Thyroid gland -

Follicular adenoma

This article was last reviewed and updated on June 9, 2019.
by Jason Wasserman, MD PhD FRCPC

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.

  • Follicular adenoma is the most common type of thyroid tumour in adults.

The thyroid gland

The thyroid is a U-shaped gland located on the front of the neck. It is responsible for making thyroid hormone. Thyroid hormone is produced inside small structures called follicles that are lined by specialized follicular cells.


What is a follicular adenoma?

A follicular adenoma is a non-cancerous (benign) tumour that is made up of the same kind of follicles found  in the normal thyroid gland. Follicular adenoma is the most common type of tumour to develop in the 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?

When examined under the microscope, the cells in a follicular adenoma can look very similar to the cells in a 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 abnormal cells have broken through the capsule and have entered the surrounding normal thyroid gland.  

The diagnosis of follicular adenoma can only be made after the entire tumour is removed and sent to a pathologist for examination under the microscope. By examining the entire tumour, your pathologist can make sure that none of the abnormal cells have broken through the capsule.


Why is this important? It is only after examining the entire tumour that your pathologist can determine if the tumour is a follicular adenoma or a follicular carcinoma.

Histologic type

There are different types of follicular adenomas and each one is called a histologic type. The histologic type of based on the size, shape, and colour of the tumour cells and the way they stick together as they grow.


The histologic type can only be determined after the tumour has been examined under the microscope.


The most common special histologic type of follicular adenoma is the oncocytic type. These cells are larger and are more pink looking than normal follicular cells.

Why is this important? All types of follicular adenomas are non-cancerous.

Tumour size

This is the size of the tumour measured in centimeters (cm). Tumour size will only be described in your report after the entire tumour has been removed. 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

Often, a small sample of tissue is removed in a procedure called a biopsy before the entire tumour is removed. The biopsy 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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