S60 morbidity and mortality. It is characterized by sustained megakaryocyte proliferation that increases the number of circulating platelets. To diagnose ET, it requires the combination of excluding causes of reactive thrombocytosis and ruling out other myeloproliferative disorders. Further genetic workup may require evaluating Jak2 mutation, which is present in 55% of cases in ET. We present an unusual presentation of essential thrombocytosis which initially presented as iron deficiency anemia (IDA) and thrombocytopenia. Case presentation: This is a 72 year old female who presented with chest pain. Initial lab evaluation revealed severe anemia with hemoglobin of 3.4 and mean corpuscular volume (MCV) of 55, with the rest of complete blood count and basic metabolic profile unremarkable. In light of severe anemia, patient had a work up that showed severe IDA with ferritin at 6. She was thereafter started on intravenous (IV) iron treatment. Initially at presentation, she was thrombocytopenic with platelets count of 95,000, but after 4 days of IV iron replacement, her platelets count increased to 700,000. In spite of discontinuing the IV iron, her platelets continued to rise to more than 1 million 14 days after admission. Her prior labs eight years ago showed platelet count of more than a million. Patient admitted to being noncompliant on iron supplements for the last several years. She underwent bone marrow biopsy which showed hypercellular bone marrow with increased megakaryocytes. Additional work up with fluorescence in situ hybridization-flow cytometry and further genetic testing were negative. Discussion: IDA normally presents with reactive thrombocytosis, but usually it responds to iron replacement therapy by normalization. There have been few case reports of IDA presenting as thrombocytopenia and the pathogenesis remain unknown. Additionally, iron replacement therapy demonstrated platelet level normalization. In light of persistent and extreme thrombocytosis is response to iron replacement therapy, this case demonstrates unusual presentation of iron deficiency anemia masking ET. Conclusion: Our patient represents a unique case presentation of ET which was masked by severe iron deficiency anemia and unmasked through iron replacement therapy.