Well, to find the cause, get the complete hematocrit done—MCV, MCH, MCHC, etc along with complete ferritin profile. At times low iron causes the platelets to rise. This can also be due to dehydration or an incidental finding. Repeat test after a week and see. If it remains persistently high, then it can be an inflammatory condition like rheumatoid arthritis or due to anemia or excessive alcohol drinking. It is also increased in some bone marrow disorders like myeloproliferative disorders (polycythemia vera, essential thrombocytosis, idiopathic myelofibrosis and chronic myeloid leukemia).
So, first see whether low iron is the cause and then all the above mentioned causes will need to be looked into. Please discuss with your doctor. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.