n. 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, *cytotoxic drugs, *interferon, or drugs that inhibit *megakaryocyte maturation.
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