Struma ovarii

by Jason Wasserman MD PhD FRCPC
January 3, 2025

Struma ovarii is a type of teratoma, a noncancerous tumour in the ovary. The term teratoma is used to describe a tumour made up of germ cells – specialized cells in the ovary that have the potential to form many different types of tissue. While most teratomas contain a mixture of tissues, such as hair, skin, or even bone, struma ovarii is unique because it is made up primarily or entirely of thyroid tissue.

What are the symptoms of struma ovarii?

Most women with struma ovarii have no symptoms, and the tumour is found by chance during imaging or surgery for another condition.

When symptoms do occur, they may include:

• A lump or swelling in the lower abdomen.
• Abdominal discomfort or pain.
• Symptoms of hyperthyroidism, such as weight loss, sweating, or a rapid heartbeat, if the tumour produces thyroid hormones.

What causes struma ovarii?

The exact cause of struma ovarii is not well understood. It is believed to develop from germ cells, which are specialized cells in the ovary that have the potential to form different types of tissue, including thyroid tissue. This process occurs as part of a group of tumours called teratomas.

What conditions are associated with struma ovarii?

Struma ovarii is associated with conditions related to thyroid function. In rare cases, the tumour can cause hyperthyroidism, a condition where too much thyroid hormone is produced. This is known as hyperfunctioning struma ovarii. Struma ovarii can also occasionally be associated with thyroid cancer, particularly papillary thyroid carcinoma, when malignant transformation occurs within the tumour.

How is this diagnosis made?

The diagnosis of struma ovarii is usually made after the tumour is surgically removed and examined under a microscope by a pathologist. Imaging studies, such as ultrasound or CT scans, may suggest the presence of a tumour in the ovary, but the diagnosis requires microscopic examination. In some cases, blood tests may show elevated thyroid hormone levels if the tumour is producing thyroid hormones.

What does struma ovarii look like under the microscope?

Under the microscope, struma ovarii is made up of thyroid tissue, which looks similar to the thyroid gland found in the neck. This tissue often forms small circular structures called follicles, which are filled with a material called colloid. If cancer is present, the tumour may show features of papillary thyroid carcinoma, including complex branching structures and overlapping nuclei.

Immunohistochemistry

Immunohistochemistry is a special test that pathologists use to confirm the diagnosis of struma ovarii.

Typical immunohistochemical features of struma ovarii include:

• Positive staining for thyroid markers such as thyroglobulin and TTF-1 (thyroid transcription factor-1).
• Negative staining for markers that rule out other types of ovarian tumours, such as inhibin and calretinin, which are markers of sex cord-stromal tumours.

These results help confirm that the tumour is made up of thyroid tissue.

What is the prognosis for a person diagnosed with a struma ovarii?

The prognosis for struma ovarii is generally excellent. Most tumours are non-cancerous and can be completely cured with surgical removal. Even in cases where cancer is found, the outcome is usually favourable, especially if the tumour is detected early and treated promptly. Regular follow-up with your doctor is important to monitor for any potential recurrence or related thyroid issues.