Oncocytic Adenoma of the Thyroid Gland: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
May 31, 2026

Oncocytic adenoma of the thyroid gland is a benign (non-cancerous) thyroid tumor. It arises from follicular cells, the cells in the thyroid gland that normally produce thyroid hormone. The tumor is composed mostly of oncocytic cells, a type of cell that contains numerous energy-producing structures called mitochondria and has a distinctive bright pink appearance under the microscope. Oncocytic adenoma is the non-cancerous counterpart of oncocytic carcinoma of the thyroid gland, which is made up of similar cells but shows evidence of invasion through the tumor’s capsule or into nearby blood vessels.

Until recently, oncocytic adenoma of the thyroid gland was called Hürthle cell adenoma, named for the early descriptions of these cells. The 2022 World Health Organization (WHO) classification changed the name to oncocytic adenoma of the thyroid gland and reclassified this tumor as a distinct entity separate from follicular adenoma. For many decades, oncocytic adenoma was treated as a subtype of follicular adenoma; research has since shown that oncocytic tumors have unique molecular features and biology, which is why they are now recognized as a tumor type of their own. The older name Hürthle cell adenoma still appears on some pathology reports during the transition between the two names.

This article will help you understand the findings in your pathology report, what each term means, and why those findings matter for your care.

Where does oncocytic adenoma start?

Oncocytic adenoma starts in the thyroid gland, the butterfly-shaped gland in the front of the lower neck that produces hormones that control metabolism and growth. The tumor arises from follicular cells, the same cells that give rise to follicular adenoma, follicular thyroid carcinoma, papillary thyroid carcinoma, and oncocytic carcinoma of the thyroid gland.

What causes an oncocytic adenoma?

The exact cause is not fully understood. Most cases develop sporadically, meaning they appear without a known trigger and are not caused by anything the person did or was exposed to. Risk factors that have been associated with oncocytic and follicular thyroid tumors include exposure to ionizing radiation, particularly during childhood, and long-standing iodine deficiency. Most patients with oncocytic adenoma have no history of either.

At the genetic level, oncocytic tumors of the thyroid have a distinctive molecular signature that is different from that of follicular adenoma. This signature usually includes large losses of genetic material from many chromosomes and changes in mitochondrial DNA (the genetic material inside the cell’s mitochondria). These molecular differences are part of the reason WHO 2022 reclassified oncocytic tumors as distinct from follicular tumors.

Most oncocytic adenomas are not inherited. A small number occur in patients with inherited conditions that raise the risk of thyroid tumors, such as Cowden syndrome (PTEN hamartoma tumor syndrome) or DICER1 syndrome.

What are the symptoms?

Most patients with oncocytic adenoma do not have symptoms. The tumor is often discovered when a thyroid nodule is found on physical examination or by chance on an imaging test performed for another reason. When symptoms do occur, they are usually related to the size of the tumor pressing on nearby structures and can include:

A painless lump or fullness in the front of the neck.
Difficulty swallowing.
A feeling of pressure or tightness in the neck.
Difficulty breathing, when the tumor is large enough to press on the airway (especially when lying down).
Hoarseness or a change in voice, when the tumor is large.

Thyroid hormone levels are usually normal because the tumor cells, although abnormal, generally do not produce enough hormone to cause symptoms.

How is the diagnosis made?

The workup usually begins when a thyroid nodule is found on physical examination or on imaging. A neck ultrasound is then used to evaluate the size, shape, and internal features of the nodule. Oncocytic adenomas usually appear as solid nodules with a surrounding capsule. They cannot be reliably distinguished from oncocytic carcinoma of the thyroid gland by ultrasound alone.

A fine-needle aspiration biopsy (FNAB) is often performed to sample cells from the nodule. FNAB can identify oncocytic features but cannot determine whether the tumor is benign (oncocytic adenoma) or malignant (oncocytic carcinoma). The key features that separate these two diagnoses are capsular invasion (tumor cells crossing the capsule surrounding the tumor) and vascular invasion (tumor cells within blood vessels). Neither feature can be reliably assessed on the small samples obtained by FNAB. For this reason, FNAB reports in this setting are often described as follicular neoplasm, oncocytic type, and surgery is needed to make the final diagnosis.

The diagnosis of oncocytic adenoma is made after the tumor is surgically removed and examined under the microscope by a pathologist. The surgery is most often a lobectomy (removal of one lobe of the thyroid). Under the microscope, the pathologist confirms that more than 75 percent of the tumor cells are oncocytic, examines the entire fibrous capsule around the tumor for any tumor cells crossing it, and examines blood vessels within and around the tumor for any tumor cells within them. When no capsular or vascular invasion is found, the diagnosis is oncocytic adenoma. When either is found, the diagnosis is oncocytic carcinoma.

Immunohistochemistry, which uses antibodies to detect specific proteins in tissue, may be used to confirm the thyroid origin of the tumor (using markers such as thyroglobulin, TTF-1, and PAX8) or to rule out other tumor types that can look similar.

Microscopic features that may be described in your report

Under the microscope, oncocytic adenoma has a distinctive appearance. The pathology report may describe one or more of the following features:

Well-defined capsule — The tumor is surrounded by a thin fibrous border called a capsule, which separates it from the surrounding normal thyroid tissue. The capsule is examined carefully along its entire length to confirm that no tumor cells have crossed through it.
Oncocytic cells — The tumor is made up of cells with abundant pink granular cytoplasm packed with mitochondria. By definition, oncocytic cells make up more than 75 percent of the tumor.
Solid or trabecular growth pattern — The tumor cells are typically arranged in sheets, cords (trabeculae), or small follicles. The cell nuclei are round with prominent nucleoli.
No capsular invasion — No tumor cells are seen crossing through the tumor’s fibrous capsule. This is one of the features that separates oncocytic adenoma from oncocytic carcinoma.
No vascular invasion — No tumor cells are seen inside blood vessels. This is the other key feature that separates oncocytic adenoma from oncocytic carcinoma.
Low mitotic activity — Dividing cells are rare, consistent with a slow-growing, benign tumor.
No necrosis — No areas of dead tumor cells are seen. Necrosis would suggest a more aggressive tumor.

Difference between oncocytic adenoma and oncocytic carcinoma

Oncocytic adenoma and oncocytic carcinoma look very similar under the microscope. Both are composed of oncocytic cells, are usually surrounded by a fibrous capsule, and can show low mitotic activity. The only reliable way to tell them apart is to examine the entire capsule of the tumor and the blood vessels in and around it for evidence of invasion:

Oncocytic adenoma — No capsular invasion and no vascular invasion.
Oncocytic carcinoma — Capsular invasion, vascular invasion, or both.

Because this distinction depends on examining the entire tumor and its capsule, an FNAB sample alone cannot reliably distinguish between them. Surgery is needed to make the final diagnosis.

What is the prognosis?

Oncocytic adenoma is a benign tumor with an excellent prognosis. The tumor does not spread to other parts of the body, and an oncocytic adenoma that has been completely removed almost never returns. The diagnosis itself is not associated with any reduction in life expectancy. The main long-term issue is the small chance that another thyroid nodule may develop over time, which is not specific to oncocytic adenoma and applies to many thyroid conditions.

What happens after this diagnosis?

The treatment plan is straightforward for a benign oncocytic adenoma. The treatment team typically considers:

Lobectomy as definitive treatment — A lobectomy (removal of the lobe of the thyroid containing the tumor) is usually sufficient. No further surgery is needed once the diagnosis is confirmed.
No need for radioactive iodine — Because oncocytic adenoma is benign, radioactive iodine therapy is not part of the treatment plan.
No need for total thyroidectomy — Removing the entire thyroid gland is not generally recommended for oncocytic adenoma, although it may be considered if there are other thyroid nodules or related conditions on the opposite side of the gland.
Thyroid function monitoring — Thyroid hormone replacement is often not needed after a lobectomy, because the remaining lobe usually produces enough hormone. Blood tests check thyroid function and decide whether hormone replacement is appropriate.
Follow-up imaging — Periodic neck ultrasound may be used to monitor the remaining thyroid for new nodules, particularly when other thyroid abnormalities are present.

Questions to ask your doctor

Was the tumor completely removed during my surgery?
How does the pathologist decide between oncocytic adenoma and oncocytic carcinoma, and why was my tumor diagnosed as the benign type?
If my FNAB result suggested an oncocytic neoplasm before surgery, did that change anything about my treatment plan?
Do I need any additional surgery, such as a total thyroidectomy?
Will I need thyroid hormone replacement after my surgery?
How will my thyroid function be monitored after surgery?
How often will I need follow-up examinations or imaging?
What is the chance that this tumor will return, or that I will develop another thyroid nodule?