Levothyroxine
"I was just diagnosed with an under-active thyroid last week. I thought I was just so tired all the time because I was getting older and had gained some weight.  Within a few days of taking the medicines I felt like my old self. I felt like I was 21 again.  I had more energy than I'd had in years and was in a good mood [who know?!].  I never attributed any of my problems to a medical condition, but if this keeps me feeling the way I am now, I couldn't be more grateful."