ET has exclusive new details about the illness that caused Jeremy Piven to withdraw from his hit Broadway show, 'Speed-the-Plow.'

Dr. Carlon Colker, a doctor who has been treating the SAG award nominee and Emmy award-winner, tells ET that Jeremy is suffering from extreme mercury toxicity. He informs us that a test revealed that Jeremy had the highest level of mercury that he has ever seen, which amounts to six times a healthy amount of mercury, in his system.

The doctor says that Jeremy is disappointed that members of the production staff of 'Speed-the-Plow' and other members of the entertainment community have not rallied behind him during this health crisis.

Colker tells ET that a major symptom of mercury poisoning is extreme fatigue. In addition, Jeremy began experiencing neuro-muscular dysfunction late last week, which led to extreme difficulty in lifting his arms and legs. Then, this past Sunday, he began feeling dizzy. Now, the doctors have ordered enforced rest. Jeremy has spent three days in the hospital recently, and the doctor tells us exclusively that the actor is no longer in New York.

Colker tells ET that Jeremy has been an avid sushi eater for many years, regularly eating sushi twice in one day. He notes that Jeremy has also taken certain Chinese herbs, and that, in combination with the frequent sushi consumption, could have led to these elevated mercury levels.

ET reported earlier today that Jeremy had left "Speed-the-Plow" and last night, his "Entourage" co-star Kevin Connolly talked to ET about his health.