Thanks for writing in.
broadly classified as primary and secondary. Primary is when there is no known underlying cause. Secondary hypothyroidism has underlying cause.
Secondary hypothyroidism does not need lifelong treatment if underlying cause is corrected.
However primary hypothyroidism do need lifelong treatment.
But heart rate will not fast. If you have experienced increased heart beats means you need to consult your physician again. Make sure that you are not taking higher doses of thyroid supplement which can lead to increased heart rate
After treatment starts, you should get your hormones tested at interval of 3 months twice. Then every yearly to make sure proper doses of supplements.
I hope I have clarified your doubt.