A 48-year-old woman is seen for evaluation of a slowly enlarging lower neck mass. Three months previously, she had symptoms consistent with an upper respiratory tract infection that resolved within 3 to 4 days, then noticed a lump in her lower neck just to the left of midline. She was treated with two courses of antibiotics, but the mass continued to enlarge. She had leftsided otalgia develop but no hoarseness, dysphagia, weight loss, fever, chills, or night sweats. Evaluation before referral included the normal thyroid-stimulating hormone (TSH) level and two fine-needle aspirations (FNAs) that were nondiagnostic. Ultrasound examination revealed a 2.9-cm 2.8-cm complex, thick-walled cyst adjacent to, but not clearly involving, the lower pole of the left lobe of the thyroid gland. A CT scan of the neck and upper mediastinum was obtained and is shown in Figure 1. Her medical history was significant in that she had no history of kidney stones, bone pain, depression, constipation, or prior radiation to the neck. She was an otherwise healthy nonsmoker, nondrinker. There was no family history of thyroid or parathyroid disease. Physical examination was remarkable for a thin, healthy-appearing woman with an approximately 33-cm mass to the left of the trachea above the sternal notch. The mass was firm, nontender, and elevated superiorly with swallowing. There was no lymphadenopathy on either side of the neck, and vocal fold mobility was normal bilaterally. The remainder of her head and neck examination was completely normal.