A 28-year-old woman presents with a 2-week history of anterior neck and pharyngeal pain, accompanied by heat intolerance, palpitations, hyperhidrosis, and generalized fatigue. Physical examination reveals a temperature of 38°C, weight loss, and moist skin. No exophthalmos is noted. The thyroid gland is enlarged to grade I, with bilateral tenderness. No bruit is heard, but fine tremor in both hands is present. Which of the following treatments is inappropriate for this condition?