A 44-year-old woman presented to the otolaryngology clinic with a 6-month history of a right neck mass with compressive symptoms. She first noticed swelling that progressed to pressure and dyspnea while supine. She reported fatigue but denied pain, voice change, weight loss, hemoptysis, and dysphagia. A physical examination revealed a visible and palpable fullness to the right neck without overlying cutaneous or sinus tract changes. Results of a complete blood cell count, a thyroid stimulating hormone test, triiodothyronine and levorotatory thyroxine testing, and a complete metabolic panel were normal. Ultrasonography showed a 7.9 × 6.3-cm septate cystic mass lateral to the right thyroid gland. A contrast-enhanced computed tomography scan of the neck demonstrated a large, nonenhancing cystic-appearing lesion abutting the lateral and posterior margins of the right thyroid lobe and extending from the level of the piriform sinus to the thoracic inlet (Figure). Ultrasound-guided fine-needle aspiration revealed clear fluid with a parathyroid hormone (PTH) level of 67 pg/mL (reference range, <100 pg/mL; to convert to ng/L multiply by 1.00). A barium swallow study showed no communication between the cyst and the piriform sinus. Excision in the operating room was performed, and right thyroid lobectomy was necessary because of the intimate association of the lesion to the thyroid lobe.