
So sometimes you are dealt a poor hand in terms of genetics - but it is possible to lead a pretty normal life if you can find suitable treatment that improves your problems, and make your own lifestyle changes to support your health.
Please don't think that life will be terrible as a result of your diagnosis - it will be different, but it can be manageable.