Propylthiouracil
💊 Propylthiouracil (PTU) is a thioamide antithyroid drug.
✅ Preferred in the management of thyroid storm.
⚠️ Like carbimazole, it carries a serious risk of agranulocytosis.
🤰 Often used in pregnancy, but may cause neonatal hypothyroidism.
📖 About
- Check the BNF entry here for full prescribing advice.
- Belongs to the thioamide group of antithyroid drugs.
- Acts as a thyroid peroxidase inhibitor and also reduces peripheral T4→T3 conversion.
⚙️ Mechanism of Action
- Inhibits iodination of tyrosine residues in thyroglobulin.
- Inhibits coupling of iodotyrosine residues (preventing thyroxine synthesis).
- Reduces peripheral conversion of thyroxine (T4) to triiodothyronine (T3).
- May have a minor immunosuppressive effect on thyroid autoimmunity.
🎯 Indications & Dose
- Hyperthyroidism (especially if carbimazole not tolerated).
- Thyroid storm — PTU is first-line due to rapid T4→T3 inhibition.
- Pregnancy (Graves’ disease) — lowest effective dose recommended; balance maternal benefit vs fetal risk.
- Initial dose: 200–400 mg once daily orally until euthyroid.
- Maintenance dose: 50–150 mg/day.
- Equivalence: Carbimazole 1 mg ≈ PTU 10 mg.
⛔ Contraindications
- Active thyroiditis.
- Large obstructive goitre (risk of worsening obstruction).
- Caution in liver or renal impairment — dose adjustments may be required.
⚠️ Side Effects
- 💢 Common: Rash, nausea, arthralgia, transient leucopenia, goitre enlargement.
- 🩸 Agranulocytosis: Rare but life-threatening.
➡️ CSM guidance: patients must receive written advice to stop immediately and seek urgent care if they develop fever, sore throat, or mouth ulcers.
- 🫁 Hepatotoxicity: PTU can cause fulminant hepatic failure (especially in children).
- 🧠 Others: Nephritis, vasculitis.
- 🤰 Pregnancy: Risk of fetal and neonatal hypothyroidism with prolonged use.
📚 Clinical Pearls
- PTU is usually reserved for thyroid storm and early pregnancy (1st trimester) — carbimazole/methimazole preferred otherwise.
- Always provide clear agranulocytosis safety netting advice.
- Switch back to carbimazole after pregnancy if tolerated to reduce risk of hepatotoxicity.
- Monitor LFTs, FBC, and thyroid function during therapy.
