Review Question - QID 210086

An 18-year-old girl comes to the clinic because she is concerned about her weight. She is on her school’s cheerleading team and is upset because she feels she is the “fattest” girl on the team despite her healthy diet. In the last 2 weeks since practice began, she has lost 2 lbs. The patient has bipolar disorder. Her medications include lithium and a combined oral contraceptive pills that were recently started by her gynecologist, because “everyone is on it." Her mother has hypothyroidism and is treated with levothyroxine. Her temperature is 98.6°F (37.0°C), blood pressure is 117/78 mmHg, pulse is 90/min, respirations are 14/min, and oxygen saturation is 98% on room air. The patient’s BMI is 23.2 kg/m2. Thyroid function labs are drawn and shown below:

Thyroid-stimulating hormone (TSH): 4.0 mIU/L
Serum thyroxine (T4): 18 ug/dL
Free thyroxine (Free T4): 1.4 ng/dl (normal range: 0.7-1.9 ng/dL)
Serum triiodothyronine (T3): 210 ng/dL
Free triiodothyronine (T3): 6.0 pg/mL (normal range: 3.0-7.0 pg/mL)

Which of the following is the most likely cause of the patient’s abnormal lab values?