A 34-year-old female presents to the psychiatric clinic with a 2-month history of severe anxiety, racing thoughts, decreased need for sleep (sleeping only 3-4 hours yet feeling energized), pressured speech, and increased goal-directed activity. She has reorganized her entire house, started three new projects at work, and has been unusually talkative and confident. She has no prior psychiatric history. On examination, she is thin with a resting heart rate of 112 bpm and a fine tremor of outstretched hands. Her eyes appear prominent with mild proptosis and lid lag on downward gaze. She has a diffusely enlarged, nontender thyroid gland. Her skin is warm and moist, and she reports heat intolerance, unintentional weight loss of 12 pounds, and increased frequency of bowel movements. Deep tendon reflexes are brisk with shortened relaxation phase. Which diagnostic test is most critical to obtain before initiating psychiatric treatment?