I guess the subtle symptoms I ignored included: a mild hand tremor, feeling tired, not liking the heat. I didn't think anything of it for quite a long time... then mentioned it as an "aside" when at the doctors for something else then BAM "you could die from this!".

I think when you are young you think illness is primarily about avoiding the flu. As you grow older you have to watch out for other health issues... things can creep up on you more slowly and it's not as obvious as vomitting.