An abnormal increase in the number of circulating platelets.
A disease in which there is an abnormal proliferation of the cells that produce blood platelets (megakaryocytes), leading to an increased number of platelets in the blood. This may result in an increased tendency to form clots within blood vessels (thrombosis); alternatively the function of the platelets may be abnormal, leading to an increased tendency to bleed. Treatment is by radiotherapy or by cytotoxic drugs.