Pharmacology · patient case question
What is the mechanism of action of propylthiouracil?
Patient case: Reviewing a long medication list
Patient: Female, 55 years old
Chief Complaint: Presents for a comprehensive examination.
Background: Graves disease, hypertension, asthma. She carries a naloxone kit because a family member has opioid use disorder.
Medications: Propylthiouracil · lisinopril · albuterol inhaler · naloxone (rescue kit)
Vital Signs: BP 132/84 mmHg · HR 84 bpm · RR 16
Show answer and explanation
Correct answer: D. Inhibits thyroid peroxidase, blocking thyroid hormone synthesis
- A. Beta-emitting destruction of follicular cells is radioactive iodine (I-131), a different treatment modality.
- B. The sodium-iodide symporter is inhibited by perchlorate and thiocyanate, not by thionamides.
- C. Replacing thyroxine is levothyroxine — the treatment for hypothyroidism, the opposite problem.
- D. CORRECT. Propylthiouracil inhibits THYROID PEROXIDASE, the enzyme that oxidises iodide and couples iodotyrosines. Without it, T3 and T4 cannot be made. Propylthiouracil has a second, distinctive action — it also blocks peripheral 5'-deiodinase conversion of T4 to the more active T3, which is why it is favoured in thyroid storm. Methimazole shares the peroxidase block but not the peripheral effect.