SUMMARY Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer. On the contrary, primary thyroid lymphoma (PTL) is a rare disease, accounting for 2% to 5% of all thyroid malignancies. Despite several cases in which both PTC and PTL arise in the setting of Hashimoto's thyroiditis (HT), the coexistence of both tumors in HT patients is very rare. Herein we report the case of a 66-year-old woman with long-standing nodular HT under replacement therapy, who presented with a fast, painless enlargement in the right anterior side of the neck. Thyroid ultrasonography demonstrated increased growth of a hypoechoic nodule in the right lobe measuring 32 × 20 mm. A total thyroidectomy was performed, and histology revealed a diffuse large B-cell lymphoma (DLBCL) on a background of florid HT. Moreover, a unifocal papillary microcarcinoma, classical variant (7 mm, pT1aNxMx), was discovered. The patient was then treated with chemotherapy for the PTL, but she did not undergo radioactive iodine ablation treatment for the microPTC as per guidelines. Two years after surgery, the patient had no evidence of recurrence of either malignancy. This rare case highlights the importance of monitoring HT patients with nodular lesions, especially if they have long-standing disease. In addition, PTL should be considered for differential diagnosis in elder HT patients who present with sudden thyroid enlargement.

In patients affected by differentiated thyroid cancer, the whole-body scan (WBS) with 131-radioiodine, especially when performed after a therapeutic activity of131I, represents a sensitive procedure for detecting thyroid remnant and/or metastatic disease. Nevertheless, a wide spectrum of potentially pitfalls has been reported. Herein we describe a 63-year-old woman affected by follicular thyroid cancer, who was accidentally found to have an abdominal mass at post-dose WBS (pWBS). pWBS showed abnormal radioiodine uptake in the upper mediastinum, consistent with lymph-node metastases, and a slight radioiodine uptake in an abdominal focal area. Computed tomography revealed an inhomogeneous mass in the pelvis, previously unrecognized. The lesion, surgically removed, was found to be a typical dermoid cyst of the ovary, without any evidence of thyroid tissue. By immunohistochemistry, a moderate expression of the sodium-iodine symporter (NIS) was demonstrated in the epithelial cells, suggesting a NIS-dependent uptake of radioiodine by the cyst.