A 40-year-old woman presents with a 3-month history of intermittent palpitations, sweating, anxiety, and a 5kg weight loss despite a good appetite. On examination, she has a fine tremor of her outstretched hands, a pulse rate of 110 bpm (regular), and a diffusely enlarged, non-tender thyroid gland. What is the most appropriate initial blood test to confirm the suspected diagnosis?