Rapidly enlarging solitary thyroid Langerhans cell histiocytosis in an adult: a case report

Solitary thyroid involvement in Langerhans cell histiocytosis (LCH) is extremely rare. When thyroid nodules exhibit rapid enlargement, they typically raise the suspicion of aggressive malignancies, such as anaplastic carcinoma, lymphoma, or metastatic thyroid tumors. However, the cytological features of LCH are often non-specific and inconclusive, making preoperative diagnosis quite challenging. In the present case report, we highlighted adult solitary thyroid-LCH as a rare but important differential diagnosis and demonstrated the diagnostic value of early surgical intervention. A 54-year-old Japanese woman with chronic thyroiditis presented with an isthmic thyroid nodule that enlarged from 1.0 to 3.0 cm over a three-month period. Ultrasonography revealed an ill-defined heterogeneously hypoechoic lesion suggestive of an unusual malignancy or inflammatory lesion. Fine-needle aspiration cytology revealed atypia of undetermined significance (Bethesda III), indicating the possibility of a lymphoma or anaplastic carcinoma. Thyroid function was normal and no symptom of diabetes insipidus was observed. Since a high-grade malignancy could not be excluded, the patient underwent subtotal thyroidectomy with central compartment dissection. Histology revealed the proliferation of epithelioid cells with irregular nuclei mixed with eosinophils. Immunohistochemistry revealed immunoreactivity for S-100 protein, langerin, and CD1a, establishing the diagnosis of LCH. The Ki-67 labeling index was approximately 10%. BRAF V600E variants were not detected. Postoperative FDG-PET/CT revealed no extrathyroidal lesion, suggesting a single-system disease confined to the thyroid. Follow-up at six months showed no evidence of recurrence. Adult solitary thyroid-LCH should be considered in the differential diagnosis of rapidly enlarging thyroid nodules. When cytology is inconclusive, and an aggressive malignancy cannot be excluded, immunohistochemical characterization without delay is essential, and surgical excision is an option for achieving this.