Background: Parathyroid cysts (PCs) are rare, constituting less than 0.5% of parathyroid lesions. PCs are often asymptomatic or present with non-specific symptoms, posing challenges for accurate diagnosis. Case Presentation: We report a case of a 22-year-old female presenting a painless, growing left-sided neck mass. Imaging, including contrast-enhanced neck CT, MRI, and high-resolution ultrasound, revealed a purely cystic lesion in the anterior upper mediastinum. Subsequent ultrasound-guided fine needle aspiration cytology (FNAC) confirmed the cystic nature. In situ PTH levels were elevated, leading to the diagnosis of a non-functioning parathyroid cyst. Surgical intervention was avoided, and symptomatic relief was achieved through US-guided cyst aspiration. Conclusion: This case emphasizes the rarity of non-functioning PCs and underscores the importance of a comprehensive diagnostic approach, including imaging studies and FNAC with in situ dosage of PTH levels.

Keywords: Parathyroid cyst- FNAC- In situ PTH- Diagnosis-Case report.