TSH Stimulation Reveals Additional Bone Metastases on [18F]FDG PET/CT in Metastatic Follicular Thyroid Carcinoma

A 62-year-old woman with metastatic follicular thyroid carcinoma underwent 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET/CT before a second course of radioactive iodine therapy because of a markedly elevated serum thyroglobulin level (498.0 ng/mL). The initial PET/CT was performed under thyroid-stimulating hormone (TSH) suppression (0.01 μIU/mL) and demonstrated FDG uptake in known osseous metastases involving the T10 and L5 vertebrae, while additional skeletal metastases were not conspicuous. Repeat PET/CT performed 6 days later after recombinant human TSH (rhTSH) stimulation (TSH, 70.96 μIU/mL) showed substantially increased FDG uptake in the known T10 and L5 lesions, with SUVmax increasing from 3.33 to 6.31 and from 3.98 to 11.61, respectively. In addition, metastases in the T9 vertebra and sternum became conspicuous only after TSH stimulation and remained evident on subsequent follow-up imaging, supporting their metastatic nature. This paired within-patient comparison directly demonstrates that, in this patient, osseous lesions inconspicuous under TSH suppression became clearly visualized after rhTSH stimulation and persisted on follow-up imaging, illustrating the potential influence of TSH status on the apparent skeletal metastatic burden on [18F]FDG PET/CT.