Review Question - QID 214776

A 42-year-old woman presents to the clinic with complaints of premature menopause. She describes episodes of “hot flashes” with associated heart palpitations for the past 3 months. However, she has continued to have regular menstrual periods during this time. She also endorses fatigue, increased appetite, and an unintentional 5-lb. weight loss. Past medical history includes diabetes that is currently diet-controlled and an upper respiratory infection 3 weeks ago that has resolved. Her temperature is 99.6°F (37.6°C), blood pressure is 145/90 mmHg, pulse is 115/min, and respirations are 21/min. A physical examination demonstrates tachycardia and a nontender palpable nodule at the neck. Laboratory studies are shown below, and a radioactive iodine uptake test (RAIU) is shown in Figure A.

Hemoglobin: 14 g/dL
Hematocrit: 39%
Leukocyte count: 10,900/mm^3 with normal differential
Platelet count: 210,000/mm^3

Serum:
Na+: 138 mEq/L
Cl-: 3.7 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): 0.1 uU/mL
Free T4: 37 ug/dL
Anti-thyroid peroxidase antibody (anti-TPO): 10 IU/mL (Normal: < 35 IU/mL)

Which of the following is the most likely explanation for this patient’s findings?