Abstract:

The clinical data of a patient with differentiated thyroid carcinoma （DTC） who developed physiological thymic uptake after postoperative iodine-131 （¹³¹I） therapy were analyzed， and the 3-year follow-up changes in the patient’s condition were reviewed. Combined with the literatures and the diagnosis and treatment process， the causes of possible false positives in whole-body scans after iodine therapy for DTC and the mechanism， clinical features， and identification methods of benign thymic ¹³¹I uptake were discussed to improve clinicians’ understanding and diagnostic ability regarding such conditions and avoid unnecessary multiple iodine treatments. The patient， a 28-year-old female， showed mediastinal imaging after the first ¹³¹I treatment， with more pronounced mediastinal iodine uptake during the second treatment. SPECT/CT localized the uptake to enlarged thymus tissue. The stimulated thyroglobulin （Tg） levels before two ¹³¹I treatments were high but gradually decreased. Apart from thymic uptake， no other examination evidence suggested DTC metastases. Subsequent follow-up for 3 years showed no pathological changes in the thymus， confirming physiological thymic uptake. Thymic ¹³¹I uptake is a common cause of false-positive whole-body scans in post-thyroidectomy patients. When post-¹³¹I therapy whole-body imaging shows only mediastinal uptake， especially in the young patients undergoing multiple ¹³¹I treatments where thymic ¹³¹I uptake intensity increases with successive treatments， even with elevated Tg levels， comprehensive use of imaging results such as SPECT/CT is essential to determine if it is normal thymus， thereby avoiding unnecessary repeated therapies.

Key words: Differentiated thyroid carcinoma, Iodine-131 therapy, Thymus, Physiological uptake, Case report