What is struma ovarii?

What is Struma Ovarii?

Struma ovarii is a rare monodermal ovarian teratoma that contains more than 50% thyroid tissue, accounting for 3% of all ovarian teratomas and typically presenting as an incidental finding in postmenopausal women. 1

Definition and Epidemiology

• Struma ovarii is a specialized variant of mature cystic teratoma (dermoid cyst) where thyroid tissue comprises >50% of the tumor mass 1
• These tumors are of endodermal origin with thyroid or C-cell differentiation, arising within teratomas 1
• They represent approximately 3% of all ovarian teratomas 1
• While mature cystic teratomas are common (accounting for up to 20% of ovarian tumors), only about 15% contain any thyroid tissue 1

Clinical Presentation

• Most cases are diagnosed incidentally during surgery, as preoperative diagnosis is rarely achieved 1
• The incidence peaks postmenopausally, with most diagnoses occurring in the fifth and sixth decades of life 1
• Patients typically present with subacute pelvic pain, pelvic pressure from a mass, or menstrual irregularities 1
• Struma ovarii is usually unilateral and more commonly arises in the left ovary 1
• Clinical hyperthyroidism occurs in only approximately 8% of cases, despite the presence of functional thyroid tissue 2, 3

Malignant Potential

• The incidence of malignant transformation is uncommon, estimated at 0.1% to 0.3% 1
• When malignancy occurs, papillary thyroid carcinoma is the most common histological type 4, 5, 3
• Metastases are very rare, occurring in probably <5% of cases 1
• An important diagnostic pitfall: metastatic thyroid carcinoma from the thyroid gland to the ovary must be excluded in all cases, though this is extremely rare 1

Diagnostic Workup

• Diagnostic evaluation should include pelvic ultrasound and abdomino-pelvic CT scan 1
• Imaging findings are generally nonspecific, making definitive preoperative diagnosis difficult 4
• A rare exception is when abnormal thyroid blood tests (elevated thyroglobulin) are found with an ovarian teratoma, especially when no thyroid gland pathology coexists 4
• Thyroid scintigraphy in hyperthyroid cases shows low or absent radioiodine uptake in the thyroid gland but uptake in the pelvis on whole body scanning 2

Surgical Management

For postmenopausal women or those who have completed childbearing, hysterectomy and bilateral salpingo-oophorectomy is advised when struma ovarii is diagnosed unexpectedly 1

• Conservative surgery (unilateral oophorectomy or cystectomy) may be considered in younger women with no extra-capsular spread and no associated mature cystic teratoma 1
• Some experts recommend completion surgery when childbearing is complete, even after initial conservative management 1
• In cases with obvious spread, more aggressive surgery must be pursued 1
• Surgical excision is the mainstay of treatment for both benign and malignant forms 4, 6

Management of Malignant Struma Ovarii

For malignant struma ovarii, postoperative care should involve referral for discussion of total thyroidectomy and management as for differentiated thyroid carcinoma 1

• Thyroidectomy is an essential prerequisite to radio-iodine imaging and ablation, as any administered radio-iodine will be taken up preferentially by the thyroid gland rather than strumal tissue 1
• Whole body imaging with radio-iodine is performed to search for functioning tissue and destroy residual thyroid tissue, allowing thyroglobulin to be used as a tumor marker 1
• While some argue this approach may be excessive, series reporting this as standard of care demonstrate the best outcomes 1

Follow-up Protocol

Prognosis is usually very good if treated optimally, with lifelong follow-up recommended 1

• Clinical examination and thyroid hormone replacement in doses that fully suppress TSH is required 1
• Serial measurement of thyroglobulin has replaced whole body radio-iodine scintigraphy for surveillance 1
• Thyroglobulin levels should be undetectable; rising levels or development of new thyroglobulin antibodies warrants investigation for possible relapse 1, 4
• PET/CT scanning has no reported value to date 1
• Follow-up is lifelong, principally to monitor thyroid function and thyroglobulin levels 1

Important Clinical Pitfalls

• In patients with pre-existing Hashimoto's thyroiditis on levothyroxine, struma ovarii can mask underlying hypothyroidism by producing autonomous thyroid hormone 2
• After resection, severe hypothyroidism may develop, requiring immediate levothyroxine replacement 2
• The co-existence of struma ovarii with other thyroid pathology creates diagnostic complexity requiring careful interpretation of thyroid function tests 2