well lets rule that one out em; from what you have described, it is most unlikely that you have any cancerous condition.....but I'll be blunt in that I wouldn't at all be surprised if that is what has been nagging away at you, deep inside that brain of yours?
Am I right?
I know it is not easy but a degree of relaxation may help you enormously, even if just breathing exercises or a good regular brisk walk....as to reading read up on hyper and hypo-thyroidism; there is a lot there on the subject and nothing to be worried once they are ontop of it and, as daisya says, there can be the occasional swing. My mother is generally hypo and on 125g of Levothyroxine once a day but she has had a couple of times when she has been hyper as she has aged...just the occasional check-up (blood work) needed for her now and you also need to watch your BP as often thyroidism and high BP go together and link through to a higher risk of strokes (and hence risk of vascular dementia too).