I have just been diagnosed with this myself, and I had never heard of it there is a lot of information available on the internet and of course the forum can always help. Take care and good luck.
Hashimoto's is an auto-immune disease named after the Person/Doctor that identified the disease. Your body's immune system (T-Cells) do not recognize your thyroid, and seeing it as as alien and attacks it,and causes it to become inflamed and enlarged, when this happens it reduces the thyroids function to secrete hormones into your system, hence the need for thyroid medication (Hormone Replacement) to make up for what your system is lacking. The lack of the hormones can cause a multitude of symptoms (You can Google Hashimoto's for a list of these symptoms) If you need any more info, just post and ask.
Good Luck FTB4