There are different types of thrombocythemia and their respective conditions vary, some of which are curable and cannot be generalised to be terminal.
Reactive thrombocythemia is secondary thrombocytosis due to a number of non-hematological factors, which include infections, rheumatic immune diseases, the effects of medications, iron deficiency, etc. When these factors are removed the platelet count returns to normal.
Spontaneous thrombocytosis is caused by a number of haematological disorders that result in increased platelet counts and require aggressive treatment of these underlying disorders, the prognosis of which varies greatly from case to case.
Primary thrombocythemia is a benign disease that progresses slowly, and the main concern is to be aware of the risk of bleeding and thrombosis, especially in people who are older and have cardiovascular disease of their own.