Mixed phenotypic acute leukemia (MPAL) is a rare clinical entity. MPAL associated with myeloidsarcoma (MS) is still rarer with only three cases mentioned in English literature. MS has been described in myriads of location, most commonly in skin, gums and lymph nodes. Although theoritically possible, it is very rare to find MS involving the thyroid gland. The diagnosis of MS can be elusive, very often masquerades and mislabeled as lymphoma. A high index of clinical suspicion coupled with PET/CT findings along with morphological clues and thorough peripheral blood, and bone marrow evaluation is mandatory for arriving at the definitive diagnosis. We report the case of a 58-year-old female presenting with thyroid swelling that was subsequently diagnosed to be MS of the thyroid with underlying MPAL (mixed myeloid/B-cell) only after 18F-FDG PET/CT, which revealed an unusual abnormal pattern of multifocal intense FDG uptake in the thyroid gland.