Q. A 45-year-old male patient with a history of Graves' disease presents to the clinic for a follow-up visit. The patient has been taking Propylthiouracil for the past 6 weeks to treat his hyperthyroidism. The patient reports improvement in his symptoms of palpitations, tremors, and weight loss. The patient's physical examination is notable for mild tachycardia and an enlarged thyroid gland. Laboratory tests reveal a normal serum thyrotropin (TSH) level and a normal free thyroxine (T4) level. What is the mechanism of action of propylthiouracil?