What causes thrombocytosis?

Primary or secondary: Elevated platelets (thrombocytosis) can be primary (driven by a bone marrow disorder) or secondary (eg, reactive). Reactive/secondary is the most common, with some frequent associated conditions including iron deficiency anemia, reactive (eg, pneumonia, rheumatoid arthritis, post surgical). Thrombocytosis that goes away is likely reactive. Primary can be evaluated for by labs & bone marrow biopsy.

Many things: It could be reactive, such as infection, recent surgery, malignancy, iron deficiency. Other causes can include myeloproliferative disorders. These include essential thrombocytosis, polycythemia, myelofibrosis, chronic myelogenous leukemia. Hope this helps.