It is the central communicator for many components of the functions of the body. As Barb indicated it is the driving force of metobolism and body temp. But it's level of functions can be endless for most patients.
Testing often and balancing meds is the only way to balance the gland to work properly.
You posted the same questions 3 times, so I'm only going to respond once.
The thyroid is a gland located in the front of the neck that controls body temp and metabolism, among other things. Hypothyoidism is when a person's thyroid gland is underactive (doesn't produce enough thyroid hormone). You can do a web search for it to come up with tons of information (including pictures of the thyroid gland, symptoms, causes, treatments, etc). There is no "cure" for it, but it can be controlled with meds that replace the thyroid hormone. Generally, a person is put on meds and must take them for the rest of their life, but your information is way too vague.
What are her symptoms? What tests were done? What were the results (with lab ranges)? What did her doctor do (other than tell her she's hypothyroid?)? Did s/he prescribe meds? If so, what kind? If not, did s/he say why?