Hyalinizing Trabecular Tumor of the Thyroid Gland: Understanding Your Pathology Report

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

Hyalinizing trabecular tumor (HTT) is an uncommon thyroid tumor that arises from follicular cells, the cells in the thyroid gland that normally produce thyroid hormone. The current World Health Organization (WHO) classification, published in 2022, lists HTT as a benign thyroid neoplasm. The older name, hyalinizing trabecular adenoma, may still appear on some pathology reports and means the same thing as hyalinizing trabecular tumor.

HTT is clinically important because its cells share several features with papillary thyroid carcinoma, including changes in the appearance of the cell nucleus. These overlapping features mean that HTT is sometimes mistaken for papillary thyroid carcinoma on a needle biopsy, which can lead to more treatment than is needed. Once the diagnosis of HTT is confirmed on the surgical specimen, treatment is much simpler.

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 hyalinizing trabecular tumor start?

Hyalinizing trabecular tumor 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 the well-differentiated thyroid cancers (papillary thyroid carcinoma, follicular thyroid carcinoma, and oncocytic carcinoma of the thyroid gland). Despite arising from the same cell type, HTT behaves in a benign way.

What causes a hyalinizing trabecular tumor?

The exact cause of hyalinizing trabecular tumor is not known. HTT is not linked to any known environmental risk factors and is not associated with inherited syndromes that increase the risk of other thyroid tumors. HTT is more often diagnosed in middle-aged women but can occur in adults of any age.

At the genetic level, almost all hyalinizing trabecular tumors carry a specific gene fusion, either PAX8::GLIS3 or, less commonly, PAX8::GLIS1. A gene fusion occurs when two genes that are normally separate become joined, producing an abnormal protein. This particular fusion is highly specific for HTT and is considered the defining genetic alteration of this tumor. Importantly, the gene changes most often seen in thyroid cancers (such as BRAF, RAS, and RET/PTC) are typically absent in HTT.

What are the symptoms of a hyalinizing trabecular tumor?

Most patients with hyalinizing trabecular tumor do not have symptoms. The tumor is usually discovered when a thyroid nodule is found on a physical examination or by chance on an imaging test performed for an unrelated reason. When symptoms do occur, the most common is a painless lump or fullness in the front of the neck. Thyroid hormone levels are typically normal because the tumor does not produce thyroid hormone.

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 HTT.

A fine-needle aspiration biopsy (FNAB) is often performed to sample cells from the nodule. The cells of HTT have nuclear features (such as pale nuclei and nuclear grooves) that closely resemble those of papillary thyroid carcinoma. As a result, an FNAB sample from an HTT is often interpreted as suspicious for or even diagnostic of papillary thyroid carcinoma. This misinterpretation is a recognized pitfall in thyroid cytology and one reason the final diagnosis of HTT can only be made after the tumor is surgically removed and examined under the microscope by a pathologist.

Once the tumor is removed (usually by a lobectomy), the pathologist evaluates the entire mass. HTT is recognized by its characteristic microscopic features and growth pattern, described in the next section. Two additional tests are often used to confirm the diagnosis:

Immunohistochemistry for Ki-67 — Ki-67 is a protein found only in cells that are dividing. In most tumors, Ki-67 is detected only inside the nucleus of the cell. HTT shows a very unusual pattern in which Ki-67 also stains the surface (membrane) of the tumor cells. This membranous Ki-67 staining is highly characteristic of HTT and helps distinguish it from papillary thyroid carcinoma, which shows the more typical nuclear-only pattern.
Molecular testing for the PAX8::GLIS fusion — When the diagnosis is uncertain, testing for the PAX8::GLIS3 or PAX8::GLIS1 fusion can confirm HTT. The fusion is found in nearly all HTTs and is not found in papillary thyroid carcinoma or other thyroid tumors that share the nuclear features.

Microscopic features that may be described in your report

Under the microscope, hyalinizing trabecular tumor has a distinctive appearance. Your pathology report may describe one or more of the following features:

Well-defined capsule — The tumor is usually surrounded by a thin fibrous border called a capsule, which separates it from the surrounding normal thyroid tissue.
Trabecular growth pattern — The tumor cells are arranged in long, thin cord-like groups called trabeculae, rather than in the small round follicles seen in normal thyroid tissue. The word trabecular in the name of the tumor comes from this pattern.
Hyalinized stroma — Between the cords of tumor cells, the supporting tissue contains a glassy, bright pink material called hyaline. The word “hyalinizing” in the tumor name describes this material.
Elongated or oval cells — The tumor cells are typically larger and more elongated than normal follicular cells and have abundant pink cytoplasm.
Papillary-like nuclear features — The cell nuclei often look pale, have grooves, and may contain small round inclusions (pseudoinclusions). These features mimic those of papillary thyroid carcinoma and are the reason HTT can be mistaken for cancer on a needle biopsy.
Low mitotic activity — Dividing cells are uncommon, consistent with the slow growth of this tumor.
Yellow bodies — Small round structures sometimes seen within the cytoplasm of tumor cells. They are not specific for HTT but are a frequently described feature.

Biomarker and molecular testing

HTT has a single defining molecular feature, and biomarker testing is most useful for confirming the diagnosis when the microscopic appearance is not clear-cut.

PAX8::GLIS fusion

Nearly all hyalinizing trabecular tumors carry a PAX8::GLIS3 or PAX8::GLIS1 gene fusion. This fusion is highly specific for HTT and is not found in papillary thyroid carcinoma, follicular thyroid carcinoma, oncocytic thyroid tumors, or other thyroid neoplasms that may share some features with HTT. Testing can be performed on the tumor tissue using methods such as RNA sequencing, reverse transcription PCR, or fluorescence in situ hybridization (FISH). A positive result confirms the diagnosis. The fusion is somatic, meaning it develops only in the tumor cells and is not present elsewhere in the body, and it is not inherited.

Ki-67 membranous staining

Membranous staining of Ki-67 on immunohistochemistry, as described above, is a highly characteristic finding in HTT. Although molecular testing is now considered the most specific diagnostic test, Ki-67 immunohistochemistry remains widely used because it is faster, less expensive, and available in most pathology laboratories.

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

What is the prognosis?

The prognosis for hyalinizing trabecular tumor is excellent. The tumor behaves in a benign way in the vast majority of cases. Rare cases of spread to lymph nodes or distant sites have been reported in the literature, which is the reason the WHO classification calls it a tumor rather than an adenoma, but these cases are exceptional. A correctly diagnosed HTT that has been completely removed does not usually require further treatment, and long-term outcomes are similar to other benign thyroid tumors.

What happens after this diagnosis?

The treatment plan depends on the diagnostic certainty and the size of the tumor. 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 generally needed when the diagnosis is confirmed.
Avoidance of overtreatment — Because HTT is benign, radioactive iodine therapy and total thyroidectomy are not required. If an FNAB result suggested papillary thyroid carcinoma and the patient underwent a total thyroidectomy before the final HTT diagnosis was made, the additional surgery is not harmful but was not necessary.
Confirmatory testing in uncertain cases — When the microscopic findings overlap with those of papillary thyroid carcinoma, immunohistochemistry for Ki-67 and molecular testing for the PAX8::GLIS fusion are used to confirm the diagnosis and prevent overtreatment.
Thyroid hormone replacement — Required for life only if a total thyroidectomy was performed. After a lobectomy, thyroid hormone replacement is often not needed, but blood tests are used to check thyroid function and decide whether hormone replacement is appropriate.
Follow-up monitoring — Because HTT is benign and rarely recurs, intensive follow-up is generally not necessary. A simple clinical examination and occasional thyroid blood tests are usually sufficient.

Questions to ask your doctor

Was the diagnosis confirmed by Ki-67 immunohistochemistry, molecular testing for the PAX8::GLIS fusion, or both?
Was the tumor completely removed by the lobectomy?
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?
If my FNAB result suggested papillary thyroid carcinoma but the final diagnosis is HTT, does that change my treatment plan?