Open Question: Could anyone please give me info about Overactive Thyroid?

I found out yesterday I have an Overactive Thyroid which caused me to loose alot of weight.. I'm going to my doctor again tomorrow.. but could you just fill me in on some of the information about Overactive Thyroid and what it can cause? Info about me: - female - 15 years of age - feeling tired all the time - extreme weightloss - nautious - sleeping problems - unusual hair loss then normal - lighter periods then usual Thanks in advance.