by Jason Wasserman MD PhD FRCPC, reviewed on March 1, 2020
The thyroid is a small U-shaped gland located in the front of the neck. It is made up of two halves, called lobes, which lie along the trachea and are joined together by a narrow band of thyroid tissue, known as the isthmus.
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormone. Once made, the thyroid hormone is released into the blood stream and transported throughout the body where they control the body’s metabolism (e.g. the conversion of oxygen and calories to energy).
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.
Nodular follicular disease is a non-cancerous type of growth involving the thyroid gland. The abnormal growth can affect one half of the gland (one lobe) or the entire gland (both lobes and the isthmus). Patients with nodular follicular disease may notice a lump in their thyroid gland or that their entire thyroid gland is enlarged. Doctors call these lumps thyroid nodules. Nodular follicular disease is the most common cause of nodules in the thyroid gland.
Doctors will often recommend a test called an ultrasound for patients with a lump in their thyroid gland or if the entire gland is enlarged. This test allows the doctor to see inside the thyroid gland. The ultrasound also allows the doctor to remove a small sample of tissue in a procedure called a fine needle aspiration (FNA). This tissue is sent to a pathologist who examines it under the microscope.
The diagnosis of nodular follicular disease can be made after part or all of the thyroid gland is surgically removed and sent to a pathologist for examination. The examination involves looking at the thyroid gland with and without a microscope. When examined without a microscope, the thyroid gland looks larger than normal and light-coloured nodules can be seen replacing the normal dark brown thyroid tissue.
When examined under the microscope, nodular follicular disease is made up of abnormal follicles that range in size from small to very large. The growth causes the thyroid gland to become divided into small round nodules. The follicular cells in these abnormal follicles look very similar to the follicular cells in the normal thyroid gland.
Some pathology reports will use the word adenomatoid to describe the nodules seen in nodular follicular disease. Adenomatoid means that the nodules looked similar to a non-cancerous type of growth called a follicular adenoma. Unlike follicular adenomas, adenomatoid nodules are not completely surrounded and separated from the normal thyroid tissue by a thin layer of tissue called a capsule. The word dominant is used to describe the largest adenomatoid nodule.
A thyroid gland that is enlarged as a result of nodular thyroid hyperplasia will show signs of injury that pathologists describe as degenerative change. When examined under the microscope these changes include old blood (hemosiderin deposition), scar formation (fibrosis), and the development of small open spaces called cysts.
If you had an FNA performed prior to having your thyroid gland removed, your pathologist will be able to see the changes caused by needle when the tissue is examined under the microscope. These changes usually include bleeding and the formation of a scar along the path of the needle. If your pathologist is unsure if you had an FNA performed before the thyroid gland was removed, they may describe these changes as “FNA-like”.
Metaplasia is a word pathologists use to describe a group of cells that have changed from one specialized type of cell into another. Metaplasia is a non-cancerous change. There are two types of metaplasia that are commonly seen in nodular follicular disease.
Pathologists use the term reactive atypia to describe follicular cells that look abnormal in shape, size, or colour. Reactive atypia can be caused by inflammation or prior fine needle aspiration. Reactive atypia is a non-cancerous change that is commonly associated with nodular follicular disease.
Other names for nodular follicular disease include nodular thyroid disease, nodular thyroid hyperplasia, and adenomatous hyperplasia. Non-pathologists use the word goiter to describe the changes caused by nodular follicular disease.