What Causes Sleeplessness,weakness And Palpitations While Suffering From Hyperthyroidism?

I am a Thyroid patient for about last 4 years. Initially it was Hyperthyroidism. My Doctor put me on drug and converted to Hypothyroidism for better management . Treatment continued at periodical review as advised and on September, 2013 I was put on Thyronorm 37.5 and I was OK, But recently I developed sleeplessness, lethargy, weakness, palpitation (racing heartbeat) off & on, sometime mild pain on left chest, etc. Recently I got my Thyroid profile blood test done. It revealed TSH level =12 (and FT4, T3, T4 are within normal limit . Please advise my Thyronorm dosing level.