Why to they need to test thyroid for?

So yesterday I went to the doctor because I was a little curious about my heart I has been feeling funny, So they took blood from me and I had to urinate in a cup, I was suppose to get a call from them today about the results but I called up there and they said They were going to do a thyroid test? I'm curious to what that means or does, my mom said we have a history of overactive? thyroids