Could I have Thyroid Disease?

Hey there :]
So, my name is Danielle and I am sixteen years old. I am VERY overweight and have been most of my life. I would say genetics, BUT try as I might I cannot lose weight! The more I try to lose weight, it seems like the more I gain.
My friend is studying to become a CNA and she said I should go to my doctor and see if I have a problem with my thyroid. I honestly didn’t even know what a thyroid was, or what it did until I started doing some research.
I have a good amount of the symptoms that are shown on http://kidshealth.org/teen/diseases_conditions/growth/thyroid.html#
-I have depression, but that could be because of genetics also.
-I normally am very tired/sluggish unless I get 12+ hours of sleep.
-My periods are very irregular
-I have a poor memory
-And I have a lot of trouble concentrating in school.
I am going to go to the doctor soon because I am due for a check up, and I am going to ask him about this, but I like getting ya’ll’s answers too. :]
Have a good day/night