Pembrolizumab Exacerbates Thyroid Diseases Shown on FDG PET/CT.

A 51-year-old woman with a history of sarcomatoid hepatocellular carcinoma, status post resection, underwent FDG PET/CT. In addition to abnormal activity elsewhere, the images showed focal activity in the isthmus of the thyroid, suggestive of malignant involvement, and diffuse mild activity in the left lobe of the thyroid, suggestive of thyroiditis. After a 10-week anti-PD-1 pembrolizumab therapy, the follow-up FDG PET/CT scan demonstrated much more intense activity in both the isthmus and the left lobe of the thyroid. Pathological examination revealed papillary thyroid cancer in the isthmus and thyroiditis in the left lobe of the thyroid.