Familial thrombocytosis is a type of thrombocytosis, a sustained elevation of platelet numbers, which affects the platelet/megakaryocyte lineage and may create a tendency for thrombosis and hemorrhage but does not cause myeloproliferation. The disease is familial thrombocytosis (MONDO_0019111, a Monarch Disease Ontology entry). Also known as: THCYT, familial thrombocythemia, hereditary thrombocythemia, hereditary thrombocytosis, hereditary thrombocytosis disease.