Solitary Flat Bone Metastasis of Follicular Thyroid Carcinoma: Complete Molecular and Partial Structural Response After 2 ¹³¹ I Therapies

We report a case of follicular thyroid carcinoma presenting with a solitary skeletal metastasis in right iliac bone that remained structurally persistent after 2 high-dose ¹³¹I treatments. However, the lesion showed no uptake on [¹⁸F]FDG PET/CT, ¹³¹I scintigraphy, or [⁶⁸Ga]Ga-FAPI PET/CT, suggesting the absence of metabolic or receptor activity. This discordance between anatomic persistence and lack of molecular expression highlights the potential of molecular imaging to detect early treatment response, even when the lesion is structurally persistent. This case highlights the importance of integrating molecular imaging into treatment response evaluation to better assess disease activity and guide further management in differentiated thyroid carcinoma.