Hyperthyroidism will do this. I am experiencing the internal shakiness now and think maybe my almost two year hiatus from hyperthyroidism has possibly returned. My levels were literally just checked this month and we're fine but ..... however I have had periods where I think it has come back because I will have increased anxiety and spiking heart rate and some more weight loss and my levels will be fine. It could just be doing it frequently and not being caught in the moment. Not sure. I have lost more weight since my levels stabilized. It also caused my appetite to go way down and food hasn't tasted that good since..also my appetite never really returned, which I am not too bothered by, but the taste of food...wish that was better.