Thrombocythemia, also known as thrombocytosis, is a chronic and uncommon blood disorder characterized by a high number of platelets in the blood. It is more common in older adults over 50 years of age and women, though younger patients may also develop this condition. 

Platelets enable blood clotting, which means high levels of platelets can lead to an increased risk of unwanted clots forming in the blood. High platelet counts may be primary or secondary: primary or essential thrombocythemia is caused by a condition such as essential thrombocytosis or chronic myelogenous leukemia; secondary causes include inflammation, surgery, iron deficiency anemia, or another secondary factor. 

Symptoms of thrombocythemia include headaches, dizziness, chest pain, fainting, temporary vision changes, numbness or tingling in the hands and feet, blood clotting, and fatigue. In rare cases, it may also lead to bleeding in the form of nosebleeds, bruising, bloody stools, or bleeding from the mouth or gums. Thrombocythemia can also cause blood clots in the arteries supplying the brain, leading to a transient ischemic attack or stroke.

There is no cure for thrombocythemia, but treatment and medication are available to mitigate the symptoms of the condition and maintain normal platelet levels in the blood. Drug classes often used to treat thrombocythemia include platelet reducing agents, other antimetabolites antineoplastic agents, and interferons, alpha.