Hyalinizing trabecular tumor (HTT) is a rare neoplasm arising from thyroid follicular cells. Tumor cells are arranged in a trabecular (cord-like) pattern with prominent intercellular hyaline material deposition. Despite showing nuclear features similar to papillary thyroid carcinoma (nuclear grooves, intranuclear pseudoinclusions, ground-glass chromatin), its biological behavior is benign or carries very low malignant potential. It is recognized as an independent entity in the WHO 2022 classification. It is frequently misdiagnosed as papillary carcinoma or medullary carcinoma on FNA.
Age Range
30-60
Peak Age
45
Gender
Female predominant
Prevalence
Rare
The pathogenesis of HTT is not fully understood, but it has been shown to originate from follicular cells and share common RET/PTC genetic rearrangements with papillary carcinoma. Tumor cells deviate from normal follicular architecture and organize in a trabecular pattern — this arrangement mimics paraganglioma-like morphology. The intercellular hyaline material consists of type IV collagen and basement membrane components — this deposition causes prominent amorphous matrix between trabeculae. Nuclear features (grooves, pseudoinclusions) share a common mechanism with papillary carcinoma as a consequence of RET/PTC activation; however, HTT lacks invasion or metastatic capacity. On ultrasonography, it typically appears as a solid, hypoechoic, well-defined nodule — this appearance mimics medullary carcinoma and follicular neoplasm. Due to hyaline material deposition, the tumor has firm consistency and may show high stiffness on elastography — this feature also increases malignancy suspicion. On CT and MRI, it appears as a homogeneously enhancing, well-defined solid mass. Calcification is rare and usually appears as thin peripheral calcification.
The 'signature finding' of HTT is actually pathological but is captured through the US + FNA combination in clinical practice. Well-defined, hypoechoic, solid nodule without microcalcifications on US → PTC-like nuclear features (grooves, pseudoinclusions) on FNA → suspicious papillary carcinoma diagnosis is made but HTT is confirmed on surgical specimen with trabecular pattern + hyaline material + MIB-1 membranous staining. Although US findings are not specific, the discrepancy between 'malignant-appearing FNA + benign-appearing US' should raise HTT suspicion.
Well-defined, homogeneous or mildly heterogeneous hypoechoic solid thyroid nodule. Smooth margins and thin halo are seen. Microcalcifications are typically absent — this feature is an important distinguishing point from papillary carcinoma. The nodule is usually wider-than-tall in shape. Size is typically between 1-3 cm. Due to hyaline material deposition, internal echotexture shows a homogeneous solid pattern; cystic component is generally absent.
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A __ x __ mm well-defined, homogeneous hypoechoic solid nodule is seen in the [right/left] thyroid lobe. A thin halo is present. No microcalcifications are identified. It is wider-than-tall in shape, and TI-RADS score is assessed as __.
Moderate intranodular vascularity is seen on color Doppler. Perinodular vascularity may also be present. Vascular pattern is regular, different from the chaotic neovascularization seen in malignant tumors. On elastography, the nodule may show high stiffness (increased strain ratio or high shear-wave velocity) — this finding reflects the fibrosis-like stiffness of hyaline material deposition and may misleadingly increase malignancy suspicion clinically.
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On Doppler examination, moderate intranodular vascularity is seen in the nodule; vascular pattern is regular. The nodule shows high stiffness on elastography (strain ratio: __ / shear-wave: __ kPa). Note: Hyalinizing trabecular tumor may show misleadingly high stiffness on elastography due to hyaline deposition.
On contrast-enhanced CT, a well-defined, homogeneously enhancing solid nodule in the thyroid lobe. It generally enhances less than surrounding normal thyroid tissue. Calcification is rare; if present, it is in the form of thin peripheral calcification. Extrathyroidal extension or lymphadenopathy is not expected. Distinguishing HTT from other benign or low-grade thyroid nodules on CT is not reliable — US and pathology are more determinative.
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On contrast-enhanced CT, a __ x __ mm well-defined, homogeneously enhancing solid nodule is seen in the [right/left] thyroid lobe. It is relatively hypodense compared to surrounding thyroid parenchyma. No calcification/extrathyroidal extension/lymphadenopathy is identified.
Solid nodule showing intermediate signal intensity on T2-weighted sequences. It may be mildly hypointense or isointense relative to normal thyroid tissue. Homogeneous signal pattern reflects the uniform trabecular structure and hyaline material deposition of the tumor. T2 signal is generally lower than papillary carcinoma and at a similar level to follicular neoplasms.
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On T2-weighted sequences, the nodule in the [right/left] thyroid lobe shows intermediate signal intensity. A homogeneous signal pattern is seen. It is mildly hypointense relative to normal thyroid parenchyma.
May show mildly high signal on DWI; ADC values are generally at intermediate levels (1.0-1.5 x 10⁻³ mm²/s). Marked diffusion restriction is not expected — this differs from the low ADC values (<0.9) shown by aggressive malignancies. The low mitotic activity and non-aggressive biology of HTT keep cellularity levels low and do not create marked diffusion restriction.
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Mild signal increase is seen in the thyroid nodule on DWI; ADC value is at an intermediate level (__ x 10⁻³ mm²/s). No marked diffusion restriction is identified. This finding does not support aggressive malignancy.
On Tc-99m pertechnetate scintigraphy, HTT generally appears as a cold or warm nodule. Hot nodule (autonomous function) is not expected. Similarly, iodine scintigraphy (I-123) shows low or normal uptake. The scintigraphic finding is not specific and is not distinguishing from other solid thyroid nodules.
Report Sentence
On Tc-99m pertechnetate thyroid scintigraphy, decreased/normal radiopharmaceutical uptake is seen in the region corresponding to the known nodule in the [right/left] lobe (cold/warm nodule). This finding is not specific and should be evaluated with FNA.
Criteria
Typical trabecular growth pattern, prominent intratrabecular hyaline material deposition, PTC-like nuclear features. Encapsulated, no invasion. Conforming to WHO 2022 definition.
Distinct Features
Well-defined, homogeneous hypoechoic nodule on US. Thin capsule, no microcalcifications. Benign biological behavior — lobectomy is curative, total thyroidectomy is not needed. No recurrence has been reported.
Criteria
HTT variant carrying prominent PTC-like nuclear features (grooves, pseudoinclusions, ground-glass chromatin) and RET/PTC rearrangement. Subtype with the highest rate of being reported as papillary carcinoma on FNA.
Distinct Features
On FNA, almost always reported as Bethesda V-VI (suspicious for/malignant). RET/PTC positivity indicates shared genetic mechanism with papillary carcinoma but no invasive behavior exists. Confirmed on surgical specimen with MIB-1 membranous staining pattern + hyaline material. Clinical significance: HTT awareness is critical to prevent unnecessary total thyroidectomy + RAI.
Criteria
Very rare case reports of HTT showing capsular or vascular invasion exist. WHO 2022 classification categorizes it as 'neoplasm with very low malignant potential' — however, reports of true metastasis are virtually nonexistent.
Distinct Features
Cannot be differentiated from classic HTT on imaging — distinction is made on pathological specimen. Follow-up is recommended in the presence of capsular invasion but aggressive treatment is not needed. The existence of this subtype is why HTT is classified as 'very low malignant potential' rather than 'definitely benign.'
Distinguishing Feature
Papillary carcinoma shows microcalcifications, taller-than-wide shape, and irregular margins; HTT is without microcalcifications, wider-than-tall, with smooth margins. Both show PTC-like nuclear features on FNA, but HTT is confirmed on surgical specimen with trabecular pattern + hyaline + MIB-1 membranous staining. 'Malignant FNA + benign US' discrepancy should raise HTT consideration.
Distinguishing Feature
Medullary carcinoma also appears as a hypoechoic solid nodule and may mimic HTT; however, coarse calcifications (amyloid deposition) and elevated calcitonin are pathognomonic in medullary carcinoma. Calcitonin is normal and calcification is generally absent in HTT. Medullary carcinoma originates from C cells, HTT from follicular cells — definitive distinction is made by immunohistochemistry (calcitonin/thyroglobulin).
Distinguishing Feature
Follicular adenoma and HTT appear similar on US — both are well-defined, hypoechoic, solid nodules. On FNA, follicular adenoma is reported as Bethesda III-IV (atypia/follicular neoplasm), while HTT is frequently reported as Bethesda V-VI (malignancy suspicion due to PTC-like nuclear features). Differentiation is made on surgical specimen by HTT's distinct trabecular pattern and hyaline material.
Distinguishing Feature
Hürthle cell carcinoma also appears as a hypoechoic solid nodule; however, it is generally larger (>4 cm), shows thick incomplete halo, and presents with high FDG-PET avidity. HTT is generally smaller (1-3 cm), shows thin halo, and significant FDG-PET avidity is not expected. Pathologically, HTT's trabecular pattern and hyaline material differ from the oncocytic cytoplasm of Hürthle cells.
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
12-monthThe most clinically significant problem with HTT is misdiagnosis as papillary carcinoma on FNA and unnecessary aggressive surgery (total thyroidectomy + RAI + long-term TSH suppression). The WHO 2022 classification has defined HTT as an independent entity with very low malignant potential. HTT should be considered in nodules reported as Bethesda V-VI on FNA but appearing benign on US (well-defined, no microcalcifications, wider-than-tall). Treatment is diagnostic lobectomy — if HTT is confirmed on frozen section or final pathology, total thyroidectomy is not needed and completion surgery is not required. RAI therapy is not indicated. Prognosis is excellent after lobectomy — recurrence rate near 0%, no metastasis reported. Long-term follow-up is recommended but aggressive surveillance is not needed (annual US is sufficient). Pathologist awareness is critical — MIB-1 membranous staining pattern is specific to HTT and confirms surgical adequacy.
Hyalinizing trabecular tumor is a benign tumor cured by surgery. Confusion with papillary carcinoma on FNA may lead to unnecessary total thyroidectomy. MIB-1 (Ki-67) membranous staining pattern is specific for HTT and helps distinguish from papillary carcinoma. Lobectomy is adequate treatment.