Follicular Neoplasm of the Thyroid Gland: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
May 31, 2026

Follicular neoplasm is a preliminary diagnosis used by pathologists to describe a growth in the thyroid gland made up of cells that look like the follicular cells of the thyroid. This term is most often used after a procedure called a fine-needle aspiration biopsy (FNAB), in which a small sample of cells is taken from a thyroid nodule with a thin needle. Follicular neoplasm is not a final diagnosis. It tells the treatment team that the nodule may be either non-cancerous or cancerous, and that more information is needed to know which.

Follicular neoplasm sits in the middle of the standard reporting system used for thyroid FNAB results, called the Bethesda System for Reporting Thyroid Cytopathology. In the current third edition of this system, the category is called follicular neoplasm. The older term follicular neoplasm or suspicious for a follicular neoplasm (FN/SFN) means the same thing and still appears on many reports.

This article will help you understand what this preliminary diagnosis means, what conditions it may turn out to be, and what the next steps usually look like.

What is a fine-needle aspiration biopsy?

A fine-needle aspiration biopsy is a procedure used to obtain a small sample of cells from a lump or nodule in the thyroid gland. A thin needle is inserted into the nodule (usually under ultrasound guidance), and a small number of cells are removed and examined under a microscope. The procedure is safe, generally well-tolerated, and does not require general anesthesia.

FNAB results are reported using the Bethesda System for Reporting Thyroid Cytopathology, which divides results into six categories. Each category carries an estimated risk of malignancy and a recommended next step. Follicular neoplasm is category IV. Other categories include non-diagnostic (category I), benign (category II), atypia of undetermined significance (category III), suspicious for malignancy (category V), and malignant (category VI).

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. The final diagnosis is established by examining the entire nodule after surgery.

Adenomatoid nodule

An adenomatoid nodule is a non-cancerous growth of thyroid cells. These nodules usually form as part of a condition called follicular nodular disease, in which multiple nodules grow in the thyroid over time. Adenomatoid nodules are benign and most do not require any treatment beyond monitoring.

Follicular adenoma

Follicular adenoma is a benign tumor of the thyroid. The cells look similar to normal follicular cells but form a well-defined lump surrounded by a thin fibrous capsule. The cells do not grow beyond the capsule into the surrounding thyroid tissue.

Follicular thyroid carcinoma

Follicular thyroid carcinoma is a type of thyroid cancer. Under the microscope, the cells look very similar to those in a follicular adenoma. The key difference is that in follicular thyroid carcinoma, tumor cells grow through the capsule and into the surrounding thyroid tissue, or into blood vessels. This kind of invasion can only be seen by examining the entire tumor after it has been removed, which is why FNAB alone cannot make the diagnosis.

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

NIFTP is a tumor that has some features of thyroid cancer but behaves in a non-aggressive way. It is no longer considered cancer. Like a follicular adenoma, NIFTP is surrounded by a capsule, with no invasion of the surrounding thyroid tissue or blood vessels. The cells have nuclear features that resemble papillary thyroid carcinoma, but without evidence of spread.

Follicular subtype of papillary thyroid carcinoma

The follicular subtype of papillary thyroid carcinoma (also called follicular variant in older reports) is a type of thyroid cancer that shares features with both follicular thyroid carcinoma and papillary thyroid carcinoma. The tumor cells grow in follicles but also have the nuclear features of papillary thyroid carcinoma. The diagnosis depends on finding invasion (capsular or vascular) after the tumor is removed.

Oncocytic adenoma and oncocytic carcinoma

When the cells in a thyroid nodule have abundant bright pink cytoplasm (called oncocytic cells), the FNAB result may be reported as follicular neoplasm, oncocytic type. After surgery, the nodule may turn out to be an oncocytic adenoma (non-cancerous) or an oncocytic carcinoma (cancerous) of the thyroid gland. As with the other categories, the distinction depends on finding invasion under the microscope.

Microscopic features that lead to a diagnosis of follicular neoplasm

When viewed under the microscope, follicular neoplasms consist of cells that closely resemble normal thyroid follicular cells. Several features lead a pathologist to use the term:

Microfollicles — The cells are arranged in small, tightly packed follicles (small round structures), rather than the larger, more open follicles seen in normal thyroid tissue.
Crowded or repetitive cell groups — The cells appear in cohesive clusters with little of the colloid (thyroid hormone storage material) that surrounds normal follicles.
Mild cellular atypia — The cells may be slightly larger than normal or have nuclei that vary somewhat in size and shape. The term atypical means the cells look abnormal, but the changes are not specific enough to identify the tumor type.
No nuclear features of papillary thyroid carcinoma — If the nuclei are clearly pale, grooved, or show the other typical features of papillary thyroid carcinoma, the result is usually reported in a different Bethesda category.

When the cells in the sample have a predominantly oncocytic appearance, the result is reported as a follicular neoplasm, oncocytic type, a separate subcategory of Bethesda category IV.

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

A fine-needle aspiration biopsy samples only a small number of cells. This allows the pathologist to see the types of cells present, but it does not show the architecture of the entire tumor, the presence or absence of a capsule, or whether tumor cells have invaded the surrounding thyroid tissue or blood vessels. Because invasion is the key feature that separates non-cancerous from cancerous follicular neoplasms, the entire tumor must be removed and examined under the microscope to make a final diagnosis.

What is the risk of cancer with this diagnosis?

Across published studies, the risk of malignancy in nodules reported as follicular neoplasm (Bethesda category IV) is generally 25 to 40 percent. Because NIFTP is no longer considered cancer, the actual risk of an aggressive cancer being found at surgery is lower than older studies suggested, and current estimates are often in the range of 10 to 30 percent. The risk is somewhat lower for the oncocytic subcategory in some studies.

Several factors can raise or lower the risk for a given nodule, including the patient’s age, the nodule’s size and ultrasound features, the results of molecular testing (described below), and the presence of related thyroid conditions. The treatment team uses all of this information together rather than relying on the FNAB result alone.

Molecular testing

Molecular testing is increasingly used to refine the risk of cancer before surgery in patients with a follicular neoplasm. The test is performed on cells left over from the FNAB sample. Common commercial tests include the Afirma Genomic Sequencing Classifier (GSC), ThyroSeq, and ThyGeNEXT/ThyraMIR. These tests look for genetic changes and gene expression patterns that are more often seen in benign or cancerous thyroid tumors. The result is usually reported as a risk estimate rather than a definitive benign or malignant result.

Molecular testing is most useful in two situations:

Lowering the risk — A “benign” or “negative” molecular result can substantially lower the estimated risk of cancer and may support a decision to avoid diagnostic surgery and follow the nodule with imaging instead.
Identifying high-risk changes — A “positive” or “suspicious” molecular result, especially when specific mutations such as BRAF V600E or TERT promoter changes are found, raises the suspicion of cancer and may guide the choice of surgery (for example, total thyroidectomy rather than lobectomy).

Molecular testing is not available in every center and is not required to make decisions, but it can add useful information when available.

What happens after this diagnosis?

The treatment plan depends on the FNAB result, the nodule’s imaging features, any molecular testing results, the patient’s other medical conditions, and their preferences. The treatment team typically considers:

Diagnostic lobectomy — Removal of one lobe of the thyroid is the most common approach. The entire nodule and the surrounding lobe are examined under the microscope for capsular or vascular invasion. This is also a definitive treatment for many benign nodules and for small low-risk cancers.
Total thyroidectomy — Removal of the entire thyroid gland may be considered when the nodule is large, when molecular testing identifies high-risk changes, when there are other thyroid nodules or related conditions, or when the patient has a strong preference for removing the whole gland.
Active surveillance — In selected patients with small low-risk nodules and reassuring molecular testing results, the nodule may be monitored with repeat ultrasound and clinical examination rather than removed immediately.
Further evaluation — Additional imaging or repeat FNAB may be considered if the original sample was small or the findings were ambiguous.

Once the nodule has been removed, the final diagnosis guides any further treatment. For benign nodules and NIFTP, no further treatment is usually needed. For confirmed cancers, additional treatment may include further surgery, radioactive iodine therapy, or thyroid hormone replacement, depending on the type and stage of the cancer.

Questions to ask your doctor

What Bethesda category was assigned to my FNAB result?
Was the result reported as follicular neoplasm, follicular neoplasm oncocytic type, or another category?
What is the estimated risk of cancer for a nodule with this result?
Is molecular testing available for my sample, and would it help guide treatment decisions?
Will I need surgery, and if so, will it be a lobectomy or a total thyroidectomy?
Is active surveillance an option in my case?
If surgery is recommended, what are the risks and what is the recovery like?
How long will it take to get a final diagnosis after surgery?
Will I need thyroid hormone replacement after surgery?
What follow-up testing or imaging do I need over the long term?