Follicular adenoma of the thyroid gland

by Jason Wasserman MD PhD FRCPC
March 26, 2024

A follicular adenoma is a non-cancerous type of thyroid tumour. The tumour is made up of specialized follicular cells and is separated from the normal thyroid gland by a thin layer of tissue called a tumour capsule.

What are the symptoms of a follicular adenoma?

Most follicular adenomas present as a painless lump or growth on the front of the neck. For many patients, the tumour is discovered incidentally (by accident), when imaging of the neck is performed for another reason. Large tumours may cause difficulty swallowing or breathing, especially when lying down.

What causes a follicular adenoma?

Radiation exposure as a child and iodine deficiency are both associated with an increased risk of developing a follicular adenoma. However, for most patients, the cause remains unknown.

What does it mean if the tumour is described as encapsulated?

Encapsulated means that the tumour is surrounded by a thin layer of tissue called a capsule. The capsule separates the cells in the tumour from the cells in the normal thyroid gland.

tumour capsule

How is this diagnosis made?

To make the diagnosis of a follicular adenoma, the entire tumour must be removed and sent to a pathologist for examination under a microscope. Your pathologist will then make sure that the tumour cells have not spread past the tumour capsule into the surrounding normal thyroid gland and that no tumour cells are seen in a blood vessel. Because the entire tumour capsule must be examined, the diagnosis cannot be made after only part of the tumour is removed in a procedure called a fine needle aspiration biopsy (FNAB).

What is the difference between follicular adenoma and follicular carcinoma?

The cells in a follicular adenoma and a follicular carcinoma look very similar when examined under a microscope. However, in contrast to the cells in a follicular adenoma, the cells in a follicular carcinoma have broken through the tumour capsule and spread into the surrounding thyroid gland. Pathologists describe this as tumour capsule invasion.

Follicular Adenoma

Follicular carcinoma

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

Other helpful resources

American Thyroid Association (ATA)
American Cancer Society

Learn more pathology

Atlas of Pathology
A+ A A-