Hashimoto nodule is a benign thyroid nodule developing in the background of Hashimoto thyroiditis (chronic lymphocytic thyroiditis). Hashimoto thyroiditis is the most common autoimmune thyroid disease and is 10-15 times more common in women than men. Diffuse lymphocytic infiltration, follicular damage, and fibrosis in Hashimoto parenchyma create a heterogeneous, micronodular structure. Pseudonodules (inflammatory pseudonodule, regenerative nodule) and true adenomatous nodules may develop in this background. The clinical significance is that malignant nodules (especially papillary carcinoma and lymphoma) are seen with increased frequency in Hashimoto background — papillary carcinoma risk is 1.5-3 fold and lymphoma risk is 40-80 fold increased compared to the general population. Therefore, every prominent nodule in Hashimoto background should be evaluated according to ACR TI-RADS criteria.
Age Range
20-60
Peak Age
40
Gender
Female predominant
Prevalence
Common
In Hashimoto thyroiditis, the autoimmune process causes T and B lymphocyte infiltration into thyroid parenchyma through anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies. Chronic lymphocytic infiltration damages follicular cells and leads to oxyphilic (Hürthle/oncocytic) metaplasia — these cells appear more echogenic on ultrasonography due to mitochondrial accumulation. Fibrous bands develop between damaged follicles and divide the parenchyma into septations — creating the heterogeneous, micronodular 'septated' pattern on ultrasonography. Regenerative nodules form as a result of compensatory hyperplasia within damaged tissue — focal adenomatous hyperplasia develops in response to TSH stimulation. These nodules are generally well-defined, iso-hyperechoic, and described as 'white knight' sign — a relatively echogenic focal area within the heterogeneous hypoechoic Hashimoto parenchyma. Pseudonodules are not true neoplasms but focal concentrations of inflammatory infiltration and regeneration areas. Coarse calcifications within fibrous bands may develop as dystrophic calcification over time.
A relatively echogenic (isoechoic-hyperechoic), well-defined, oval-shaped benign nodule within the heterogeneous, generally hypoechoic parenchyma of Hashimoto thyroiditis — stands out like a bright 'knight' on the dark background. This appearance results from the preserved follicular architecture of regenerative adenomatous hyperplasia showing higher echogenicity than the surrounding damaged parenchyma. The white knight sign is highly characteristic of benign Hashimoto nodule and helps assess malignancy risk as low.
'White knight' sign: A relatively echogenic (isoechoic-hyperechoic), well-defined, oval-shaped focal lesion within the heterogeneous, generally hypoechoic parenchyma of Hashimoto thyroiditis. The nodule appears brighter than surrounding parenchyma — this is the result of regenerative adenomatous hyperplasia or Hürthle cell metaplasia. A thin echogenic rim (pseudocapsule) may accompany. Suspicious TI-RADS findings such as microcalcifications, taller-than-wide shape are absent. In ACR TI-RADS, an isoechoic solid nodule scores 1 point (benign appearance).
Report Sentence
A ... mm isoechoic-hyperechoic, well-defined, oval-shaped nodule ('white knight' sign) is seen in the [right/left] thyroid lobe within a parenchyma showing Hashimoto thyroiditis background, without suspicious TI-RADS findings; consistent with benign Hashimoto nodule.
Diffusely heterogeneous, micronodular thyroid parenchyma — the characteristic ultrasonographic background of Hashimoto thyroiditis. Diffuse hypoechogenicity, micronodular structure separated by fibrous septa ('shattered glass' or 'honeycomb' pattern), and scattered hyperechoic fibrous bands are seen in the parenchyma. Glandular size may be enlarged (early stage) or atrophic (late stage). This heterogeneous background may complicate detection and characterization of focal nodules.
Report Sentence
Diffusely heterogeneous, micronodular parenchyma with hypoechoic areas separated by fibrous septa is seen in both thyroid lobes; these findings are consistent with Hashimoto thyroiditis.
Hashimoto nodule shows perinodular vascular pattern or minimal vascularity on color Doppler — no prominent intranodular hypervascularity. Diffusely increased vascularity in surrounding Hashimoto parenchyma ('thyroid inferno' pattern — in active inflammatory phase) may be seen. Perinodular halo flow reflects vessels in the pseudocapsule and is a benign finding.
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The thyroid nodule shows perinodular vascular pattern on color Doppler without prominent intranodular hypervascularity; this pattern is consistent with benign nodule. Diffusely increased vascularity related to Hashimoto thyroiditis is present in the surrounding parenchyma.
Diffusely decreased density of thyroid parenchyma on non-contrast CT — normal thyroid has 60-90 HU density due to high iodine content, while Hashimoto parenchyma decreases to 30-50 HU due to reduced iodine concentration from follicular damage. Focal nodule(s) may be seen within this diffusely hypodense parenchyma. Nodules may be isodense or mildly hyperdense (preserved follicular architecture).
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The thyroid parenchyma shows diffusely decreased density (... HU), consistent with Hashimoto thyroiditis; a ... mm focal nodule is seen within the parenchyma.
Hashimoto parenchyma shows heterogeneous signal on T2-weighted MRI — lymphocytic infiltration areas show high T2 signal (free water content), fibrous bands show low T2 signal, and preserved follicular areas show intermediate signal. Hashimoto nodules generally show intermediate-to-high T2 signal intensity. Malignant nodules (papillary carcinoma) generally show lower T2 signal — this may help in differential diagnosis.
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The thyroid parenchyma shows heterogeneous signal on T2-weighted MRI, consistent with Hashimoto thyroiditis; a ... mm well-defined nodule with intermediate-to-high T2 signal intensity is seen in the [right/left] lobe.
Hashimoto nodule (regenerative/adenomatous) shows similar or increased uptake compared to surrounding parenchyma on scintigraphy (warm or hot nodule). Hyperplastic follicular cells concentrate radiopharmaceutical in response to TSH stimulation. Malignancy risk in hot nodules is very low (1-2%). Cold (nonfunctional) nodules require more careful evaluation for malignancy — particularly for papillary carcinoma or lymphoma.
Report Sentence
A hot/warm nodule showing increased uptake compared to surrounding parenchyma is seen on Tc-99m pertechnetate scintigraphy in the [right/left] thyroid lobe, correlating with palpation; consistent with benign Hashimoto nodule.
Criteria
Benign nodule formed as compensatory hyperplasia in response to TSH stimulation. Histologically contains hyperplastic follicular cells and preserved follicular architecture. Not a true neoplasm.
Distinct Features
Isoechoic-hyperechoic, well-defined, oval. White knight sign. Warm/hot on scintigraphy. Benign follicular cells + Hürthle cells on FNA. Small sizes (<10 mm) generally do not require FNA.
Criteria
Focal concentration of lymphocytic infiltration and regeneration areas — not a true neoplasm or adenomatous hyperplasia. Considered as focal intensification of diffuse Hashimoto changes.
Distinct Features
More indistinct margins, more gradual transition with surrounding parenchyma. Size may change over time (may shrink or disappear). Minimal vascularity on Doppler. Lymphocytic infiltration + few follicular cells on FNA.
Criteria
Nodule composed of follicular cells showing oxyphilic (Hürthle/oncocytic) metaplasia. Hürthle cells are characterized by granular cytoplasm due to dense mitochondrial accumulation. Distinction between benign Hürthle cell nodule and Hürthle cell neoplasm may be difficult on FNA.
Distinct Features
More heterogeneous appearance on US, may be mildly hypoechoic. FNA may result in Bethesda III (atypical) or IV (follicular neoplasia). Distinction from Hürthle cell adenoma is made histopathologically in cases requiring surgery. Hürthle cell nodules are common in Hashimoto background but the vast majority are benign.
Distinguishing Feature
Papillary carcinoma is seen with increased frequency in Hashimoto background. Papillary carcinoma is hypoechoic (even more hypoechoic than surrounding Hashimoto parenchyma), shows microcalcifications, taller-than-wide shape, irregular margins, intranodular hypervascularity. Hashimoto nodule is isoechoic-hyperechoic (white knight), well-defined, oval, without microcalcifications.
Distinguishing Feature
Lymphoma is a rapidly growing, markedly hypoechoic (pseudocystic) mass showing posterior acoustic enhancement with very low ADC on DWI. Hashimoto nodule is slowly growing, iso-hyperechoic, well-defined, without posterior enhancement. Clinically, rapid growth suggests lymphoma while stability suggests benign nodule.
Distinguishing Feature
Follicular adenoma is a well-defined, thick-haloed, iso-hypoechoic nodule. Definitive distinction from Hashimoto nodule is made by FNA and histopathology. Follicular adenoma may result in Bethesda IV (follicular neoplasia) on FNA. Ultrasonographic distinction may be difficult; clinical context (Hashimoto history, anti-TPO positivity) is helpful.
Urgency
surveillanceManagement
surveillanceBiopsy
Not NeededFollow-up
12-monthNodules detected in Hashimoto background should be evaluated according to ACR TI-RADS criteria — Hashimoto background does not automatically make a nodule benign. Isoechoic-hyperechoic, well-defined, oval (white knight) nodules receive low TI-RADS score (TR2-TR3) and if <15 mm, FNA is not required; annual ultrasonographic surveillance is sufficient. However, hypoechoic, microcalcified, irregularly margined, or taller-than-wide nodules receive high TI-RADS score and FNA is recommended based on size threshold. Any rapidly growing nodule in Hashimoto patients should be urgently evaluated for lymphoma. Anti-TPO, anti-thyroglobulin antibodies, and TSH level should be checked. If hypothyroidism is present, levothyroxine should be started.
Hashimoto pseudonodules are benign and require no treatment. If hypothyroidism develops, levothyroxine replacement is started. Hashimoto thyroiditis increases lymphoma risk (60-80x) — lymphoma should be excluded in rapidly growing mass. ACR TI-RADS TR1-TR2.