shaky left thumb

i have noticed my left thumb shaking for prob 6-7 years now, since i was 19, but lately it has got a bit worse. its kinda constantly shakes on its own unless i rest it on my hand or table. if i attempt to lift my thumb up off a resting posistion it shakes quiet badly, like as if you were giving someone the thumbs up! when my arm is down by my side it doesnt shake. im very healthy otherwise i dont smoke or drink(though i did used to drink heavily in college though have not in 2 years now) and im in training for a marathon. im not sure whats wrong and kinda fear the worse incase i mite have parkinsons or something. any suggestions or info would be greatly appreciated. thanks