What can cause high platelets for several months that are slowly increasing? no infection, rbc and wbc are normal. 22 y/o female with history of thyroid cancer?

Platelets can elevate when there is inflammation present. Inflammation can be present from many etiologies including infection, cancer, autoimmune disorders, etc Has the thyroid cancer been treated? Your doctors can diagnose whatever the source of inflammation is and monitor the platelets while treating whatever needs treatment. Best of luck to you!

I recommend consulting a hematologist to determine the severity,type and cause of your thrombocytosis. It could be reactive or could be a primary bone marrow disorder. A hematologist would be best to advise you and provide appropriate followup as well.