by Jason Wasserman MD PhD FRCPC
April 20, 2026
Struma ovarii is a specialized type of mature teratoma of the ovary in which thyroid tissue makes up the predominant or only component of the tumor. The word “struma” is a Latin term that has historically referred to the thyroid gland. Most cases of struma ovarii are noncancerous and behave in a completely benign way. Rarely, a cancer — most commonly a papillary thyroid carcinoma — can develop within the thyroid tissue inside the tumor, a situation called malignant struma ovarii. This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.
What are the symptoms?
Most people with struma ovarii have no symptoms at all, and the tumor is found by chance during imaging or surgery performed for another reason. When symptoms do occur, they typically relate to the size of the ovarian mass and may include abdominal or pelvic discomfort, pain, swelling, or pressure in the lower abdomen.
In a minority of cases, the thyroid tissue within the tumor produces thyroid hormones — the same hormones normally made by the thyroid gland in the neck — in excess amounts. This is called hyperfunctioning struma ovarii. The symptoms of excess thyroid hormone (hyperthyroidism) include unexplained weight loss, a rapid or irregular heartbeat, sweating, feeling too warm, tremors, or anxiety. If you experienced any of these symptoms before your diagnosis, let your medical team know, as thyroid hormone levels in the blood can be checked to determine whether the tumor was producing hormones.
What causes struma ovarii?
Struma ovarii develops from germ cells — specialized cells in the ovary that normally develop into eggs and have the unusual ability to produce many different tissue types. In a mature teratoma, this gives rise to a mixture of tissues including skin, hair, fat, and others. In struma ovarii, the thyroid tissue component has grown to dominate or entirely replace the other tissue types. The reason this happens in some teratomas but not others is not well understood. Struma ovarii is not associated with any known hereditary cancer syndromes, BRCA mutations, or Lynch syndrome.
How is the diagnosis made?
The diagnosis is made after the tumor is surgically removed and examined under the microscope by a pathologist. Imaging studies — such as ultrasound, CT, or MRI — can detect an ovarian mass but cannot confirm the presence of thyroid tissue. In some cases, imaging and blood tests showing elevated thyroid hormones raise suspicion before surgery, but the definitive diagnosis always requires microscopic examination.
Under the microscope, struma ovarii is composed of thyroid tissue that appears essentially identical to normal thyroid tissue from the neck. The tissue forms small circular structures called follicles — the same structures that make up the normal thyroid gland — which are filled with a material called colloid. Colloid is a gel-like substance that stores the building blocks of thyroid hormones inside the follicle. The cells lining the follicles are uniform, flat to cuboidal in shape, and look normal rather than abnormal.
To confirm the diagnosis and rule out other ovarian tumors that can appear similar, the pathologist uses immunohistochemistry (IHC) — a technique that uses antibodies to detect specific proteins in the cells. Struma ovarii stains positively for thyroglobulin — a protein produced exclusively by thyroid follicular cells — and for TTF-1 (thyroid transcription factor-1), a protein found in the nuclei of thyroid and lung cells. These positive results confirm that the tumor is made up of thyroid tissue. The tumor is typically negative for markers of sex cord-stromal tumors (such as inhibin and calretinin), which helps exclude other ovarian tumor types that may enter the differential diagnosis.
A critical part of the pathologist’s examination is carefully evaluating the thyroid tissue for any features of malignancy — in particular, the nuclear features of papillary thyroid carcinoma. This is described in detail in the malignant struma ovarii section below.
Histologic grade
Struma ovarii is not assigned a histologic grade. Grading systems used for ovarian carcinomas are designed for epithelial cancers and do not apply to this tumor type. For benign struma ovarii, the tumor is noncancerous by definition, and no grade is appropriate. If malignant transformation is identified — meaning a cancer has developed within the thyroid tissue — the malignant component is evaluated and reported according to thyroid cancer criteria rather than ovarian cancer grading systems.
Malignant struma ovarii
In rare cases — estimated at approximately 5% of struma ovarii — a cancer develops within the thyroid tissue inside the tumor. This is called malignant struma ovarii. The most common type of cancer to arise is papillary thyroid carcinoma, the same common and generally favorable type of thyroid cancer that develops in the thyroid gland in the neck. Less commonly, follicular thyroid carcinoma or other types can occur.
The pathologist recognizes malignant transformation by identifying the characteristic nuclear features of papillary thyroid carcinoma within the follicular cells: the nuclei become enlarged, pale, and overlapping, with fine chromatin and folds or grooves along their surfaces — features that are distinct from the flat, uniform nuclei of normal thyroid follicles. In some cases, the malignant component forms complex branching structures called papillae. The diagnosis is supported by immunohistochemistry showing the same thyroid marker profile (thyroglobulin-positive, TTF-1-positive) as the benign component.
An important concept is that malignant struma ovarii is a form of thyroid cancer that happens to be located in the ovary rather than the neck. Its biology is essentially that of thyroid cancer rather than ovarian cancer. This means that the treatment approach, staging, and follow-up for malignant struma ovarii are guided by thyroid oncology principles rather than the ovarian cancer protocols used for most other ovarian malignancies.
The prognosis for malignant struma ovarii is generally favorable, reflecting the inherently good prognosis of well-differentiated thyroid carcinoma. Most patients with malignant struma ovarii do well after surgery, with or without additional treatment. However, accurate diagnosis is important because it changes management significantly compared to benign struma ovarii.
If malignant struma ovarii is identified in your pathology report, your care team will likely involve an endocrinologist or thyroid cancer specialist in addition to your gynecologic oncologist to help guide treatment decisions.
What is the prognosis?
The prognosis for struma ovarii depends on whether the tumor is benign or malignant:
- Benign struma ovarii — The prognosis is excellent. Complete surgical removal is curative in virtually all cases. Recurrence after complete removal is extremely rare. If hyperfunctioning struma ovarii was causing thyroid hormone excess, hormone levels typically normalize after the tumor is removed.
- Malignant struma ovarii (papillary or follicular thyroid carcinoma within the teratoma) — The prognosis is generally favorable, consistent with the excellent prognosis of well-differentiated thyroid carcinoma. Most patients are cured with surgery and, in selected cases, radioactive iodine therapy. Outcomes depend on the extent of disease, the specific type of thyroid carcinoma present, and whether spread beyond the ovary has occurred. Long-term follow-up with thyroid cancer monitoring is required.
What happens after the diagnosis?
Management depends entirely on whether the struma ovarii is benign or malignant.
For benign struma ovarii: No further treatment is needed after surgical removal. The surgical approach depends on the clinical situation and the patient’s age and reproductive plans — options include cystectomy (removal of the cyst while preserving the ovary) or oophorectomy (removal of the entire ovary). If hyperfunctioning struma ovarii was present, thyroid function tests should be checked after surgery to confirm that hormone levels have returned to normal. No specific follow-up imaging is required in the absence of symptoms or abnormal findings.
For malignant struma ovarii: Additional treatment is typically required and is guided by thyroid oncology principles. The standard approach for well-differentiated thyroid carcinoma of the ovary follows a similar pathway to thyroid cancer arising in the neck:
- Complete surgical removal of the ovarian tumor is the first step and has usually already been accomplished at the time of diagnosis.
- If the native thyroid gland in the neck is intact, removal of the thyroid (thyroidectomy) is often recommended to allow subsequent radioactive iodine therapy and to enable serum thyroglobulin monitoring — a blood test that serves as a tumor marker for thyroid cancer.
- Radioactive iodine (RAI) therapy — a treatment in which radioactive iodine is taken by mouth and selectively absorbed by any remaining thyroid tissue, including metastatic deposits — may be recommended depending on the stage and features of the malignancy. Because thyroid cells (including malignant ones) naturally absorb iodine, RAI is an effective tool for eliminating residual disease.
- Long-term surveillance includes serum thyroglobulin monitoring and imaging, similar to the follow-up used for thyroid cancer arising in the neck.
Your care team will include a gynecologic oncologist and, in cases of malignant struma ovarii, an endocrinologist or thyroid cancer specialist. Together, they will guide you through the appropriate management pathway for your specific situation.
Questions to ask your doctor
- Was my struma ovarii benign, or was any evidence of malignant transformation (cancer within the thyroid tissue) identified?
- Was the tumor completely removed, and was a cystectomy or oophorectomy performed?
- If malignant struma ovarii was found, what type of thyroid cancer is present — papillary, follicular, or another type?
- If malignant struma ovarii was found, has it spread beyond the ovary?
- Is the removal of the thyroid gland in my neck recommended as part of my treatment?
- Is radioactive iodine therapy recommended, and what does that involve?
- Should my thyroid hormone and thyroglobulin levels be checked after surgery?
- Were there any symptoms of excess thyroid hormone before my diagnosis, and have those resolved after surgery?
- What follow-up schedule do you recommend, and does it involve a thyroid cancer specialist?
- Do I need any further treatment, or is surgery alone sufficient?
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