The normal number of blood platelets is between 150,000 and 450,000 in each microliter of blood, according to Johns Hopkins Medicine. Thrombocytosis refers to having more than the standard number, while thrombocytopenia means there are too few.
In primary thrombocytosis, the bone marrow contains abnormal cells that produce excessive amounts of platelets, Johns Hopkins explains. However, the cause is not identified as of 2015. Secondary thrombocytosis is the result of another condition, such as anemia, cancer, inflammation or infection. A typical symptom of thrombocytosis is blood clots in the arms or legs that form without any injury.
Thrombocytopenia is triggered by certain medicines, chemotherapy, leukemia, lymphoma, kidney disease or excessive consumption of alcohol, says Johns Hopkins. A person with thrombocytopenia bruises easily and bleeds from the gums, nose or gastrointestinal tract.