Iodine-131 (131I) uptake in bilateral ovarian teratomas of a papillary thyroid carcinoma patient
Letter to the Editor

Iodine-131 (131I) uptake in bilateral ovarian teratomas of a papillary thyroid carcinoma patient

Xiaojing Ren1,2#, Ding Nan1,2#, Zairong Gao1,2, Xiao Zhang1,2

1Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China; 2Hubei Key Laboratory of Molecular Imaging, Wuhan, China

#These authors contributed equally to this work.

Correspondence to: Zairong Gao, MD, PhD; Xiao Zhang, MD, PhD. Department of Nuclear Medicine, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, No. 1277 Jiefang Ave., Wuhan 430022, China; Hubei Key Laboratory of Molecular Imaging, Wuhan, China. Email: gaobonn@163.com; zhangxiao199204@foxmail.com.

Submitted Apr 01, 2025. Accepted for publication Jul 04, 2025. Published online Aug 11, 2025.

doi: 10.21037/qims-2025-797

Introduction

Differentiated thyroid carcinoma (DTC), including papillary and follicular thyroid carcinoma, is the most common type of thyroid cancer. DTC cells typically retain the iodine-absorbing capability of thyroid follicular cells, making radioactive iodine-131 (131I) therapy an effective treatment modality (1). By internalizing 131I, the DNA of DTC cells is disrupted, leading to cellular death. Radioactive iodine therapy is commonly administered after thyroidectomy to eliminate residual thyroid tissue and microscopic metastatic foci, thereby reducing disease recurrence. This targeted approach selectively destroys cancer cells while minimizing damage to surrounding healthy tissues (2).

In addition to its therapeutic role, 131I emits gamma rays, which can be detected using single-photon emission computed tomography/computed tomography (SPECT/CT). SPECT/CT integrates functional imaging from SPECT with anatomical imaging from CT, providing detailed insights into lesion localization and biological behavior. This fusion imaging technology enhances diagnostic accuracy, particularly in cases where conventional CT or SPECT alone is inconclusive (3).

Here, we present a case of unexpected bilateral 131I uptake in the pelvic cavity, detected during a postoperative whole-body scan (WBS) following radioactive iodine therapy for papillary thyroid carcinoma (PTC).

Case presentation

A 33-year-old female was incidentally found to have a thyroid nodule during a routine physical examination. She was entirely asymptomatic with normal thyroid function, and there was no significant family history of thyroid cancer or prior exposure to ionizing radiation. Subsequent thyroid ultrasound revealed a 1.9-cm diameter thyroid isthmus nodule with calcification, outward protrusion, and vascular encasement. Fine-needle aspiration confirmed PTC, classified as Bethesda Category VI (Malignant), based on the Bethesda System for Reporting Thyroid Cytopathology (4). One month later, she underwent total thyroidectomy for PTC. Histopathological examination confirmed classic PTC with extrathyroidal extension and metastasis in 2 of 3 resected right central lymph nodes, staged as T3bN1aM0 (8th Edition American Joint Committee on Cancer Tumor-Node-Metastasis Staging) (5). The patient was classified as intermediate-risk per American Thyroid Association guidelines (6). Nine months postoperatively, after withholding levothyroxine for 3 weeks, the patient’s stimulated thyroglobulin level was measured at 17.60 IU/mL. She subsequently received oral 131I therapy (150 mCi), followed by a post-therapy WBS 3 days later.

Post-treatment WBS demonstrated intense uptake at the thyroid bed (Figure 1, blue arrows), indicating residual thyroid tissue. Notably, focal 131I uptake was observed in the pelvic region, prompting further evaluation with pelvic SPECT/CT imaging. Two well-defined masses were identified in the bilateral ovarian regions (right side: 5.4 cm × 3.4 cm × 3.4 cm; left side: 3.9 cm × 3.6 cm × 3.7 cm) with avid 131I uptake (Figure 1, red arrows). The masses exhibited fat and calcified densities, consistent with typical teratoma features. The patient subsequently underwent bilateral ovarian lesions resection, and pathological examination confirmed as mature cystic teratomas.

Figure 1 Post-therapy 131I whole-body scan and fused SPECT/CT showing physiological thyroid-bed uptake and bilateral ovarian teratomas with focal 131I uptake. (A,B) Anterior and posterior images of the post-therapy 131I WBS. Blue arrows: focal uptake in the thyroid bed. Red arrows: focal uptake in the pelvic cavity (bilateral ovarian teratomas). Black arrows: urinary contamination. Yellow arrows: physiologic uterine uptake. (C) Transversal (axial) SPECT/CT imaging revealing 131I uptake in the bilateral ovarian regions (red arrows). (D) Transversal (axial) SPECT/CT imaging demonstrating 131I uptake in the uterine physiological uptake (yellow arrows) and areas of urine contamination. (E) H&E staining (original magnification ×40): cystic lesion with characteristic components including sebaceous material, hair shafts, and osseous tissue, with focal neuroglial differentiation. 131I, iodine-131; SPECT/CT, single-photon emission computed tomography/computed tomography; WBS, whole-body scan; H&E, hematoxylin and eosin.

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Discussion

As far as we know, this is the first case that reported abnormal 131I uptake in bilateral ovaries due to mature teratomas containing ectopic thyroid tissue. SPECT/CT identified two ovarian masses with iodine uptake, ultimately diagnosed as mature ovarian teratomas containing ectopic thyroid tissue. This case highlights the importance of differentiating abnormal iodine uptake in the pelvic region from common causes such as benign lesions or physiological retention, and emphasizes the role of multimodal imaging in providing accurate diagnosis and preventing misinterpretation as metastatic thyroid cancer.

Abnormal pelvic 131I accumulation requires careful differential diagnosis (Table 1). The most common causes of pelvic iodine uptake are physiological retention and benign lesions. Physiological retention can occur when radioactive iodine metabolites accumulate in the bladder or intestines, leading to localized uptake in the pelvic region (14). Furthermore, in women of reproductive age, hormonal fluctuations can cause increased blood flow to the ovaries and endometrium, resulting in slight iodine uptake during certain stages of the menstrual cycle (15). During pregnancy, the fetus and placenta may also exhibit radioactive iodine uptake, particularly in the late stages when the fetal thyroid begins to take up iodine. Benign lesions such as uterine fibroids (16), functional ovarian cysts (7), and pelvic inflammatory diseases are common causes of pelvic iodine uptake. In rare cases, malignant diseases (like neuroendocrine tumors), inflammatory bowel diseases, or autoimmune diseases may alter the distribution of iodine (17,18). Additionally, medications or contrast agents may interfere with iodine metabolism, leading to abnormal iodine distribution. Therefore, differential diagnosis is essential to avoid misdiagnosis and unnecessary examinations or treatments when interpreting abnormal pelvic radioactive iodine uptake on post-treatment scans (19).

Table 1

Causes and imaging characteristics of pelvic radioactive iodine uptake

Uptake site Etiology Uptake mechanism Auxiliary imaging findings
Intestine – Physiological uptake –
Bladder – Physiological retention –
Uterus Uterine leiomyoma Unclear CT: non-enhanced scans show density similar to the uterus, which may decrease with fibroid degeneration. Enhanced scans demonstrate variable enhancement
MRI: compared to the uterine myometrium, it shows slightly low signal intensity on both T1WI and T2WI, with heterogeneous enhancement on contrast scans
Ultrasound: abnormal echoic mass, mostly hypoechoic, with larger myomas exhibiting posterior acoustic shadowing; well-demarcated from the normal myometrium
Ovary Ovarian cyst Passive diffusion of iodine or inflammation-induced uptake (7) CT: round or oval homogeneous water-like density shadows
MRI: uniform T1WI hypointensity, T2WI hyperintensity, with no enhancement
Ultrasound: an anechoic mass with posterior echo enhancement and typically no blood flow signals
Endometrial cyst Passive diffusion or partial active transport (8) Ultrasound: irregular thick-walled mass with echogenic foci that shift during menstruation and body movement; minimal blood flow signals on the capsule wall, no blood flow signals within the cyst
Mature teratoma NIS expression in thyroid tissue (9) CT: mostly round or oval, with characteristic fat density shadows, calcifications, teeth, etc.
Ultrasound: heterogeneous mass with hyperechoic calcifications. Features include the “dough, wall-standing nodule, complex structure, lipid-fluid level and starburst” signs
Struma ovarii NIS expression in thyroid tissue (10) CT: high-density cystic lesions, with calcification of cyst wall and septa, and soft tissue density inside
MRI: the viscous colloidal material inside the cyst shows very low signal on T2WI, and the solid components enhance significantly on enhanced scanning
Ultrasound: one or more well-defined round hyperechoic masses within a cystic area, with detectable blood flow signals inside. The floating punctate strong echoes within the cyst with comet-tail signs
Ovarian endometrioma Passive diffusion of iodine or inflammation-induced uptake (11) CT: round or oval homogeneous low-density shadows, occasionally with hyperdense foci due to hemorrhage
MRI: classic T1WI hyperintensity due to hemorrhagic content, T2 hypointense “shading sign” and no post-contrast enhancement
Ultrasound: hypoechoic mass with homogeneous “ground-glass” echoes, posterior acoustic enhancement, and absent vascularity on Doppler
Pelvic Serous cystadenoma NIS expression and fluid retention Ultrasound: the mass appears as anechoic or sparsely echoic, with fine punctate echoes within areas of hemorrhage if present, and is often unilocular
Mucinous cystadenoma NIS expression and fluid retention (12) Ultrasound: most often appearing as cloudy or sparsely hypoechoic masses, multilocular
Cystadenofibroma NIS expression and passive adsorption of fibrous tissue (13) CT: round or oval-shaped isodense or hypodense lesions, with visible septations within
MRI: markedly hypointense on T2WI with mild progressive enhancement post-contrast
Dermoid cyst NIS expression in thyroid tissue (9) CT: presents as a round low-density lesion with sharp margins, no visible cyst wall, and no enhancement effect
Neuroendocrine tumor NIS expression CT: isodense or slightly hypodense lesion, with uniform or heterogeneous density. Calcification, cystic change and necrosis are rare. Significant arterial phase enhancement is noted post-contrast
MRI: high T2WI signals, restricted diffusion and marked enhancement on contrast scans
Ultrasound: hypoechoic with uniform echotexture well-defined margins, and a relatively rich blood flow signal within the tumor

CT, computed tomography; MRI, magnetic resonance imaging; NIS, sodium/iodide symporter; T1WI, T1-weighted imaging; T2WI, T2-weighted imaging.

Mature ovarian teratomas are germ cell tumors characterized by differentiated tissues such as skin, hair, teeth, and sebaceous material, without immature embryonic components. They account for approximately 20% of all ovarian tumors and can occur at any age, though they are more common in women of reproductive age (20). Notably, if thyroid tissue is present within a mature teratoma, it can retain the capability to take up iodine, which may contribute to pelvic iodine uptake. One such rare condition is struma ovarii, a specialized ovarian teratoma containing >50% thyroid tissue, which represents <5% of all ovarian teratomas (21).

In this case, the pelvic radioactive iodine uptake was suspected due to thyroid tissue within the teratoma, which retained the normal thyroid’s ability to absorb iodine, forming a focus of radioactive iodine accumulation (20). Moreover, the thyroid-like tissue in the teratoma can produce thyroid hormones (thyroxine and free triiodothyronine) and respond to thyroid-stimulating hormone (22). Therefore, in some cases, this tissue may secrete excessive thyroid hormones, causing hyperthyroid symptoms (23). These tissues can resemble normal thyroid tissue on imaging, displaying characteristic signals on ultrasound, CT, or magnetic resonance imaging (MRI). Ectopic thyroid tissues, like those within teratomas, have many clinical implications. It may affect thyroid function, and during radioactive iodine scans for thyroid cancer, it may be mistaken for metastatic thyroid cancer (24). Although rare, ectopic thyroid tissue can also undergo malignant transformation, leading to conditions such as malignant struma ovarii or DTC within the teratoma. In such cases, 131I can be used not only for diagnostic purposes but also to indicate possible therapeutic options, as the ectopic thyroid tissue can respond to radioactive iodine therapy (21).

SPECT/CT following 131I therapy improves the diagnostic accuracy of WBS and plays a key role in the postoperative management of thyroid cancer (25). Multimodal imaging techniques such as SPECT/CT are increasingly vital for improving diagnostic precision and guiding clinical decisions (26). In this case, SPECT/CT not only identified ovarian lesions but also distinguished them from PTC metastases, demonstrating its value in resolving ambiguous pelvic iodine uptake. While technological advancements promise greater diagnostic accuracy, further studies are necessary to validate the broader clinical benefits of such integrated approaches.

Acknowledgments

None.

Footnote

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-797/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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Cite this article as: Ren X, Nan D, Gao Z, Zhang X. Iodine-131 (131I) uptake in bilateral ovarian teratomas of a papillary thyroid carcinoma patient. Quant Imaging Med Surg 2025;15(9):8668-8673. doi: 10.21037/qims-2025-797

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