Follicular neoplasm of the thyroid gland

By Jason Wasserman MD PhD FRCPC
July 2, 2025

Follicular neoplasm is a term used by pathologists to describe a growth in the thyroid gland that is made up of cells that look like normal thyroid follicular cells. This term is most often used as a preliminary diagnosis after a test called a fine-needle aspiration biopsy (FNAB). Because the cells from a follicular neoplasm look similar in both non-cancerous and cancerous thyroid conditions, additional testing is usually needed to make a final diagnosis.

What is a fine-needle aspiration biopsy?

A fine-needle aspiration biopsy is a procedure used to obtain a small sample of tissue or fluid from a lump or nodule in the thyroid gland. A thin needle is inserted into the nodule, and cells are removed and examined under a microscope. This test helps doctors decide whether the nodule is likely to be benign (non-cancerous) or malignant (cancerous), and whether surgery may be needed.

When the sample shows features of a follicular neoplasm, it means the pathologist sees cells that are arranged in small groups or follicles, but they cannot determine whether the tumour is benign or cancerous based on the biopsy alone.

What conditions are included under follicular neoplasm?

The term follicular neoplasm can refer to several different conditions. Some are non-cancerous, while others are types of thyroid cancer.

Adenomatoid nodule

An adenomatoid nodule is a non-cancerous (benign) growth of thyroid cells. These nodules usually form as part of a condition called nodular thyroid hyperplasia, where multiple nodules grow in the thyroid over time.

Follicular adenoma

Follicular adenoma is a benign tumour of the thyroid. The tumour cells look similar to normal follicular cells, but they grow in a well-defined lump that is surrounded by a thin layer of fibrous tissue called a capsule. The cells do not grow beyond the capsule into surrounding tissue.

Follicular carcinoma

Follicular carcinoma is a type of thyroid cancer. The tumour cells look very similar to those in a follicular adenoma. The key difference is that in follicular carcinoma, the tumour cells break through the capsule and invade surrounding thyroid tissue or blood vessels. This type of invasion can only be seen by examining the entire tumour after it has been removed.

Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP)

NIFTP is a tumour that has some features of thyroid cancer but behaves in a non-aggressive way. It is not considered cancer. Like follicular adenoma, it is surrounded by a capsule, and there is no invasion into surrounding tissue. The cells in NIFTP have nuclear features that resemble papillary thyroid carcinoma but without evidence of spread.

Follicular variant of papillary thyroid carcinoma

Follicular variant of papillary thyroid carcinoma is a type of thyroid cancer that shares features with both follicular carcinoma and papillary thyroid carcinoma. Like other follicular neoplasms, it is usually surrounded by a capsule. However, unlike NIFTP, the tumour cells invade beyond the capsule into the surrounding thyroid gland, confirming its cancerous nature.

Why can’t a definite diagnosis be made from a fine-needle aspiration biopsy?

A fine-needle aspiration biopsy only samples a small number of cells from the tumour. This allows the pathologist to see the types of cells present, but it does not show whether the tumour has a capsule or if the cells are invading surrounding tissue. Since invasion is the key feature that separates non-cancerous from cancerous follicular neoplasms, the entire tumour must be removed and examined under the microscope to make a final diagnosis.

What happens after a diagnosis of follicular neoplasm?

If your biopsy shows a follicular neoplasm, your doctor will likely recommend surgery to remove the part of the thyroid that contains the tumour. This is usually done by removing one lobe of the thyroid (called a lobectomy). Once the entire tumour has been removed, a pathologist will carefully examine the tissue to look for signs of invasion.

Depending on the final diagnosis, your doctor may recommend no further treatment (if the tumour is benign), or additional surgery, radioactive iodine therapy, or follow-up testing (if the tumour is cancerous).

Microscopic features of a follicular neoplasm

When viewed under the microscope, follicular neoplasms are made up of cells that closely resemble normal thyroid follicular cells. However, in a neoplasm, the follicles are often smaller and more tightly packed. These small follicles are called microfollicles. The cells may also appear in small groups or as single, detached cells. Your pathologist may use the word atypical to describe cells that look abnormal in size, shape, or color. Atypical cells may suggest a tumour, but they can also result from other causes such as inflammation, infection, radiation, or certain medications.

Questions to ask your doctor

• Will I need surgery to remove the tumour?

• How will we know if it is cancer?

• What are the next steps after surgery?