Review Question - QID 214796

An otherwise healthy 35-year-old woman presents to the clinic with complaints of a neck mass. She noticed the mass about 6 months ago when she had a cold and thought it was an inflamed lymph node. However, the mass slowly grew and is now uncomfortable. She reports fatigue, mild dysphagia, and malaise but denies fever, chills, weight change, shortness of breath, or chest pain. She has no significant past medical history and takes no medications. Her temperature is 98.6°F (37.0°C), blood pressure is 115/70 mmHg, pulse is 70/min, and respirations are 12/min. Physical examination demonstrates a hard, non-tender, and symmetrically enlarged thyroid that is immobile with swallowing. Laboratory testing is shown below.

Hemoglobin: 15 g/dL
Hematocrit: 37%
Leukocyte count: 10,300/mm^3 with normal differential
Platelet count: 190,000/mm^3

Serum:
Na+: 137 mEq/L
Cl-: 3.8 mEq/L
K+: 4.2 mEq/L
HCO3-: 25 mEq/L
BUN: 11 mg/dL
Glucose:134 mg/dL
Creatinine: 0.9 mg/dL
Thyroid-stimulating hormone (TSH): 8.6 uIU/mL
Free T4: 0.2 ug/dL
Anti-thyroid peroxidase antibody (anti-TPO): 10 IU/mL (Normal: < 35 IU/mL)

Which of the following is the most likely underlying etiology of this patient’s symptoms?