My mum and I both have underactive thyhroids, It's really no big deal, the main effect is that you're cold ALL THE TIME. mine isn't too underactive, so I don't have to take anything for it, but my mum's is a lot worse, so she takes a couple of pills every morning.
The only thing is that thyhroid problems can actually be a symptom of adrenal gland fatigue, which is a bit more serious, it makes it so everything stresses you out, eg, someone jumping out in front of you and saying "boo." My mum has pretty bad adrenal fatigue, and sometimes if something unexpected happens, she'll just sit down, and start crying. Adrenal fatigue usually turns into chronic fatigue syndrome, which makes it really difficult to cope with everyday life, and things like having a job....... thyroid issues don't always mean that that is whats happening though..... that's really only for a small percentage of the population, and if you catch the problem early, it can be very easily contained :)