What Is The Right Dosage Of Synthroid Prescribed For Hypothyroidism?

My sister and I just discovered that we both went from 150mcg to 75 mcg Synthroid & Lev etc generic form.Neither of us are feeling optimal. I got my doc to move my med to 112 mcg but my sister is still at 75. We are having the same symptoms for hypothyrodism as we originally had. Why was this done and can we get our doseage bumped up? white, female both 65+ good health otherwise