Marked neutrophilia associated with neoplasia is a relatively rare finding and has been considered as a paraneoplastic manifestation. Thyroid cancer seldom presents with paraneoplastic leukocytosis. We report on a case of a 69-year-old man who presented with paraneoplastic leukocytosis seven months after undergoing total thyroidectomy and I-131 therapy for treatment of papillary thyroidcarcinoma. We found neither bone marrow involvement of malignant cells nor hematologic malignancy. Based on elevated levels of serum granulocyte colony-stimulating factor, we concluded that the cause was paraneoplastic leukocytosis. Another interesting point was the anaplastic transformation in the pleural metastatic site. It usually occurs in the intrathyroid or regional lymph node.

• Jeong Su Baek(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Sung Hyun Lee(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Ae Ri Cho(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Joo Young Lee(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Jeong Mi Jang(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Seung Suk Lee(Pathology, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences)
• Hyo Rak Lee(Department of Internal Medicine, Korea Cancer Center Hospital, Korea Institute of Radiological and Medical Sciences) Corresponding author