Follicular adenoma of the thyroid gland

by Jason Wasserman MD PhD FRCPC
July 3, 2025

Follicular adenoma is a non-cancerous tumour of the thyroid gland. It is made up of follicular cells, the same type of cells that produce thyroid hormones. These tumours are usually surrounded by a thin layer of tissue called a capsule, which helps keep the tumour separated from the rest of the thyroid gland.

Follicular adenomas are classified as neoplasms, which means they are true tumours made up of cells that grow and multiply more than normal. However, unlike cancer, the cells in a follicular adenoma do not invade surrounding tissues or spread to other parts of the body.

Where does a follicular adenoma occur?

Follicular adenomas arise from the thyroid gland, which is located at the front of the neck. They usually appear as a solitary nodule (single lump) within the gland, although more than one may be present. Rarely, follicular adenomas may occur in thyroid tissue located in unusual areas, such as in a thyroglossal duct cyst, lingual thyroid, or in the ovary as part of a condition called struma ovarii.

Who gets follicular adenoma?

Follicular adenoma can occur at any age but is most common in adults between the ages of 45 and 55. It is more common in women than men. The tumour is usually found incidentally during a routine physical exam or imaging study such as an ultrasound or CT scan. Most people with follicular adenoma do not have symptoms, although large tumours may cause pressure in the neck, difficulty swallowing, or shortness of breath.

Most patients with follicular adenoma have normal thyroid hormone levels (a condition called euthyroid). Rarely, the tumour may produce excess thyroid hormone, leading to hyperthyroidism.

What causes follicular adenoma?

The exact cause of follicular adenoma is not always known. Most tumours are thought to occur sporadically, without a clear inherited cause.

However, some cases are linked to:

• Radiation exposure during childhood

• Iodine deficiency

• Genetic syndromes, such as PTEN hamartoma tumour syndrome (Cowden syndrome), DICER1 syndrome, and Carney complex.

If multiple follicular adenomas are found, especially in a young person or in combination with other findings, your doctor may recommend evaluation for a hereditary condition.

How is follicular adenoma diagnosed?

Most follicular adenomas are first found during ultrasound imaging of the thyroid. They often appear as well-defined, solid, and oval-shaped nodules. A common feature seen on ultrasound is a halo surrounding the nodule, representing the capsule.

To further evaluate the nodule, a doctor may perform a fine-needle aspiration biopsy (FNAB), which involves using a thin needle to remove cells from the nodule for examination under a microscope. However, follicular adenoma cannot be diagnosed by biopsy alone, because a definitive diagnosis requires showing that the tumour does not invade the capsule or nearby blood vessels. This can only be determined after the nodule is surgically removed and examined under the microscope.

What does a follicular adenoma look like under the microscope?

Under the microscope, follicular adenoma is made up of follicular cells that closely resemble normal thyroid cells. These cells are arranged in small round structures called follicles, which may be filled with a pink fluid called colloid. The tumour may also show solid, trabecular, or microfollicular growth patterns.

Important microscopic features include:

• A thin capsule that separates the tumour from the surrounding thyroid tissue.

• No invasion into the capsule or nearby blood vessels.

• Cells with round, uniform nuclei and pink cytoplasm.

• A low number of dividing cells (low mitotic activity).

• No tumour necrosis (areas of dead tumour cells).

Occasionally, the tumour may exhibit unusual features, such as clear cells, fat (lipomatous metaplasia), or bizarrely appearing cells. These changes are not signs of cancer and do not affect the benign diagnosis.

How is follicular adenoma different from cancer?

The main difference between follicular adenoma and follicular thyroid carcinoma (a type of thyroid cancer) is the presence of invasion. In follicular carcinoma, tumour cells break through the capsule or enter blood vessels. In follicular adenoma, this type of invasion is absent.

Because invasion can only be identified by examining the entire capsule under a microscope, a diagnosis of follicular adenoma cannot be made until the tumour is fully removed and carefully examined by a pathologist.

Are there different types of follicular adenoma?

Most follicular adenomas share similar microscopic features, but there are some uncommon subtypes based on their appearance:

• Clear cell follicular adenoma: Made up mostly of cells with clear cytoplasm.

• Follicular adenoma with papillary hyperplasia: Shows finger-like projections but lacks features of papillary thyroid carcinoma.

• Lipoadenoma: Contains areas of fat within the tumour.

• Spindle cell follicular adenoma: Contains long, thin cells resembling spindle shapes.

• Black follicular adenoma: Contains dark pigment, sometimes seen after treatment with medications like minocycline.

• Follicular adenoma with bizarre nuclei: It contains very large, dark nuclei that may look abnormal, but are not a sign of cancer.

These subtypes do not behave differently and are all considered benign.

What is the prognosis for follicular adenoma?

Follicular adenoma is a benign tumour, which means it will not spread to other parts of the body and is not cancerous. Once the tumour is removed, no further treatment is usually needed, and it does not come back if completely excised.

Patients with follicular adenoma generally have an excellent prognosis. However, regular follow-up may be recommended, especially if the tumour was part of a larger goiter or if other nodules are present in the thyroid.

Questions to ask your doctor

• Will I need surgery to remove the nodule?

• How is follicular adenoma different from follicular carcinoma?

• Was the tumour completely removed?

• Do I need thyroid hormone replacement after surgery?

• Should I be tested for a hereditary condition?

• Will I need follow-up imaging or blood tests?

Other helpful resources

American Thyroid Association (ATA)

American Cancer Society