Incidental thyroid tissue in cervical lymph nodes in patients with fibrosing variant of Hashimoto thyroiditis

Bland thyroid follicles are occasionally identified within cervical lymph nodes, raising the differential diagnosis of benign thyroid inclusions versus metastatic papillary thyroid carcinoma. Proposed criteria for benign thyroid inclusions include lack of cytologic atypia, subcapsular or central nodal location, and involvement of only one or two lymph nodes. We observed several cases of fibrosing variant Hashimoto thyroiditis with multifocal thyroid tissue in cervical lymph nodes despite the absence of identifiable primary carcinoma. The pathology archives from 2014 to 2024 were searched for cases of fibrosing variant Hashimoto thyroiditis. Cases were reviewed for occult papillary thyroid carcinoma and thyroid tissue within sampled lymph nodes. IgG4 staining results were recorded. HBME-1, BRAF V600E, and NRAS Q61R immunostains were performed on nodal thyroid tissue. Thirty-six patients were identified (28 females, 8 males; median age 54 years, range 26-83). Increased IgG4-positive plasma cells were present in 29 cases. Lymph nodes were sampled in 23 patients, and thyroid tissue was identified in 8. No primary papillary thyroid carcinoma was identified in 6 of these 8 patients, including 4 in whom the entire thyroid gland was submitted for microscopic examination. HBME-1, BRAF V600E, and NRAS Q61R were negative in all nodal thyroid tissue. Our findings suggest that multifocal thyroid tissue within cervical lymph nodes is relatively common in fibrosing variant Hashimoto thyroiditis. The absence of primary carcinoma in most cases, together with bland cytology and negative immunohistochemical findings, supports the possibility that these represent benign thyroid inclusions that fall outside traditionally accepted diagnostic criteria.