What are the essential features of thionamides?
Examples
- Thiouracils:
- Propylthiouracil
- Imidazoles:
- Methimazole
- Carbimazole
Indications
- Thyroid storm
- After radioactive iodine treatment
- Before radioactive iodine treatment or thyroidectomy
Key Considerations
- Slow onset of action (3–4 weeks)
- In pregnancy
- 1st trimester: Propylthiouracil is recommended.
- 2nd/3rd trimester: carbimazole or methimazole recommended
Mechanism of Action
- Inhibits thyroid hormone production via inhibition of thyroid peroxidase
- Blocks iodide oxidation and organification
- Propylthiouracil also lowers peripheral conversion of T4 to T3 by inhibiting 5′-deiodinase.
Toxicity & Side Effects
Significant clinical side effects include:
- Agranulocytosis (<0.5%)
- Transient leucopaenia (10%)
- Mucocutaneous rash
- Hepatotoxicity
- Vasculitis
- Myopathy