Welcome to HCM,
You have been treated for Ca Thyroid and taking Thyroxine ,the symptoms you have described points out that you are taking excess dose and having Hyperthyroid state that is palpitation
, and some relief with Beta blocker
Please Visit a Physician and get T3, T4,( FT3 and FT4) and TSH your TSH is expected to come low in hyperthyroid state.For symptomatic relief Propranolol
may be taken with consultation of Physician.
You can reduce or stop Thyroxine for some days and test yourself as soon possible then dose can be adjusted.
Please take care