Review Question - QID 213867

A 35-year-old woman presents to her primary care physician complaining of decreased energy. She says she has been sleeping more than usual and is unable to "keep up with taking care of my family." On review of systems, she states she has gained 10 lbs, despite no change in appetite, and frequently feels constipated. She has no past medical history and is taking a daily multi-vitamin and fish oil. She works as a car mechanic and has had trouble bending her neck forward during routine labor. She has a 10-year smoking history and drinks alcohol occasionally. Her temperature is 97.9°F (36.6°C), blood pressure is 110/85 mmHg, pulse is 58/min, respirations are 18/min, and BMI is 31 kg/m^2. On physical exam, she has a mobile, nontender anterior neck mass with non-labored breathing. Her heart and lung sounds are normal. Her abdomen is soft with no masses. She has dry skin and 2+ deep tendon reflexes with delayed relaxation. The rest of the physical exam is unremarkable. Laboratory studies are shown below.

Serum:
Na+: 138 mEq/L
Cl-: 110 mEq/L
K+: 4.1 mEq/L
HCO3-: 24 mEq/L
BUN: 12 mg/dL
Glucose: 98 mg/dL
Creatinine: 0.7 mg/dL

TSH: 4.2 µU/mL
T4: 14 ug/dl
T3: 200 ng/dl

Ca2+: 8.5 mg/dL
AST: 16 U/L
ALT: 18 U/L

Which of the following is the most likely cause of this patient's current symptoms?