Pharmacology · Antithyroid agents
Propylthiouracil !
An antithyroid tablet kept for the first trimester of pregnancy, thyroid storm or methimazole intolerance, because it can severely injure the liver.
Updated 2026-11-01
Also called: PTU
No human thyroid peroxidase structure with the drug exists; the figure shows propylthiouracil bound to lactoperoxidase (goat), a closely related heme peroxidase.
Mechanism of action
Propylthiouracil inhibits thyroid peroxidase, blocking organification of iodide and coupling of iodotyrosines, so no new T4 or T3 is made. Unlike methimazole, it also inhibits the peripheral deiodinase that converts T4 to the more active T3, which gives it a faster effect in thyroid storm. It does not affect hormone already stored in the gland.
- Propylthiouracil inhibits thyroid peroxidase in the gland
- New T4 and T3 synthesis stops
- In tissues it also blocks conversion of T4 to active T3
Pharmacokinetics
- Absorption
- Absorbed by mouth
- Distribution
- Crosses the placenta and can cause fetal goitre
Indications
- Hyperthyroidism in the first trimester of pregnancy or just before it Graves disease (hyperthyroidism) →
- Thyroid storm (severe thyrotoxicosis)
- Graves disease or toxic goitre in people who cannot take methimazole (except for agranulocytosis or hepatitis) Goitre →
Adverse effects
Common
- Skin rash and itching
- Nausea
- Joint pain
- Hypothyroidism if overtreated
Contraindications
- Hypersensitivity to propylthiouracil
Key interactions
- Warfarin
- Its effect may increase; INR is checked more often
- Beta-blockers
- Clearance falls as the patient becomes euthyroid; the dose may need lowering
- Digoxin
- Levels may rise as the patient becomes euthyroid
- Theophylline
- Clearance falls as the patient becomes euthyroid
What is monitored
- Liver tests and symptoms of liver injury
- Complete blood count if fever or sore throat appear
- Free T4, T3 and TSH
Administration and safety
- Given by mouth; doses are adjusted to thyroid tests
- After the first trimester, a switch to another antithyroid drug may be advisable
Nursing care
- Teach the patient to report fever, sore throat or mouth ulcers at once: possible agranulocytosis
- Ask about yellow skin, dark urine, itching or right upper abdominal pain (liver injury)
- Check heart rate, weight and symptoms of hyper- or hypothyroidism at each visit
- Ask about pregnancy plans: the choice of drug changes by trimester
Pregnancy and breastfeeding
Preferred in the first trimester because methimazole is linked to malformations; after that a switch is often advised because of liver risk.
Test yourself
Think of the answer, then tap the question to see it.
Which action does propylthiouracil have that methimazole lacks?
It inhibits peripheral deiodination of T4 to T3, which lowers active hormone faster in thyroid storm.
What boxed warning does propylthiouracil carry?
Severe liver injury and acute liver failure, sometimes fatal or needing a liver transplant.
When is propylthiouracil preferred over methimazole in pregnancy?
During or just before the first trimester, when methimazole carries the highest risk of malformations.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Propylthiouracil: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2017); DailyMed - PROPYLTHIOURACIL tablet (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).
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Representative model, for educational purposes only.