Poorly Differentiated Thyroid Carcinoma: Understanding Your Pathology Report

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

Poorly differentiated thyroid carcinoma (PDTC) is a rare type of thyroid cancer that arises from follicular cells, the cells in the thyroid gland that normally produce thyroid hormone. PDTC sits in the middle of the thyroid cancer spectrum: it is more aggressive than the well-differentiated thyroid cancers (papillary thyroid carcinoma and follicular thyroid carcinoma) but less aggressive than anaplastic thyroid carcinoma. PDTC can develop on its own or evolve from a less aggressive thyroid cancer that has become more advanced.

The current World Health Organization (WHO) classification, published in 2022, includes PDTC in a broader group called high-grade follicular cell-derived non-anaplastic thyroid carcinomas. A closely related entity in this group is differentiated high-grade thyroid carcinoma (DHGTC), which is discussed in a separate article.

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 poorly differentiated thyroid carcinoma start?

Poorly differentiated thyroid carcinoma starts in the thyroid gland, the butterfly-shaped gland in the front of the lower neck that produces hormones that control metabolism and growth. In a minority of cases, the tumor arises in thyroid tissue that formed outside its usual location during development, such as in the area behind the breastbone (the mediastinum).

What causes poorly differentiated thyroid carcinoma?

The exact cause is not fully understood. PDTC develops sporadically in most patients, meaning it appears without a known trigger and is not caused by anything the person did or was exposed to. A history of ionizing radiation exposure, particularly during childhood, may increase the risk, but most patients have no such history. PDTC is more common in older adults, with most patients diagnosed in their 50s, 60s, or 70s, and it is slightly more common in women than in men.

At the genetic level, PDTC commonly shows mutations in the BRAF and RAS families (HRAS, KRAS, NRAS) and in the TERT promoter. Mutations in TP53, CTNNB1, AKT1, and EIF1AX are also seen. These changes are discussed further in the biomarker section below.

Most PDTCs are not inherited. A small number occur in patients with inherited conditions that raise the risk of thyroid cancer, such as Cowden syndrome (PTEN hamartoma tumor syndrome) or DICER1 syndrome. Genetic counseling may be considered when the patient is young, when multiple thyroid tumors are present, or when there is a family history of related cancers.

What are the symptoms?

Poorly differentiated thyroid carcinoma tends to grow more quickly than well-differentiated thyroid cancers, so patients often notice a rapidly enlarging lump in the front of the neck. As the tumor grows, it can press on surrounding structures and cause:

A visible or palpable lump in the front or side of the neck.
Difficulty swallowing.
Hoarseness or a change in voice (when the tumor presses on or invades the nerve to the voice box).
Difficulty breathing.
Persistent neck or throat discomfort.
A separate lump on the side of the neck if cancer cells have spread to nearby lymph nodes.

Thyroid hormone levels are usually normal because the abnormal cells in PDTC 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. Imaging features alone cannot make the diagnosis of PDTC, but they can identify nodules suspicious enough to require further evaluation. Blood tests measure thyroid hormone levels and thyroid-stimulating hormone (TSH).

A fine needle aspiration (FNA) biopsy may be performed, in which a thin needle is used to remove cells from the nodule. FNA can sometimes suggest a high-grade thyroid cancer but cannot reliably distinguish PDTC from other types of thyroid cancer because the diagnostic criteria depend on growth pattern and high-grade features that are difficult to assess on small samples. The diagnosis is most often made after the tumor is surgically removed and examined under the microscope by a pathologist. The surgery is most often a total thyroidectomy (removal of the entire thyroid gland), sometimes with removal of nearby lymph nodes.

Under the microscope, the pathologist evaluates the tumor using internationally accepted criteria called the Turin criteria. PDTC is diagnosed when all three of the following are present:

The tumor grows in a solid, trabecular (cord-like), or insular (small nest-like) pattern.
The tumor does not show the nuclear features of papillary thyroid carcinoma (such as clear nuclei or nuclear grooves).
At least one high-grade feature is present: convoluted (twisted) nuclei, tumor necrosis (areas of dead tumor cells), or increased mitotic activity (3 or more dividing cells per 2 square millimeters of tumor).

The pathologist also considers whether the findings might better fit the related WHO 2022 entity, differentiated high-grade thyroid 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) and to rule out other tumor types, such as medullary thyroid carcinoma or tumors that have spread to the thyroid from elsewhere in the body.

Mixed and progressive tumors

Poorly differentiated thyroid carcinoma sometimes occurs alongside other types of thyroid cancer in the same tumor. The pathology report may describe areas of papillary thyroid carcinoma, follicular thyroid carcinoma, or oncocytic carcinoma of the thyroid gland within the same specimen. In some cases, parts of the tumor may show progression to anaplastic thyroid carcinoma, which is more aggressive again. Identifying these mixed features is important because the behavior of the tumor is generally determined by the most aggressive component present.

Tumor size and extrathyroidal extension

After the tumor is removed, the pathologist measures it in three dimensions and records the largest dimension on the report. Tumor size is one of the most important factors in staging the cancer.

The pathologist also looks for extrathyroidal extension, meaning the tumor has grown beyond the thyroid gland into the surrounding tissues. Two patterns are described:

Microscopic extrathyroidal extension — Tumor cells have grown a small distance into the soft tissue around the thyroid but only visible under the microscope. This finding has a small effect on prognosis and is not by itself used to increase the tumor stage.
Macroscopic (gross) extrathyroidal extension — The tumor visibly invades structures around the thyroid (such as the neck muscles, the airway, the esophagus, or major blood vessels) during surgery or on imaging. This finding raises the pathologic tumor stage and is associated with a higher risk of recurrence.

Vascular invasion

Vascular invasion, also called angioinvasion, refers to tumor cells present within a blood vessel. In PDTC, vascular invasion is an important finding because the tumor frequently spreads through the bloodstream to distant sites such as the lungs and bones. The pathology report often describes the extent of vascular invasion:

Focal vascular invasion — Tumor cells are found in fewer than four blood vessels.
Extensive vascular invasion — Tumor cells are found in four or more blood vessels.

Extensive vascular invasion is associated with a higher risk of distant spread and may lead to more intensive treatment and closer follow-up.

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 present in some PDTCs and can serve as a route for tumor cells to reach nearby lymph nodes.

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, radioactive iodine, or external beam radiation, 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. Cancer cells can travel from the thyroid to the lymph nodes through the lymphatic channels. 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.

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). Lymph node involvement is more common in PDTC than in well-differentiated thyroid cancers and is recorded as part of the pathologic stage.

Biomarker and molecular testing

Biomarker testing is increasingly important in PDTC and DHGTC because the results can identify treatment options for patients with advanced disease and can provide information about prognosis.

BRAF and RAS mutations

The BRAF V600E mutation is found in some PDTCs, especially those that arose from a pre-existing papillary thyroid carcinoma. RAS family mutations (HRAS, KRAS, and NRAS) are also common, particularly in tumors that arose from a follicular pattern. Both of these mutations are early driver changes shared with the well-differentiated thyroid cancers and have specific targeted therapies available for advanced disease.

TERT promoter mutations

Mutations in the TERT promoter (a regulatory region of the gene that makes telomerase) are common in PDTC and DHGTC. They are particularly important because, when present alongside BRAF or RAS mutations, they are associated with more aggressive disease, a higher risk of recurrence and distant spread, and a worse overall prognosis.

TP53 and other late mutations

Mutations in TP53 (the gene that encodes the p53 protein) and in other genes, such as CTNNB1, AKT1, and EIF1AX, are observed in some PDTCs. These changes are thought to occur later in cancer development and are part of the molecular evolution toward more aggressive disease.

Targetable gene fusions

Less commonly, PDTC and DHGTC may show gene fusions involving NTRK, RET, or ALK. These findings are important because targeted drugs are available for tumors with these alterations and may be considered for cancers that have recurred or spread despite standard treatment.

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 most well- and intermediate-grade thyroid cancers, including PDTC and DHGTC, the stage grouping 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 poorly differentiated thyroid carcinoma is intermediate between that of well-differentiated thyroid cancers and anaplastic thyroid carcinoma. Reported 5-year overall survival rates vary across studies but are generally in the range of 60 to 75 percent, and 10-year overall survival is approximately 40 to 50 percent. Individual outcomes vary widely depending on the stage at diagnosis, the completeness of surgical removal, and the molecular features of the tumor. Differentiated high-grade thyroid carcinoma has a broadly similar intermediate prognosis.

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

Macroscopic extrathyroidal extension — Visible invasion into structures outside the thyroid.
Extensive vascular invasion — Tumor cells in four or more blood vessels.
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. PDTC most often spreads to the lungs and bones.
Positive surgical margin — Linked to a higher risk of local recurrence.
TERT promoter mutation — Especially when combined with BRAF or RAS, associated with more aggressive disease.
Larger tumor size — Tumors greater than 4 centimeters carry a higher risk of recurrence.
Older age at diagnosis — Patients 55 years and older have somewhat worse outcomes than younger patients.
Progression to anaplastic thyroid carcinoma — The presence of even small areas of anaplastic transformation substantially worsens the prognosis.

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:

Total thyroidectomy with lymph node dissection — The mainstay of treatment is complete surgical removal of the thyroid gland, often with removal of the central neck lymph nodes and any additional lymph nodes suspected to contain tumor.
Radioactive iodine (RAI) therapy — Often considered after surgery to destroy any remaining thyroid tissue or cancer cells. PDTC and DHGTC are less iodine-avid than well-differentiated thyroid cancers, so the treatment team may use pre-treatment scans to assess whether RAI is likely to help.
External beam radiation — May be considered when the tumor has invaded structures outside the thyroid, when surgical margins are positive in locations where re-operation is not feasible, or for symptomatic local recurrence.
Thyroid hormone replacement — Required for life after total thyroidectomy. The dose is often adjusted to keep thyroid-stimulating hormone (TSH) levels low, which can reduce the chance of recurrence.
Targeted therapy and chemotherapy for advanced disease — For tumors that have recurred or spread and are not controlled by surgery or RAI, options include tyrosine kinase inhibitors (such as lenvatinib or sorafenib), targeted drugs directed at specific genetic changes (BRAF inhibitors for BRAF V600E, NTRK inhibitors for NTRK fusions, RET inhibitors for RET fusions), and systemic chemotherapy in selected cases.
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, CT, MRI, and FDG-PET scans are also used to look for new disease.
Genetic counseling — Considered when the patient is young, when multiple thyroid tumors are present, or when there is a family history suggestive of an inherited syndrome.
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 thyroid cancer center given the rarity and complexity of PDTC and DHGTC.
Palliative care — Considered alongside other treatments, particularly for patients with advanced disease, to help manage symptoms such as pain, breathing difficulty, or swallowing problems.

Questions to ask your doctor

Does my report describe poorly differentiated thyroid carcinoma, differentiated high-grade thyroid carcinoma, or features of both?
Which high-grade features were present (necrosis, mitotic activity, or both)?
Were any features of progression to anaplastic thyroid carcinoma identified?
What was the size of my tumor, and was it confined to the thyroid?
Was extrathyroidal extension present, and was it microscopic or macroscopic?
Was vascular invasion present, and how extensive was it?
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 BRAF, RAS, TERT promoter, or other gene mutations identified?
Were any targetable gene fusions (NTRK, RET, or ALK) identified?
Is radioactive iodine likely to work for my tumor, and how will the team decide?
Will I need external beam radiation, and why?
What targeted therapy or systemic treatment options might apply if the cancer returns or spreads?
How often will I need follow-up blood tests, ultrasound, and other imaging?
Would referral to a specialized thyroid cancer center help guide my care?