Hypothyroidism Quiz: Ace Your Exams

A 33-year-old woman comes to the clinic complaining of excessive fatigue for the last several weeks. The patient gave birth to her first child 7 months ago; the pregnancy and vaginal delivery were uneventful. She breastfed for the first 6 months. The patient noted that after delivery, she was constantly worried about the wellbeing of her child. She states that she had trouble sleeping, anxiety, and diminished concentration; these symptoms resolved after 2 weeks. She has recently gained 12-lb in the last 5 weeks and experienced constipation, which is well-managed with an oral laxative. Temperature is 37.0°C (98.6°F), pulse is 58/min, and blood pressure is 124/88 mm Hg. Heart and lung sounds are normal. There is mild non-pitting edema in the bilateral lower extremities. Laboratory studies reveal the following:  

Laboratory value	Result	Reference Range
Sodium	137 mEq/L	136-146 mEq/L
Potassium	4.8 mEq/L	3.5-5 mEq/L
Chloride	99 mEq/L	95-105 mEq/L
Glucose	97 mg/dL	70-110 mg/dL
Hemoglobin	12.6 g/dL	12-16 g/dL
Hematocrit	38%	36-46%

Which of the following is the most likely cause of this patient’s condition?