A Rare Case of Struma Ovarii Presenting as an Abdominal Mass in a 15-year-old Girl

ABSTRACT

Struma ovarii is an uncommon monodermal ovarian teratoma. Symptoms are often nonspecific, which may lead to misdiagnosis. We report a rare case of a 15-year-old girl who presented with a painless left abdominal mass. Imaging revealed a sizable ovarian lesion, which was then resected via a lower midline laparotomy. This case highlights the diagnostic challenge of distinguishing struma ovarii from other ovarian neoplasms in adolescents. We also describe our surgical approach and management considerations.

INTRODUCTION

Teratomas are germ cell tumors composed of derivatives from one or more germ cell layers. Ovarian teratomas are classified histologically into mature, immature, or monodermal teratomas. Struma ovarii is a monodermal ovarian teratoma that predominantly comprises thyroid tissue. It is a rare tumor that predominantly affects women of reproductive age. However, it can sometimes present in adolescents. Reports in adolescents are limited, highlighting its rarity in this age group.

CASE REPORT

A 15-year-old female presented with a 3-month history of a palpable mass in her left lower abdomen, with constipation and urinary incontinence. An abdominal mass of 10 cm was palpated in the left lower quadrant, extending to the midline. It was firm and nontender, with no overlying skin changes. An ultrasound of the pelvis showed a multilocular cystic lesion measuring 18.9 cm × 13.4 cm × 9.4 cm. It contained low-level internal echoes, with smooth inner walls and thickened separations measuring 5 mm. No septal or interval vascularity was seen. The mass effect also resulted in mild left hydronephrosis. Tumor markers and blood tests were within normal limits. The patient was also clinically euthyroid. Hence, thyroid function tests were not performed preoperatively as struma ovarii was not suspected based on initial evaluation. Based on imaging, the differential diagnosis included mature cystic teratoma, serous or mucinous cystadenoma, and, less likely, a malignant ovarian neoplasm. The patient subsequently underwent a laparoscopic-assisted left ovarian cystectomy, in view of suspicions of an ovarian tumor.

Intraoperatively, a large cystic ovarian mass was identified with no clear delineation between the cyst and the surrounding ovarian tissue. Due to poor anatomical demarcation, conversion to a lower midline laparotomy was performed. This approach was chosen due to the size of the mass and the need to preserve adjacent structures, including the fallopian tube and viable ovarian tissue. Given the patient’s young age and fertility potential, ovarian preservation was prioritized. The cystic ovarian mass, omental nodules, and serous fluid were sent for histological analysis. A total of approximately 1200 mL of serous fluid was aspirated during decompression of the cyst to facilitate handling and improve surgical visualization. This resulted in the partial collapse of the cyst, altering its gross intraoperative dimensions.

Histological examination demonstrated that the lesion was predominantly composed of thyroid tissue arranged in macrofollicles, lined by bland cuboidal to flattened epithelium with abundant colloid, consistent with struma ovarii. A focal area of keratin-filled cysts was present, in keeping with mature teratoma elements. No evidence of malignancy or secondary germ cell components was observed.

The postoperative course was uneventful, and the patient remained well at subsequent follow-ups.

DISCUSSION

Struma ovarii is a rare tumor, making up only 2.7% of ovarian teratomas and 1% of all ovarian tumors.[1] It occurs mostly in women between the ages of 30 and 50 years, typically presenting before menopause. Pediatric presentations, though rare, have been reported in the literature.

Most cases of struma ovarii present asymptomatically. Occasionally, patients present with abdominal distention due to ascites. Despite an abundance of thyroid tissue in these tumors, only 5%–8% are associated with clinical hyperthyroidism.[1] Yoo et al., in 2008, reported that 41.2% of cases were discovered on imaging. Although thyroid function testing can aid in identifying functional struma ovarii, it is not routinely performed in patients without clinical features of thyrotoxicosis. In this case, the absence of suggestive symptoms and low preoperative suspicion for struma ovarii resulted in the decision not to carry out endocrine evaluation.

The diagnosis of struma ovarii can be challenging, as there is no definitive laboratory or imaging modality. Tumor markers are typically within normal limits, and imaging findings are often nonspecific with no pathognomonic features. In the pediatric population, ultrasound remains the primary imaging modality for ovarian masses due to its accessibility, high diagnostic yield, and lack of radiation exposure. Computed tomography (CT) has a limited role and is generally reserved for cases where complications or malignancy are suspected, or when the diagnosis remains unclear following initial imaging.[2] In our patient, CT was not performed as the lesion was adequately characterized on ultrasound, and to minimize unnecessary radiation exposure in an adolescent.

The gold standard for the diagnosis of struma ovarii is the histopathological analysis of the resected mass, showing that thyroid tissue occupies over 50% of the tumor.[3] Compared to previously reported adolescent cases, our patient presented with urinary symptoms, which is less commonly described. The absence of hyperthyroidism despite predominant thyroid tissue reinforces that biochemical euthyroidism does not exclude struma ovarii. This highlights the difficulty of preoperative diagnosis, and supports the role of histopathology as the definitive diagnostic modality.

In some cases, images of struma ovarii may be mistaken for a malignant tumor due to increased vascularity and fibrosis. The apparent discrepancy between imaging and intraoperative tumor size is not uncommon in large cystic ovarian masses. This is typically attributable to intraoperative decompression and collapse of cystic components, resulting in differences in measurement planes between imaging and surgical assessment. In our case, the aspiration of approximately 1200 mL of serous fluid resulted in significant cyst decompression, explaining the smaller intraoperative appearance.

Surgical management depends on tumor size and fertility considerations. Laparoscopic surgery is preferred in young patients due to reduced morbidity and faster recovery.[4] However, larger masses may require conversion to laparotomy, as in our case.

Radical surgery is the other option for the removal of struma ovarii. This consists of total abdominal hysterectomy, bilateral salpingo-oophorectomy, peritoneal lymphadenectomy, thyroidectomy along with radio-ablation with 131I. This option should only be reserved for patients with advanced disease not amenable to laparoscopic surgery, and/or if they are postmenopausal patients.

Overall, struma ovarii has a very good prognosis. Rare complications are seen in only 5% of patients, and they include torsion, rupture, infection, and malignancy.[4] Another rare but possible complication is thyrotoxicosis, which occurs in about 8% of patients.[5] However, routine thyroid testing is not universally recommended unless clinically indicated.[5] Clinical presentation should therefore guide the need for thyroid testing. In our case, thyroid function tests were not performed as the patient was clinically euthyroid and there was low preoperative suspicion. While this limits endocrine correlation, the diagnosis was confirmed based on standard histological criteria and clinical course.

CONCLUSION

Struma ovarii should be considered in the differential diagnosis of large ovarian masses in adolescents, particularly when tumor markers are normal, and imaging findings are nonspecific. Preoperative diagnosis remains challenging, and definitive diagnosis relies on histopathology. This case highlights the importance of fertility-preserving surgical strategies and careful intraoperative decision-making in young patients.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given her consent for her images and other clinical information to be reported in the journal. The patient understands that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.