A 68-year-old woman presents with a 2-week history of progressive weakness in her right arm and leg, associated with a tremor in the right hand. She also reports feeling more tired than usual. Her general practitioner noted an elevated TSH (12 mIU/L, reference range 0.4-4.0 mIU/L) on routine blood tests done a month ago, but free T4 was within the normal range (15 pmol/L, reference range 10-20 pmol/L). On examination, she has mild right-sided hemiparesis (MRC grade 4/5) and a coarse tremor in the right hand, worse with action. Reflexes are brisk on the right. The rest of the neurological exam is unremarkable. She has mild bradycardia (55 bpm) but no other signs of overt hypothyroidism. What is the most appropriate initial investigation?