ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Thionamide Antithyroid Agent Pregnancy: Preferred in first trimester (carbimazole teratogenicity); switch back to carbimazole in T2/T3 due to PTU hepatotoxicity risk

Propylthiouracil (PTU)

Brand names: Propylthiouracil (generic)

Adult dose

Dose: Initial: 200–400 mg/day in 3–4 divided doses. Maintenance: 50–150 mg/day in divided doses.
Route: Oral
Frequency: TDS–QDS (shorter half-life than carbimazole)
Max: 600 mg/day
Second-line antithyroid agent (after carbimazole). Preferred in: first trimester of pregnancy, thyroid storm, and patients intolerant of carbimazole. More hepatotoxic than carbimazole — MHRA black box warning.

Paediatric dose

Dose: 1.67 mg/kg
Route: Oral
Frequency: Three times daily (total 5 mg/kg/day in 3 divided doses)
Max: 100 mg per dose (300 mg/day)
Concentration: 50 mg tablet mg/ml
BNFc paediatric Graves' disease (specialist initiation): 5–7 mg/kg/day in 3 divided doses initially; titrate down to maintenance with thyroid-function normalisation. MHRA black-box warning for hepatotoxicity — monitor LFTs, especially in first 6 months.

Dose adjustments

Renal

Reduce dose in severe renal impairment

Hepatic

Avoid — significant hepatotoxicity risk (fulminant hepatic failure possible)

Paediatric weight-based calculator

BNFc paediatric Graves' disease (specialist initiation): 5–7 mg/kg/day in 3 divided doses initially; titrate down to maintenance with thyroid-function normalisation. MHRA black-box warning for hepatotoxicity — monitor LFTs, especially in first 6 months.

Clinical pearls

  • MHRA safety warning: PTU causes rare fulminant hepatic failure — reserve for specific indications (first trimester, thyroid storm, carbimazole allergy)
  • Thyroid storm: PTU preferred over carbimazole as it also blocks peripheral T4→T3 conversion (additional benefit)
  • First trimester preference: carbimazole associated with aplasia cutis and choanal/oesophageal atresia — switch to PTU in T1
  • Check FBC urgently if patient reports sore throat or fever — agranulocytosis can be life-threatening

Contraindications

  • Previous PTU-induced agranulocytosis or hepatic damage
  • Severe hepatic impairment

Side effects

  • Agranulocytosis (rare but serious — check FBC urgently if fever/sore throat)
  • Hepatotoxicity (serious — monitor LFTs)
  • Rash
  • Arthralgia
  • Vasculitis/ANCA-positive vasculitis
  • Nausea
  • Hypothyroidism (over-treatment)

Interactions

  • Warfarin — hyperthyroidism increases warfarin requirement; PTU itself also has mild anticoagulant effect
  • Digoxin — dose adjustment may be needed as thyroid status changes

Monitoring

  • TFTs (T4/T3/TSH) every 4–6 weeks until stable
  • FBC if fever, sore throat, or infection symptoms
  • LFTs (baseline and periodically)
  • ANCA levels if vasculitis symptoms

Reference: BNFc; BNF; MHRA Drug Safety Update 2010; British Thyroid Association Guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Pathways