Follicular adenoma - Thyroid -

This article will help you read and understand your pathology report for follicular adenoma of the thyroid gland.

by Jason Wasserman, MD PhD FRCPC, reviewed on September 28, 2020

Quick facts:
  • Follicular adenoma is a non-cancerous thyroid gland tumour.
  • It starts from the cells normally found inside the thyroid gland.
  • These tumours are often round and firm and may be felt as a lump in the front of the neck.
Anatomy and histology of the thyroid gland

The thyroid is a U-shaped gland located in the front of the neck. The normal thyroid gland is divided into right and left lobes that are connected in the middle by the isthmus. Some people also have another small lobe above the isthmus called the pyramidal lobe.

Anatomy thyroid gland

The thyroid gland makes thyroid hormone. Most of the cells in the thyroid gland are called follicular cells. The follicular cells connect together to form small round structures called follicles. Thyroid hormone is stored in a material called colloid which fills the centre of follicles.

Normal thyroid follicles

What is a follicular adenoma?

A follicular adenoma is a non-cancerous thyroid tumour. Follicular adenoma is made up of the same kind of follicles found  in the normal thyroid gland.

The cells in a follicular adenoma are separated from the normal thyroid gland by a barrier called a capsule. Because the tumour is so well separated from the normal thyroid tissue, it usually forms a nodule that can be felt in the neck when the thyroid gland is examined. The nodule can also be seen when the thyroid gland is examined by ultrasound.

How do pathologists make this diagnosis?

The diagnosis of follicular adenoma can only be made after the entire tumour is examined by a pathologist. When viewed under the microscope, the cells in a follicular adenoma can look very similar to the cells in a type of thyroid cancer called follicular carcinoma. The only difference between a follicular adenoma and a follicular carcinoma is that all of the abnormal cells in a follicular adenoma are separated from the normal thyroid gland by the capsule. In contrast, in a follicular carcinoma, the tumour cells have broken through the capsule and have entered the surrounding normal thyroid gland. Pathologists describe this as capsular invasion.

Follicular Adenoma

Follicular carcinoma

Because the entire capsule needs to be examined, the diagnosis of follicular adenoma can only be made after the tumour has been removed and sent to a pathologist for examination under the microscope. By examining the entire tumour, your pathologist can make sure that there is no evidence of capsular invasion.

Fine needle aspiration (FNA)

A fine needle aspiration (FNA) is a procedure which removes a small amount of thyroid tissue. This tissue is then examined by a pathologist under the microscope.

The cells in a follicular adenoma may be seen in groups, connected together as follicles, or as single cells. Most tumours also contain less colloid than the normal thyroid gland.

The fine needle aspiration pathology report will describe the tumour as a follicular neoplasm, a category which includes both follicular adenoma and follicular carcinoma.

What to look for in your pathology report after the tumour has been removed

Tumour size

This is the size of the tumour measured in centimeters (cm). The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 4.0 cm by 2.0 cm by 1.5 cm, your report will describe the tumour as being 4.0 cm.

Post biopsy changes

The fine needle aspiration performed before the tumour is removed causes changes in the thyroid gland and the tumour that can be seen under the microscope. Your pathology report may describe these changes as post-biopsy changes or FNA-like changes.

These changes include bleeding (hemorrhage), cystic degeneration (the development of holes or spaces in the tissue), and abnormal (atypical) looking cells. All of these changes are normal and expected findings.

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