A 72-yeаr-оld femаle wаs admitted tо the hоspital with a week-long history of palpitations, dyspnea, and generalized weakness. She also gives history of decreased oral intake, weight loss, easy fatigability, and apathy. The patient has no significant previous medical history. On admission, vital signs include T: 98.2F, HR 130/min, BP 100/68, and RR 14/min. Chest x-ray is normal and EKG revealed atrial fibrillation. Skin appears warm and smooth. Which of the following interventions would be the most appropriate next step in the diagnostic evaluation of this patient?