Follicular neoplasm of the thyroid gland

by Adnan Karavelic MD FRCPC
March 26, 2024

Follicular neoplasm is a term pathologists use to describe a tumour in the thyroid gland. This diagnosis is usually made after a procedure called a fine-needle aspiration biopsy (FNAB).

Follicular neoplasm is a preliminary diagnosis that includes both non-cancerous and cancerous conditions. These conditions include:

• Adenomatoid nodule: Adenomatoid nodule is a non-cancerous type of growth in the thyroid gland. Most adenomatoid nodules develop as part of a condition called nodular thyroid hyperplasia.
• Follicular adenoma: Follicular adenoma is a non-cancerous type of thyroid tumour. The tumour cells are separated from the normal thyroid gland by a thin tissue barrier called a capsule.
• Follicular carcinoma: Follicular carcinoma is a type of thyroid cancer. Most tumours are at least partially separated from the normal thyroid gland by a thin barrier called a capsule. The tumour cells in follicular carcinoma look similar to the cells in a follicular adenoma. The most important difference is that in follicular carcinoma, the tumour cells cross the capsule and spread into the surrounding thyroid gland.
• Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP): NIFTP is a non-cancerous type of thyroid tumour. The tumour cells are separated from the normal thyroid gland by a thin tissue barrier called a capsule.
• Follicular variant of papillary thyroid carcinoma: Follicular variant of papillary thyroid carcinoma is a type of thyroid cancer. Most tumours are at least partially separated from the normal thyroid gland by a thin barrier called a capsule. The cells in a follicular variant of papillary thyroid carcinoma look similar to the cells in an NIFTP. The most important difference is that in follicular variant papillary thyroid carcinoma, the tumour cells cross the capsule and spread into the surrounding thyroid gland.

To determine which of these conditions you have, the entire tumour needs to be removed and examined under a microscope. In particular, the pathologist needs to be able to determine if the tumour is surrounded by a capsule and if the cells inside the tumour have crossed the capsule into the surrounding thyroid gland. This examination can only be performed after the entire tumour is removed. Look for the final diagnosis in your pathology report after the tumour is removed.

What happens next?

Most patients who receive a diagnosis of follicular neoplasm after a fine-needle aspiration biopsy will be offered surgery to remove half of the thyroid gland with the tumour. Depending on the final diagnosis, additional treatment may be offered.

Microscopic features of a follicular neoplasm

When examined under the microscope, the cells in a follicular neoplasm look similar to the follicular cells normally found in the thyroid gland. However, the follicles in the tumour are often smaller than normal follicles. These small follicles are called microfollicles. The tumour cells may also be arranged in small groups or even as single-detached cells. Your pathologist may use the term atypical to describe cells that have an abnormal shape, colour, or size compared to normal, healthy, follicular cells. Cells may become atypical as a result of cancer, infection, radiation, medications, or inflammation.

Other helpful resources

American Thyroid Association (ATA)

Learn more pathology

Atlas of Pathology