The dose of thyroxine depends on body weight of a person . Typically we give dose of thyroxine 1.6 mcg per KG . Since you have not mentioned your body weight. .
Also when your dose of was started on 75mcg you should have not reduced the dose to 50 again.
Once the target
TSH is achieved on what ever dose the same dose needs to be continued .
Most important thing is that this tablet needs to be taken on empty stomach and there should be a gap of minimum 30 mins