Case Report: Concurrent thyroid tuberculosis and tuberculous lymphadenitis initially misdiagnosed as malignancy by 18F-FDG PET/CT.

BACKGROUND: Tuberculous lymphadenitis (TBL) is a common manifestation of extrapulmonary tuberculosis (EPTB). However, establishing a definitive diagnosis can be challenging, particularly in primary healthcare settings, when the clinical presentation is atypical or when TBL coexists with another rare form of EPTB-thyroid tuberculosis (TB). Whether primary or secondary, TB remains an exceedingly uncommon clinical entity, even in the context of the persistently high overall prevalence of tuberculosis in China. Thyroid tuberculosis accounts for approximately 0.1%-0.4% of all thyroid diseases, with an even lower incidence of primary thyroid involvement. Herein, we report a case in which 1⁸F-FDG PET/CT imaging initially suggested malignant lesions involving thyroid nodules and cervical lymph nodes; however, the final histopathological assessment and subsequent favorable response to diagnostic anti-tubercular therapy confirmed the diagnosis of concurrent thyroid tuberculosis and tuberculous lymphadenitis. CASE DESCRIPTION: The patient was a 68-year-old male presenting with a 10-day history of a palpable neck mass. Thyroid ultrasonography revealed multiple bilateral thyroid nodules classified as TI-RADS category 4b, accompanied by markedly elevated serum anti-thyroglobulin antibody (TGAb) and anti-thyroid peroxidase antibody (TPOAb) levels. An enlarged lymph node was detected in the right cervical region. An 1⁸F-FDG PET/CT scan performed at an external institution demonstrated lesions with increased FDG uptake involving the thyroid gland, multiple systemic lymph nodes, and the lungs. To clarify the diagnosis, ultrasound-guided core needle biopsy (CNB) was performed on the bilateral thyroid nodules and the right cervical lymph node. Histopathological examination revealed granulomatous inflammation, and tuberculosis could not be excluded. Polymerase chain reaction (PCR) testing and fungal polysaccharide immunofluorescence (IF) staining were both negative, while acid-fast bacilli staining showed suspicious positive results. The patient exhibited no specific clinical symptoms and denied a history of exposure to tuberculosis-endemic areas. Based on these findings, diagnostic anti-tubercular therapy was initiated. A limited initial follow-up assessment demonstrated a favorable therapeutic response, supporting the diagnosis, although the patient was subsequently lost to long-term follow-up due to financial and adherence issues. CONCLUSION: This case highlights the importance of considering the possibility of thyroid tuberculosis in clinical practice. With the increasing detection of thyroid nodules during routine examinations, accurate differentiation between benign and malignant etiologies is crucial. Incorporating tuberculosis infection into the differential diagnosis may help prevent unnecessary surgical interventions for suspected thyroid malignancy.