She's been told that she can expect many horrible things, including the possibility of infertility and birth defects. Neither of us can find any information on this disease, nor did her physician tell her much else. I'm very concerned about her health. I will try to clarify these issues using your question as a jumping-off point. Hypothyroidism means that the thyroid gland in the neck is underactive, that it is not producing enough of the normal hormones.
Hyperthyroidism means that the gland is overactive, producing too many hormones.
This is most commonly caused by Grave's disease. Hashimoto's disease is by far and away the most common cause of hypothyroidism, accounting for more than 90 percent of all cases. Since hypothyroidism is so common, Hashimoto's is also a very common disorder. It is more regularly diagnosed now since sensitive tests have been developed, but it was causing hypothyroidism 50 years ago, only no one recognized it. You, your father, and your sister have probably had Hashimoto's, although it has only been recognized in your sister.