Oncocytic Carcinoma of the Thyroid Gland: Understanding Your Pathology Report

Section Editor: Jason Wasserman MD PhD FRCPC
May 29, 2026

Oncocytic carcinoma of the thyroid gland is a rare type of thyroid cancer. 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 carcinoma is the cancerous counterpart of oncocytic adenoma, a noncancerous thyroid tumor composed of similar cells.

Until recently, oncocytic carcinoma of the thyroid gland was called Hürthle cell carcinoma, named for the early descriptions of these cells. The 2022 World Health Organization (WHO) classification changed the name to oncocytic carcinoma of the thyroid gland and, equally importantly, reclassified this tumor as a distinct entity separate from follicular thyroid carcinoma. For many decades, these tumors were treated as a subtype of follicular thyroid carcinoma. Research has shown that they have unique molecular features, behave somewhat differently, and respond differently to treatment, which is why they are now recognized as a tumor type of their own. The older name Hürthle cell carcinoma 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 carcinoma of the thyroid gland start?

Almost all oncocytic carcinomas start within the thyroid gland itself. The thyroid is a butterfly-shaped gland in the front of the lower neck that produces hormones controlling metabolism and growth. In rare cases, the tumor can develop in thyroid tissue that formed outside the normal location during development, such as the base of the tongue (lingual thyroid) or behind the breastbone (the mediastinum).

What causes oncocytic carcinoma of the thyroid gland?

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. The most consistently identified risk factor is exposure to ionizing radiation, particularly during childhood. A small number of cases occur in people with inherited conditions such as Cowden syndrome (PTEN hamartoma tumor syndrome) or DICER1 syndrome.

At the genetic level, oncocytic carcinoma of the thyroid gland has a distinctive molecular signature distinct from other thyroid cancers. This signature usually includes large losses of genetic material from many chromosomes (called genome-wide haploidization), changes in mitochondrial DNA (the genetic material inside the cell’s mitochondria), and, in some tumors, mutations in genes such as TP53, NRAS, and the TERT promoter. The mutations seen in classic papillary thyroid carcinoma (particularly BRAF V600E) are uncommon in oncocytic carcinoma. These molecular differences are one of the reasons WHO 2022 reclassified oncocytic carcinoma as a distinct entity from follicular thyroid carcinoma.

What are the symptoms?

Most patients with oncocytic carcinoma of the thyroid gland do not have symptoms beyond a slowly enlarging, painless lump in the front of the neck. Many tumors are 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 can include:

A lump or swelling in the front of the neck.
A feeling of pressure or tightness in the neck.
Difficulty swallowing.
Hoarseness or a change in voice.
Difficulty breathing, when the tumor is large enough to press on the airway.

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 carcinomas usually appear as solid nodules with a surrounding capsule. They cannot reliably be distinguished from the closely related, non-cancerous oncocytic adenoma by ultrasound alone.

The next step is most often a fine-needle aspiration (FNA), in which a thin needle is used to remove cells from the nodule for microscopic examination. FNA 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 FNA. For this reason, FNA reports in this setting are often described as an oncocytic neoplasm or as suspicious for one, and surgery is needed to make the final diagnosis.

The diagnosis 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) or a total thyroidectomy (removal of the entire thyroid gland). Under the microscope, oncocytic carcinoma is typically a thickly encapsulated tumor composed of large oncocytic cells arranged in solid or trabecular (cord-like) patterns. The cells have abundant pink granular cytoplasm and round nuclei with prominent nucleoli. The pathologist confirms that more than 75 percent of the tumor cells are oncocytic and looks specifically for capsular and vascular invasion to establish the diagnosis of carcinoma.

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

Subtypes based on extent of invasion

The WHO 2022 classification divides oncocytic carcinoma of the thyroid gland into three subtypes based on the extent of invasion observed under the microscope. The subtype affects the risk of recurrence and the choice of treatment.

Minimally invasive oncocytic carcinoma — The tumor shows capsular invasion only, with no invasion into blood vessels. This subtype usually behaves indolently and has an excellent prognosis when completely removed. Additional treatment beyond surgery is often not required.
Encapsulated angioinvasive oncocytic carcinoma — The tumor remains fully encapsulated but shows invasion into one or more blood vessels. The pathology report often describes the extent of vascular involvement:
- Limited vascular invasion means tumor cells are found in fewer than four blood vessels. This subgroup has an intermediate risk of spread.
- Extensive vascular invasion means tumor cells are found in four or more blood vessels. This subgroup has a higher risk of recurrence and distant spread.
Widely invasive oncocytic carcinoma — The tumor cells extensively invade the surrounding thyroid tissue or soft tissues outside the thyroid, often with multiple areas of vascular invasion. This subtype has the highest risk of recurrence and distant metastasis, most often to the lungs, bones, or liver. More intensive treatment and long-term surveillance are typically recommended.

High-grade transformation

In rare cases, an oncocytic carcinoma can undergo high-grade transformation, meaning the tumor takes on features of a more aggressive thyroid cancer. The WHO 2022 classification recognizes this as high-grade differentiated thyroid carcinoma, oncocytic type, and it sits between conventional oncocytic carcinoma and poorly differentiated thyroid carcinoma in terms of behavior. Features that support high-grade transformation include:

Tumor necrosis — Areas of dead tumor cells within an otherwise viable tumor.
Increased mitotic activity — A higher number of dividing cells (mitotic figures) than is typical for oncocytic carcinoma. The WHO 2022 threshold is 5 or more mitoses per 2 square millimeters of tumor.
Atypical mitotic figures — Dividing cells with an abnormal shape or pattern.

Tumors with high-grade transformation are generally more aggressive, are often resistant to radioactive iodine, and have a worse prognosis than conventional oncocytic carcinoma. Identifying these features on the pathology report changes the follow-up plan and the consideration of additional treatments.

Extrathyroidal extension

Extrathyroidal extension means that tumor cells have grown beyond the thyroid gland into surrounding tissues. The pathology report distinguishes two patterns:

Microscopic extrathyroidal extension — Tumor cells extend just beyond the thyroid capsule and can only be seen under the microscope. This finding has a small effect on prognosis and is no longer used by itself to increase the tumor stage.
Macroscopic (gross) extrathyroidal extension — The tumor visibly invades nearby structures, such as the neck muscles, the airway (trachea), the esophagus, the recurrent laryngeal nerve, or major blood vessels in the neck. This finding raises the pathologic tumor stage and is associated with a higher risk of recurrence.

Vascular invasion

Vascular invasion means that tumor cells are seen inside a blood vessel. The pathologist looks for true vascular invasion (tumor cells attached to a vessel wall or mixed with blood clot material inside the vessel) rather than displaced tumor cells that may have been pushed into a vessel during tissue handling.

Vascular invasion is particularly important in oncocytic carcinoma because this tumor type spreads more often through the bloodstream than through lymphatic channels. The extent of vascular invasion (limited vs. extensive) directly affects the assignment of the subtype (encapsulated angioinvasive) and is one of the strongest predictors of distant spread to the lungs, bones, or liver.

Lymphatic invasion

Lymphatic invasion means that tumor cells are seen inside a lymphatic channel, a small thin-walled vessel that carries lymph from tissues toward the lymph nodes. The WHO 2022 classification asks pathologists to report lymphatic invasion separately from vascular invasion. Lymphatic invasion is uncommon in oncocytic carcinoma compared with classic papillary thyroid carcinoma, but it can serve as a route for tumor cells to reach nearby lymph nodes when present.

Surgical margins

A margin is the cut edge of the tissue removed at surgery. The pathologist examines the margins to determine whether the tumor was completely removed.

Negative margin — No tumor cells are seen at the cut edge. This is the most favorable result and is the strongest single predictor of long-term cure.
Positive margin — Tumor cells reach the cut edge. A positive margin indicates that some tumor may have been left behind. Additional treatment, such as further surgery or radioactive iodine, may be considered.
Close margin — Tumor cells come within a few millimeters of the cut edge without reaching it. The pathology report may give the distance in millimeters.

Lymph nodes

Lymph nodes are small bean-shaped structures throughout the body, including the neck, that filter fluid and house immune cells. The lymph nodes near the thyroid are grouped into anatomical regions called levels (numbered 1 through 7); the central neck (level 6) drains the thyroid most directly.

Lymph node involvement is less common in oncocytic carcinoma than in classic papillary thyroid carcinoma but more common than in follicular thyroid carcinoma. A neck dissection is sometimes performed when imaging or palpation suggests lymph node involvement. The pathology report will state how many lymph nodes were examined, how many contained tumor cells, the size of the largest tumor deposit in any node, and whether extranodal extension is present (meaning tumor cells have broken through the outer capsule of a lymph node into the surrounding tissue).

Biomarker and molecular testing

Biomarker testing is not required for every patient, but it can provide information about how the tumor is likely to behave, whether radioactive iodine is likely to work, and about treatment options if the cancer returns or has spread.

Mitochondrial and chromosomal changes

The molecular hallmark of oncocytic carcinoma of the thyroid gland is widespread loss of genetic material from many chromosomes (called genome-wide haploidization or near-haploidization) along with changes in mitochondrial DNA. These changes are not seen in most other thyroid cancers and contribute to the distinctive behavior of oncocytic carcinoma.

TERT promoter mutations

Some oncocytic carcinomas carry mutations in the TERT promoter (a regulatory region of the gene that makes telomerase, an enzyme that helps cells keep dividing). When present, TERT promoter mutations are associated with a higher risk of aggressive behavior, recurrence, and distant spread and indicate the need for closer follow-up.

Other genetic findings

Mutations in RAS family genes (especially NRAS) and in TP53 are found in some oncocytic carcinomas. Less commonly, the tumor may carry gene fusions involving NTRK, RET, or ALK. These findings are important because targeted drugs are available for some of these alterations and may be considered for tumors that have recurred or spread despite standard treatment.

Radioactive iodine uptake

An important practical feature of oncocytic carcinoma is that the tumor cells often take up less iodine than those in other types of thyroid cancer. This makes radioactive iodine therapy less effective in some patients with oncocytic carcinoma than in those with classic papillary or follicular thyroid carcinoma. The treatment team uses a combination of pathology features, post-surgical scans, and blood tests to determine whether radioactive iodine is likely to be effective.

For more information on biomarker testing in cancer, please visit our Biomarkers section.

Pathologic stage (pTNM)

Thyroid cancers are staged using the American Joint Committee on Cancer (AJCC) Cancer Staging Manual, 8th edition. The system has three parts: tumor (pT), nodal (pN), and metastasis (pM). For well-differentiated thyroid cancers, including oncocytic carcinoma, the stage grouping is unusual in that it also depends on the patient’s age at diagnosis (younger than 55 years vs. 55 years and older), reflecting the substantially better prognosis seen in younger patients.

Tumor stage (pT)

pT1 — Tumor 2 centimeters or smaller, confined to the thyroid. pT1a is 1 centimeter or smaller; pT1b is greater than 1 centimeter and up to 2 centimeters.
pT2 — Tumor larger than 2 centimeters and up to 4 centimeters, confined to the thyroid.
pT3 — Tumor larger than 4 centimeters confined to the thyroid (pT3a), or tumor of any size with macroscopic extrathyroidal extension into nearby strap muscles only (pT3b).
pT4 — Tumor with extensive macroscopic extrathyroidal extension. pT4a invades into the upper airway, larynx, esophagus, or recurrent laryngeal nerve; pT4b invades into the spine, prevertebral fascia, or major blood vessels in the neck.

Nodal stage (pN)

pN0 — No tumor cells in any regional lymph nodes examined.
pN1a — Tumor cells in lymph nodes of the central neck (level 6) or upper mediastinum (level 7).
pN1b — Tumor cells in lymph nodes on the side of the neck (levels 1 through 5), behind the throat, or on the opposite side of the neck.

Stage grouping

For patients younger than 55 years at diagnosis, any tumor that has not spread to distant organs is Stage I, and any tumor that has spread to distant organs is Stage II. For patients 55 years and older, the stage groupings follow the more typical pattern based on the pT and pN categories. Your treatment team can explain the specific stage and what it means in your case.

What is the prognosis?

The prognosis for oncocytic carcinoma of the thyroid gland depends most strongly on the subtype (extent of invasion), the patient’s age at diagnosis, and the response to initial treatment. Minimally invasive tumors have an excellent prognosis with surgery alone. Encapsulated angioinvasive tumors with limited vascular invasion have a good prognosis but a slightly higher risk of recurrence. Widely invasive tumors, tumors with extensive vascular invasion, and tumors with high-grade transformation carry a higher risk of distant spread and recurrence.

Pathologic and clinical features linked to a higher risk of recurrence or worse outcome include:

Widely invasive subtype or extensive vascular invasion — The strongest pathologic predictors of distant spread.
High-grade transformation — Tumors with necrosis, increased mitotic activity, or atypical mitoses.
Macroscopic extrathyroidal extension — Visible invasion into structures outside the thyroid.
Lymph node involvement — Especially when many nodes are involved or extranodal extension is present.
Distant metastasis at diagnosis — The strongest single predictor of poor outcome.
Positive surgical margin — Linked to higher risk of local recurrence.
TERT promoter mutation — Associated with more aggressive disease.
Older age at diagnosis — Patients 55 years and older have somewhat worse outcomes than younger patients with otherwise similar tumors.
Low radioactive iodine uptake — Limits the effectiveness of one of the standard adjuvant treatments for thyroid cancer.

Compared with other well-differentiated thyroid cancers, oncocytic carcinoma is more likely to spread through the bloodstream to distant sites such as the lungs and bones. Because of this, long-term surveillance is important even after apparently successful initial treatment.

What happens after this diagnosis?

The pathology findings guide the next steps in care rather than dictating a single treatment. After complete staging, the treatment team typically considers:

Type of thyroid surgery — A lobectomy may be sufficient for small, low-risk, minimally invasive tumors. A total thyroidectomy (removal of the entire gland) is generally considered for larger tumors, tumors with extensive vascular invasion, widely invasive tumors, macroscopic extrathyroidal extension, lymph node involvement, or when radioactive iodine is planned.
Radioactive iodine (RAI) therapy — RAI is considered for tumors with high-risk features (extensive vascular invasion, macroscopic extrathyroidal extension, lymph node involvement, distant spread). Because oncocytic carcinomas often take up less iodine than other thyroid cancers, the treatment team carefully weighs whether RAI is likely to help in each case. Pre-treatment scans or blood tests may be used to assess iodine uptake.
Thyroid hormone replacement — Required for life after a total thyroidectomy. The dose is often adjusted to keep thyroid-stimulating hormone (TSH) levels low, which can reduce the chance of recurrence.
Follow-up monitoring — Regular blood tests measure thyroglobulin (a protein made by thyroid cells, including thyroid cancer cells) and anti-thyroglobulin antibodies. A rising thyroglobulin level after treatment can be the first sign of recurrence. Neck ultrasound is also used to look for new disease in the thyroid bed or in the neck lymph nodes. Because oncocytic carcinoma is more likely to spread to distant sites, FDG-PET scans (which detect glucose-using tumor cells regardless of iodine uptake) may be used in selected cases.
Treatment of recurrence or metastatic disease — Options can include further surgery, additional radioactive iodine (when uptake is sufficient), external beam radiation, or targeted therapy. Targeted drugs are available for tumors with NTRK, RET, or ALK gene fusions, and tyrosine kinase inhibitors (such as lenvatinib or sorafenib) are sometimes used for advanced iodine-refractory disease.
Genetic counseling — Considered when the patient is young, when there is a family history of related cancers, or when features of an inherited syndrome are present.
Multidisciplinary care — Endocrinology, endocrine surgery, nuclear medicine, medical oncology, radiation oncology, and (when relevant) genetics work together to plan treatment and follow-up. Care is often coordinated through a specialized center with experience in thyroid cancer.

Questions to ask your doctor

Which subtype do I have (minimally invasive, encapsulated angioinvasive, or widely invasive)?
How extensive was the vascular invasion, and how many vessels were involved?
Were any features of high-grade transformation identified (necrosis, increased mitoses, atypical mitoses)?
What was the size of my tumor, and was it confined to the thyroid?
Was extrathyroidal extension present, and was it microscopic or macroscopic?
Were the surgical margins clear?
How many lymph nodes were examined, and were any involved by tumor?
What is my pathologic stage (pT, pN, and pM), and what does it mean given my age?
Were any genetic changes identified, including TERT promoter mutations or targetable fusions?
Is radioactive iodine likely to work for my tumor, and how will the team decide?
Will I need lifelong thyroid hormone replacement, and how will the dose be monitored?
How often will I need follow-up blood tests, ultrasound, and other imaging?
Will I need additional imaging, such as FDG-PET, given that oncocytic carcinoma can be less iodine-avid?
Should I be referred for genetic counseling based on my findings or family history?
Would referral to a specialized thyroid cancer center help guide my care?