Hurthle cell adenoma (oncocytic adenoma) is a rare benign thyroid neoplasm that develops from oncocytic (oxyphilic) transformation of thyroid follicular cells. It constitutes 3-5% of all thyroid neoplasms. Histologically composed of cells with abundant eosinophilic granular cytoplasm (Hurthle cells/Askanazy cells); this abundant cytoplasm reflects excessively accumulated mitochondria within the cells. Like follicular adenoma, it is surrounded by a thin fibrous capsule, and carcinoma distinction is made only histopathologically by the presence of capsular/vascular invasion. An important difference is that Hurthle cell tumors have low radioactive iodine (RAI) uptake capacity — they almost always appear as cold nodules on scintigraphy. In the 2022 WHO classification, it is classified as a separate entity as 'oncocytic thyroid carcinoma/adenoma'.
Age Range
40-70
Peak Age
55
Gender
Female predominant
Prevalence
Uncommon
Hurthle cells arise from oncocytic metaplasia of thyroid follicular epithelium. The fundamental pathological event is mutations in mitochondrial DNA — these mutations cause loss of function in oxidative phosphorylation enzyme complexes. The cell dramatically increases mitochondrial number (mitochondrial proliferation) to compensate for energy production capacity. This excessive mitochondrial accumulation appears as abundant granular eosinophilic cytoplasm on histology and contributes to the typically hypoechoic appearance on ultrasonography due to the increased acoustic impedance of the cells. The dense mitochondrial content increases cellular metabolism and requires rich vascularity — therefore, Hurthle cell tumors generally show increased intranodular Doppler signal. Due to oncocytic transformation, NIS (sodium-iodide symporter) expression decreases or is lost — this explains the low RAI uptake and cold nodule appearance on scintigraphy. Classification as adenoma (no invasion) or carcinoma (invasion present) depends on capsular integrity.
Triad for Hurthle cell tumors: (1) solid, markedly hypoechoic, encapsulated nodule — lower echogenicity than follicular adenoma, (2) increased intranodular vascularity — rich blood supply for metabolically active oncocytic cells, (3) cold on scintigraphy — cannot trap RAI due to NIS loss. This combined triad creates high suspicion for Hurthle cell neoplasm and directs toward surgery (lobectomy + histopathological invasion assessment).
Hurthle cell adenoma appears as a solid, markedly hypoechoic (significantly lower echogenicity than surrounding parenchyma) nodule on ultrasonography. Echogenicity is typically lower than classic follicular adenoma — this results from the dense mitochondrial content of oncocytic cells. A thin complete capsule (halo) is usually visible. The nodule is generally wider-than-tall with smooth margins. Internal structure tends to be homogeneous but cystic degeneration areas may develop in large tumors. In TI-RADS v2017, hypoechoic composition scores 2 points; marked hypoechogenicity (equal to surrounding muscle) scores 3 points.
Report Sentence
A solid, markedly hypoechoic nodule measuring …x…x… mm surrounded by a thin complete capsule is identified in the right/left thyroid lobe; Hurthle cell neoplasm (oncocytic neoplasm) should be considered.
On color Doppler, Hurthle cell adenoma demonstrates markedly increased intranodular vascularity — different from the typical perinodular (basket) pattern of follicular adenoma. The intense intranodular flow reflects the rich neovascular network supplying the high metabolic demands of oncocytic cells. Perinodular vascularity may also accompany. Increased intranodular vascularity, while not directly scoring points in TI-RADS, can raise malignancy suspicion and guide the clinician toward biopsy decision.
Report Sentence
Color Doppler examination demonstrates markedly increased intranodular vascularity in the nodule, consistent with a metabolically active neoplasm (Hurthle cell tumor?).
On Tc-99m pertechnetate scintigraphy, Hurthle cell adenoma almost always appears as a cold (hypofunctioning) nodule. This results from loss/decrease of NIS expression following oncocytic transformation. Cold nodule finding relatively increases malignancy risk (5-15%) but being cold is an expected finding in Hurthle cell tumors and does not alone indicate malignancy. Response to I-131 therapy is poor — therefore RAI therapy is less effective in Hurthle cell carcinoma.
Report Sentence
On Tc-99m pertechnetate scintigraphy, no radiopharmaceutical uptake is observed in the area corresponding to the nodule in the right/left thyroid lobe, consistent with a cold (hypofunctioning) nodule; Hurthle cell neoplasm should be kept in mind.
On contrast-enhanced CT, Hurthle cell adenoma demonstrates notably intense enhancement — rich neovascularity increases the degree of enhancement. On non-contrast images, it appears isodense or slightly hypodense relative to normal thyroid parenchyma. The capsule may be visible as a thin low-density rim on contrast-enhanced studies. Homogeneous enhancement is seen in small tumors, heterogeneous enhancement in large tumors (due to cystic degeneration/necrosis). CT cannot distinguish adenoma from carcinoma.
Report Sentence
On contrast-enhanced CT, a well-encapsulated solid nodule measuring …x…x… mm with notably intense enhancement is identified in the right/left thyroid lobe; hypervascular neoplasm (Hurthle cell tumor?) should be considered.
On MRI, Hurthle cell adenoma shows variable signal on T2-weighted images: iso- to hypointense (unlike follicular adenoma, T2 shortening may occur due to dense mitochondrial content) or mildly hyperintense. On T1, generally iso- to hypointense relative to parenchyma. Capsule appears as thin hypointense rim on T2. Prominent homogeneous enhancement on contrast-enhanced MRI. Mild to moderate diffusion restriction may be seen on DWI — dense cellular packing restricts water molecule movement.
Report Sentence
On MRI, the encapsulated solid nodule in the right/left thyroid lobe demonstrates iso- to hypointense signal on T2, prominent homogeneous enhancement on contrast-enhanced sequences, and mild diffusion restriction on DWI, consistent with Hurthle cell neoplasm.
Criteria
Tumor composed entirely of Hurthle cells, surrounded by thin intact fibrous capsule. No capsular/vascular invasion.
Distinct Features
Typical markedly hypoechoic, solid, encapsulated appearance on US. Increased intranodular flow on Doppler. Diagnosis confirmed after surgery.
Criteria
Cystic degeneration in large Hurthle cell adenoma due to central ischemia and necrosis. Mixed cystic-solid structure.
Distinct Features
Cystic areas avascular, solid component intensely vascular. Cystic variant of papillary carcinoma should be considered in differential.
Criteria
Atypical histological features like increased mitotic activity, cellular pleomorphism. NO capsular/vascular invasion.
Distinct Features
Cannot be distinguished from classic type on imaging. Requires close follow-up. Recurrence risk may be slightly higher.
Distinguishing Feature
Follicular adenoma tends to be less hypoechoic and typically shows perinodular (basket) vascularity. Hurthle cell adenoma is more markedly hypoechoic with more intense intranodular vascularity. Both tumors can be cold on scintigraphy but Hurthle cell tumors are almost always cold.
Distinguishing Feature
Papillary carcinoma shows microcalcifications, taller-than-wide shape, and irregular margins. Hurthle cell adenoma has smooth margins, capsule, and no microcalcifications. Cervical lymphadenopathy is common in papillary carcinoma but not expected in Hurthle cell adenoma.
Distinguishing Feature
Medullary carcinoma can be hypoechoic and hypervascular (similar to Hurthle) but shows coarse calcifications, irregular margins, and cervical LAP. Elevated serum calcitonin is pathognomonic for medullary carcinoma. Calcitonin is normal in Hurthle cell adenoma.
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
specialist-referralWhen Hurthle cells are identified on FNAB (Bethesda IV), surgery (diagnostic lobectomy) is recommended — adenoma/carcinoma distinction can only be made histopathologically. Malignancy rate of Hurthle cell tumors is somewhat higher than follicular neoplasms (15-30% vs 10-15%). Response to RAI therapy is poor — therefore, if carcinoma is diagnosed, completion thyroidectomy + external beam radiotherapy or tyrosine kinase inhibitors may be considered. In the 2022 WHO classification, Hurthle cell tumors are defined as a separate category (distinct from follicular tumors). Molecular testing (ThyroSeq, Afirma GSC) may support surgical decision.
Hurthle cell adenomas are benign tumors but cannot be distinguished from their malignant counterpart (Hurthle cell carcinoma) on imaging. FNA yields 'Hurthle cell neoplasm' and diagnostic lobectomy is recommended. Bethesda Category IV.