A 29-year-old female patient exhibited a solitary neck mass, severe hypercalcemia, and multiple skeletal lytic lesions consistent with metastatic neoplastic disease. Fine-needle aspiration (FNA) cytology of the neck lesion indicated a follicular thyroid neoplasm. CT-guided bone biopsy was non-diagnostic. Subsequent 18F-FDG PET/CT examination demonstrated avid glucose uptake within the neck mass and diffuse bony lesions of variable metabolic activity. Repeat biopsy utilizing PET/CT guidance produced core tissue with classic histologic features of a brown tumor. Postoperative histology revealed an exclusively oncocytic parathyroid adenoma. Atypical radiotracer uptake of this rare functioning adenoma subtype is illustrated with discussion of improved procedural diagnostic yield utilizing PET/CT.