A 17-year-old female presents with palpitations and hyperhidrosis for 6 months, and polydipsia and polyuria for 1 month. Physical examination reveals significant weight loss, mild exophthalmos, thyroid gland enlargement (Grade II), and vascular bruit at both upper poles. Lab results: FT<sub>3</sub> 33.5 pmol/L, FT<sub>4</sub> 40 pmol/L, TSH 0.01 mU/L (0.01 μIU/mL). Propranolol is added to her hyperthyroidism treatment. Which of the following is most critical to monitor during therapy?