AlternativePTU

Propylthiouracil (PTU)

Thionamide antithyroid

Preferred antithyroid in 1st trimester (carbimazole switch in T2).

Oral

First-line for

Hyperthyroidism (Graves') in 1st trimester
Thyroid storm

Mechanism

Inhibits thyroid peroxidase → ↓ T4/T3 synthesis. High dose also blocks peripheral T4→T3 conversion.

“P-1, C-2”

PTU in 1st trimester (less aplasia cutis)
Carbimazole in 2nd/3rd (less hepatotoxicity)
Lowest dose to keep T4 high-normal
Monitor fetal goitre on USS

Dosing

Start: 150–450 mg/day divided TDS
Maintenance: 50–150 mg/day
Thyroid storm: 200 mg q4h PO/NG + Lugol's iodine + β-blocker + steroid

Contraindications

AbsoluteSevere hepatic impairment
CautionSwitch to carbimazole at 16 weeks

Side effects

Rash, pruritus
Agranulocytosis (warn: sore throat → urgent FBC)
Hepatotoxicity (PTU > carbimazole)
Vasculitis (ANCA+)

Fetal & maternal notes

• Crosses placenta — can cause fetal hypothyroidism/goitre at high doses.
• Carbimazole: rare embryopathy (aplasia cutis, choanal atresia).

Key interactions

Warfarin
Caution
↑ anticoagulant effect
Digoxin
Caution
Levels rise as patient becomes euthyroid

Clinical pearls

💡 Aim free T4 in upper third of normal range.
💡 Breastfeeding safe at ≤300 mg/day PTU or ≤20 mg carbimazole.

Content last reviewed: REPLACE_ME (e.g. 2026-07)

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