<qa>  What is the main cause of essential thrombocythaemia?Essential thrombocythaemia is a disorder of bone marrow, which causes too many platelets to be produced. Platelets are small cells carried around in the blood, which help form blood clots. When patients have too many platelets, there is a risk of blood clots forming unnecessarily and excessive bleeding. The aim of this study is to gain additional information on the safety profile of Anagrelide Xagridr and Hydroxyurea also known as hydroxycarbamide. 