A 54-year-old female presents with a 6-month history of worsening fatigue, low mood, poor concentration, weight gain of 15 pounds despite no change in eating habits, and loss of interest in activities she previously enjoyed. She reports feeling cold even when others are comfortable, and her husband notes she has become increasingly forgetful. She has no prior psychiatric history. On examination, her speech is notably slow, her affect is flat, and she appears puffy in the face. Her skin is dry and coarse, and deep tendon reflexes demonstrate a delayed relaxation phase. Heart rate is 56 bpm. She scores 18 on the PHQ-9. Her primary care provider started sertraline 50 mg three months ago with no improvement. Which of the following is the most appropriate next step in the diagnostic workup?