I have subclinical hypothyroidism, which means my brain needs to pump out more TSH than normal in order to inspire my thyroid to - in turn - pump out enough of the thyroid hormone. That is why I have no symptoms. I have a thyroid and it's at normal size.
I was given a prescription for thyroxine very early on in my pregnancy, but a very small dose since the TSH levels were only slightly high. I had blood taken a couple days ago to get my TSH levels checked again. The doctor thinks that I might have to have my dosage increased again. I'll talk to her again soon.
I once knew the answer to your last question, but I can't remember now. This was really a surprise diagnosis as I've had no symptoms and I didn't even know I had family with thyroid problems. Sadly, it does run in one side of my family (no men, interestingly enough), but not every woman gets it. I'm just unfortunate enough.