What is thyroid ? What are the symptoms ? If untreated could it yield to enlarge of the heart or heart attack?

Six years ago, I was diagnosed to have toxic thyroid. My doctor gave me medicine and when my thyroid is in normal level, my doctor recommend a radioactive treatment. But I never undergo a radioactive treatment. Just this afternoon, I happen to met a long highschool friend who is now a successful doctor. He told me to have a medical check up immediately because my heart might start to enlarge due to non treatment of thyroid. Is there any doctor out there who can share us ?