SUN-369 - A Delayed Sequela: Papillary Thyroid Carcinoma Years after Radioactive Iodine Therapy for Graves' Disease

Background: Radioactive iodine (RAI) therapy is a widely used definitive treatment for Graves’ disease and has historically been considered safe. However, large cohort studies and meta-analyses suggest a modest, dose-dependent association between RAI exposure and solid cancer mortality, including thyroid and breast cancers, raising renewed concern regarding long-term malignancy risk.

Case: A 73-year-old male was treated for Graves’ disease in 2009 with radioactive iodine ablation (I-131, 17.2 mCi). At the time of diagnosis and treatment, there were no known thyroid nodules and no evidence of thyroid malignancy. He subsequently developed permanent hypothyroidism and remained on long-term levothyroxine therapy. Sixteen years later, imaging revealed an enlarging right thyroid nodule. Ultrasound-guided fine-needle aspiration demonstrated malignant cytology (Bethesda VI), consistent with papillary thyroid carcinoma (PTC). Preoperative imaging showed a 2–2.9 cm right thyroid nodule without pathologic cervical lymphadenopathy.

The patient underwent total thyroidectomy with neck dissection. Pathology revealed multifocal classic-type PTC measuring 2.8 cm with lympho-vascular invasion, focal extrathyroidal extension, positive posterior margins, and metastatic involvement of two right lateral neck lymph nodes with extranodal extension. An incidental microscopic noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) was identified in the contralateral lobe. Final staging was pT2N1b (AJCC 8th edition). He subsequently received adjuvant radioactive iodine therapy (I-131, 144.6 mCi) following recombinant TSH stimulation, with post-therapy imaging showing uptake confined to the thyroid bed.

Conclusion: This case illustrates the development of differentiated thyroid carcinoma more than a decade after low-dose RAI therapy for Graves’ disease. While causality cannot be established, emerging epidemiologic evidence supports a dose-response relationship between RAI exposure and solid cancer mortality. This case underscores the importance of individualized risk–benefit discussions, patient counseling regarding long-term risks, and consideration of ongoing thyroid surveillance in selected patients treated with RAI for benign disease.

*Unless otherwise noted, all abstracts presented at ENDO must not be released to the press or the public until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.*