by Jason Wasserman MD PhD FRCPC
June 3, 2022
A follicular adenoma is a non-cancerous thyroid tumour. The tumour 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 thin tissue barrier called a tumour 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.
The diagnosis of follicular adenoma can only be made after the entire tumour is removed and sent to a pathologist for examination. 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 a thin tissue barrier called a tumour capsule. In contrast, in follicular carcinoma, the tumour cells have broken through the tumour capsule and have entered the surrounding normal thyroid gland. Pathologists describe this as tumour capsule invasion.
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.
A fine-needle aspiration (FNA) is a procedure that 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 that includes both follicular adenoma and follicular carcinoma.
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 hemorrhage (bleeding), cystic degeneration (the development of holes or spaces in the tissue), and atypical (abnormal) looking cells. All of these changes are expected findings in a follicular adenoma.