What is the problem with my thyroid gland?

I have been experiencing chest pain, anxiety, palpatations, episodes of breathlessness/lightheadedness, numbness in localised areas on my arm and head. I have been to the dr and hospital, had numerous ECG's and recently had a blood test that came back fine apart from the need to test my thyroid again in a month. Why do they need to test it again? what does my thyroid do to make me have these symptoms? how can it be treated?